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19-2335; Rev 1; 6/02
Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies
General Description
The MAX1917 provides a complete power-management solution for DDR memory. It contains a synchronous buck controller and an amplifier to generate 1/2VDDR voltage for VTT and VTTR. The VTT and VTTR voltages are maintained within 1% of 1/2VDDR. The controller operates in synchronous rectification mode to ensure balanced current sourcing and sinking capability of up to 25A. With a shutdown current of less than 5A, the MAX1917 is the best choice for low-power notebook applications, as well as servers and desktop computers. An all N-FET design optimizes efficiency. The MAX1917 can also be used for generating VDDR and as a general-purpose step-down controller with variable switching frequency as high as 1MHz with few additional components. The MAX1917 uses Maxim's proprietary Quick-PWMTM architecture for fast transient response up to 96% efficiency, and the smallest external components. Output current monitoring is achieved without sense resistors by monitoring the bottom FET's drain-to-source voltage. The current-limit threshold is programmable through an external resistor. The MAX1917 comes in a space-saving 16-pin QSOP package. o 25A Sourcing and Sinking Current o Automatically Sets VTT to 1/2VDDR o VTT and VTTR Within 1% of 1/2VDDR o Smallest Output Capacitors o 4.5V to 14V (or 28V with Resistor-Divider) Input Voltage Range o 1.5V to 3.6V Input DDR Range o 200kHz/300kHz/400kHz/550kHz Preset Switching Frequencies o Variable Switching Frequency of Up to 1MHz o Overcurrent Protection Without Current-Sense Resistor o Internal Soft-Start o VTTR Reference Sources and Sinks Up to 25mA o Quick-PWM Control for Fastest Loop Response o Up to 96% Efficiency o 16-Pin QSOP Package
Features
MAX1917
Applications
DDR Memory Power Supply Processor or DSP Core Supply AGTL Bus Termination Supply Notebook Computers Desktop Computers Storage Networking Systems
Ordering Information
PART MAX1917EEE TEMP RANGE -40C to +85C PIN-PACKAGE 16 QSOP
Typical Operating Circuit
VIN (2.5V TO 15V)
Quick-PWM is a trademark of Maxim Integrated Products, Inc.
Pin Configuration
POK
EN/HSD POK VDDR DDR (4.5V TO 14V) V+ REF ILIM FSEL GND
DH VL VOUT
TOP VIEW
EN/HSD 1 DDR 2 POK 3 VTT 4 ILIM 5 FSEL 6 REF 7 GND 8 16 LX 15 DH 14 BST
MAX1917
BST LX DL PGND VTT VTTR VTTR
MAX1917EEE
13 PGND 12 DL 11 VL 10 V+ 9 VTTR
VL
QSOP
________________________________________________________________ Maxim Integrated Products
1
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies MAX1917
ABSOLUTE MAXIMUM RATINGS
V+ to GND ..............................................................-0.3V to +15V EN/HSD to GND .....................................................-0.3V to +16V VL to GND ................................................................-0.3V to +6V PGND to GND .......................................................-0.3V to +0.3V VTT, DDR, POK to GND ...........................................-0.3V to +6V REF, VTTR, DL, ILIM, FSEL to GND ................-0.3V to VL + 0.3V LX to PGND ............................................................-0.3V to +30V BST to GND ............................................................-0.3V to +36V DH to LX ......................................................-0.3V to VBST + 0.3V LX to BST..................................................................-6V to +0.3V REF Short Circuit to GND ...........................................Continuous Continuous Power Dissipation (TA = +70C) 16-Pin QSOP (derate up to +70C)..............................667mW 16-Pin QSOP (derating above +70C) .....................8.3mW/C Operating Temperature Range Extended .........................................................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering 10s.) .................................+300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V+ = 12V, VEN/HSD = VDDR = 2.5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0, ILIM = VL, PGND = LX = POK = GND, BST = VL. Specifications are for TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise specified.)
PARAMETER V+ Input Voltage Range EN/HSD Input Voltage Range DDR Input Voltage Range V+ Supply Current DDR Supply Current EN/HSD Supply Current VL Supply Current V+ Shutdown Supply Current DDR Shutdown Supply Current VL Shutdown Supply Current VL Undervoltage Lockout Threshold VTT VTT Input Bias Current VTT Feedback Voltage Range VTT Feedback Voltage Accuracy REFERENCE Reference Output Voltage Reference Load Regulation Reference UVLO VTTR VTTR Output Voltage Range IVTTR = -5mA to +5mA VTTR Output Accuracy IVTTR = -25mA to +25mA, VDDR = 1.8V IVTTR = -25mA to +25mA, VDDR = 3.6V 0 49.5 49 49.5 50 50 50 1.8 50.5 51 50.5 %VDDR V V+ = VL = 4.5V to 5.5V, IREF = 0 V+ = VL = 5V, IREF = 0 to 50A V+ = VL = 5V 1.5 1.6 1.98 2.00 2.02 10 1.7 V mV V Overload range, VDDR = 1.8V Overload range, VDDR = 3.6V VVTT = 2.5V -0.1 0 49.5 49.5 50 50 0 1.8 50.5 50.5 A V %VDDR VL = V+ = 5.5V, VTT = 2.0V EN/HSD = 0V EN/HSD = 0V VL = V+ = 5.5V Rising edge, hysteresis = 40mV 4.05 3 4.25 VTT = 2.0V VL = V+ Enabled CONDITIONS MIN 5.5 4.5 1.5 0 0.8 115 5 0.8 3 TYP MAX 14.0 5.5 15.0 3.6 1.2 250 10 1.2 5 1 5 4.40 UNITS V V V mA A A mA A A A V
2
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 12V, VEN/HSD = VDDR = 2.5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0, ILIM = VL, PGND = LX = POK = GND, BST = VL. Specifications are for TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise specified.)
PARAMETER Thermal Shutdown SOFT-START ILIM Ramp Period OSCILLATOR FSEL = VL Oscillator Frequency FSEL = unconnected FSEL = REF FSEL = GND FSEL = VL On Time (Note 1) FSEL not connected FSEL = REF FSEL = GND Minimum Off Time (Note 1) CURRENT LIMIT LX to PGND, ILIM = VL Current-Limit Threshold (Positive Direction) Current-Limit Threshold (Negative Direction) ILIM Input Current VL REGULATOR Output Voltage Line Regulation RMS Output Current Bypass Capacitor DRIVER DH Gate-Driver On-Resistance DL Gate-Driver On-Resistance (Source) DL Gate-Driver On-Resistance (Sink) Dead Time FSEL LOGIC Logic Input Current Logic GND Level Logic REF Level Logic Float Level FSEL = VREF FSEL floating 1.65 3.15 -3 3 0.5 2.35 3.85 A V V V VBST - VLX = 5V DL high state DL low state DL rising DL falling 1.4 1.6 0.75 32 30 2.5 3 1.25 ns ESR < 100m 2.2 5.5V < V+ < 14V 1mA < IVL < 35mA 5.5V < V+ < 14V, IVL = 10mA 4.8 5.0 0.2 35 5.2 V % mA F LX to PGND, RILIM = 100k LX to PGND, RILIM = 400k LX to PGND, ILIM = VL, with percentage of positive current-limit threshold 90 40 170 -90 100 50 200 -110 5 110 60 230 -130 % A mV 2.18 1.45 1.09 0.82 200 300 400 550 2.5 1.67 1.25 0.91 350 2.83 1.89 1.41 1.00 400 ns s kHz Ramps the ILIM trip threshold from 20% to 100% in 20% increments 1.7 ms CONDITIONS Rising temperature, typical hysteresis = 15C MIN TYP +160 MAX UNITS C
MAX1917
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies MAX1917
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 12V, VEN/HSD = VDDR = 2.5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0, ILIM = VL, PGND = LX = POK = GND, BST = VL. Specifications are for TA = 0C to +85C. Typical values are at TA = +25C, unless otherwise specified.)
PARAMETER Logic VL Level EN/HSD LOGIC EN/HSD Shutdown Current Logic High Logic Low POWER-OK OUTPUT Upper VTT Threshold Lower VTT Threshold Upper VTTR Threshold Lower VTTR Threshold POK Output Low Level POK Output High Leakage ISINK = 2mA VPOK = 5.5V 110 86 110 86 112 88 112 88 114 90 114 90 0.4 5 %VDDR/ 2 %VDDR/ 2 %VDDR/ 2 %VDDR/ 2 V A Max IEN/HSD for VEN/HSD = 0.8V VL = V+ = 4.5V to 5.5V, 100mV hysteresis VL = V+ = 4.5V to 5.5V 0.5 1.45 0.8 3.0 A V V FSEL = VL CONDITIONS MIN VL - 0.4 TYP MAX UNITS V
ELECTRICAL CHARACTERISTICS
(V+ = 12V, VEN/HSD = VDDR = 2.5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0, ILIM = VL, PGND = LX = POK = GND, BST = VL. Specifications are for TA = -40C to +85C, unless otherwise specified.) (Note 2)
PARAMETER V+ Input Voltage Range EN/HSD Input Voltage Range DDR Input Voltage Range V+ Supply Current DDR Supply Current EN/HSD Supply Current VL Supply Current V+ Shutdown Supply Current DDR Shutdown Supply Current VL Shutdown Supply Current VL Undervoltage Lockout Threshold VTT VTT Input Bias Current VTT Feedback Voltage Range VTT Feedback Voltage Accuracy REFERENCE Reference Output Voltage Reference Load Regulation V+ = VL = 4.5V to 5.5V, IREF = 0 V+ = VL = 5V, IREF = 0 to 50A 1.98 2.02 10 V mV Overload range, VDDR = 1.8V Overload range, VDDR = 3.6V VVTT = 2.5V -0.15 0 49.5 49.5 0 1.8 50.5 50.5 A V %VDDR VEN/HSD = 2.5V VL = V+ = 5.5V EN/HSD = 0V EN/HSD = 0V VL = V+ = 5.5V Rising edge, hysteresis = 40mV 4.05 VTT = 2.0V VL = V+ Enabled CONDITIONS MIN 5.5 4.5 1.5 0 TYP MAX 14.0 5.5 15.0 3.6 1.2 250 10 1.2 5 1 5 4.40 UNITS V V V V mA A A mA A A A V
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 12V, VEN/HSD = VDDR = 2.5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0, ILIM = VL, PGND = LX = POK = GND, BST = VL. Specifications are for TA = -40C to +85C, unless otherwise specified.) (Note 2)
PARAMETER Reference UVLO VTTR VTTR Output Voltage Range IVTTR = -5mA to +5mA VTTR Output Accuracy OSCILLATOR FSEL = VL On Time (Note 1) FSEL not connected FSEL = REF FSEL = GND Minimum Off Time (Note 1) CURRENT LIMIT LX to PGND, ILIM = VL Current-Limit Threshold (Positive Direction) Current-Limit Threshold (Negative Direction) VL REGULATOR Output Voltage RMS Output Current Bypass Capacitor FSEL LOGIC Logic Input Current Logic GND Level Logic REF Level Logic Float Level Logic VL Level EN/HSD LOGIC EN/HSD Shutdown Current Logic High Logic Low POWER-OK OUTPUT Upper VTT Threshold Lower VTT Threshold 110 86 114 90 %VDDR /2 %VDDR /2 IEN/HSD for VEN/HSD = 0.8V VL = V+ = 4.5V to 5.5V, 100mV hysteresis VL = V+ = 4.5V to 5.5V 0.5 1.45 0.8 3.0 A V V FSEL = VREF FSEL floating FSEL = VL 1.65 3.15 VL - 0.4 3 0.5 2.35 3.85 A V V V V ESR < 100m 2.2 5.5V < V+ < 14V; 1mA < IVL < 35mA 4.8 5.2 35 V mA F LX to PGND, RILIM = 100k LX to PGND, RILIM = 400k LX to PGND, ILIM = VL, with percentage of positive current-limit threshold 85 35 160 -90 110 60 230 -130 % mV 2.18 1.45 1.09 0.82 2.83 1.89 1.41 1.00 400 ns s IVTTR = -25mA to +25mA, VDDR = 1.8V IVTTR = -25mA to +25mA, VDDR = 3.6V 0 49.5 49 49.5 1.8 50.5 51 50.5 %VDDR V V+ = VL = 5V CONDITIONS MIN 1.5 TYP MAX 1.7 UNITS V
MAX1917
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies MAX1917
ELECTRICAL CHARACTERISTICS (continued)
(V+ = 12V, VEN/HSD = VDDR = 2.5V, CVL = 4.7F, CVTTR = 1F, CREF = 0.22F, VFSEL = 0, ILIM = VL, PGND = LX = POK = GND, BST = VL. Specifications are for TA = -40C to +85C, unless otherwise specified.) (Note 2)
PARAMETER Upper VTTR Threshold Lower VTTR Threshold POK Output Low Level POK Output High Leakage ISINK = 2mA VPOK = 5.5V CONDITIONS MIN 110 86 TYP MAX 114 90 0.4 5 UNITS %VDDR /2 %VDDR /2 V A
Note 1: On Time and Off Time specifications are measured from 50% point to 50% point at the DH pin with LX forced to 0V, BST forced to 5V, and a 250pF capacitor connected from DH to LX. Actual in-circuit times may differ due to MOSFET switching speeds. Note 2: Specifications to -40C are guaranteed by design and are not production tested.
Typical Operating Characteristics
(V+ = 12V, VOUT = 1.25V, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT AT 1.25V OUTPUT
MAX1917 toc01
EFFICIENCY vs. LOAD CURRENT AT 0.9V OUTPUT
MAX1917 toc02
EFFICIENCY vs. LOAD CURRENT AT 1.25V OUTPUT
90 80 EFFICIENCY (%) 70 60 50 40 30 20 VIN = 12V VIN = 5V VIN = 2.5V
MAX1917 toc03
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 0.1 1.0 LOAD CURRENT (A) CIRCUIT OF FIGURE 3 VIN = 2V VIN = 5V VIN = 2.5V
96 94 92 EFFICIENCY (%) 90 88 86 84 82 80 78 76 0.1 CIRCUIT OF FIGURE 3 1.0 LOAD CURRENT (A) VIN = 5V VIN = 2.5V
100
10 0 0.1
CIRCUIT OF FIGURE 4 1.0 LOAD CURRENT (A) 10.0
10.0
10.0
EFFICIENCY vs. LOAD CURRENT AT 0.9V OUTPUT
MAX1917 toc04
FREQUENCY vs. LOAD CURRENT AT 1.25V OUTPUT
MAX1917 toc05
FREQUENCY vs. LOAD CURRENT AT 0.9V OUTPUT
600 FREQUENCY (kHz) 550 500 VIN = 5V 450 400 350 VIN = 2.5V
MAX1917 toc06
95 90 85 EFFICIENCY (%) 80 75 70 65 60 55 50 0.1 1.0 LOAD CURRENT (A) VIN = 5V VIN = 2.5V
650 600 FREQUENCY (kHz) 550 500 450 400 350 VIN = 12V VIN = 2.5V VIN = 5V
650
CIRCUIT OF FIGURE 4 300 10.0 -8 -6 -4 -2 0
CIRCUIT OF FIGURE 3 300 2 4 6 8 -8 -6 -4 -2 0
CIRCUIT OF FIGURE 3 2 4 6 8
LOAD CURRENT (A)
LOAD CURRENT (A)
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies
Typical Operating Characteristics (continued)
(V+ = 12V, VOUT = 1.25V, TA = +25C, unless otherwise noted.)
MAX1917
FREQUENCY vs. INPUT VOLTAGE
MAX1917 toc07
FREQUENCY vs. TEMPERATURE
538 536 FREQUENCY (kHz) 534 532 530 528 526 524 ILOAD = 3.5A
MAX1917 toc08
SHUTDOWN CURRENT vs. INPUT VOLTAGE
MAX1917 toc09
650 600 IOUT = 7A 550 FREQUENCY (kHz) 500 IOUT = 0.1A 450 400 350 300 1 CIRCUIT OF FIGURE 3 3 5 7 9 11 13
540
4.0 3.5 SHUTDOWN CURRENT (A) 3.0 2.5 2.0 1.5
522 520 15 CIRCUIT OF FIGURE 3 -40 -20 0 20 40 60 80 100 1.0 1
CIRCUIT OF FIGURE 6 3 5 7 9 11 13 15
INPUT VOLTAGE (V)
TEMPERATURE (C)
INPUT VOLTAGE (V)
SHUTDOWN CURRENT vs. INPUT VOLTAGE
MAX1917 toc10
LOAD REGULATION WITHOUT DROOP RESISTOR
MAX1917 toc11
LOAD REGULATION WITH DROOP RESISTOR
1.33 1.31 OUTPUT VOLTAGE (V) 1.29 1.27 1.25 1.23 1.21 1.19 1.17 1.15 VIN = 2.5V VDDR = 2.5V VIN = 12V VIN = 5V
MAX1917 toc12
2.5 2.3 SHUTDOWN CURRENT (mA) 2.1 1.9 1.7 1.5 1.3 1.1 0.9 0.7 0.5 1 3 5 7 CIRCUIT OF FIGURE 3 9 11 13
1.32 1.31 1.30 OUTPUT VOLTAGE (V) 1.29 1.28 1.27 1.26 1.25 1.24 1.23 1.22
VDDR = 2.5V
1.35
VIN = 5V VIN = 12V
VIN = 2.5V
15
-7
-5
-3
-1
1
3
5
7
-8
-6
-4
-2
0
2
4
6
8
INPUT VOLTAGE (V)
LOAD CURRENT (A)
LOAD CURRENT (A)
TRANSIENT LOAD RESPONSE WITHOUT DROOP RESISTOR
MAX1917 toc13
TRANSIENT LOAD RESPONSE WITH DROOP RESISTOR
MAX1917 toc14
POWER-UP WITH VDDR TRACKING
MAX1917 toc15
VOUT 1V/div VVTT 50mV/div VVTT 50mV/div VPOK 5V/div VDDR 2.5V 0A IOUT 5A/div IOUT 5A/div 400s/div VEN/HSD 10V/div CIRCUIT OF FIGURE 3 20s/div
OA
CIRCUIT OF FIGURE 3 20s/div
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies MAX1917
Typical Operating Characteristics (continued)
(V+ = 12V, VOUT = 1.25V, TA = +25C, unless otherwise noted.)
CURRENT LIMIT vs. TEMPERATURE
MAX1917 toc16
INDUCTOR PEAK AND VALLEY CURRENT vs. INPUT VOLTAGE AT CURRENT LIMIT
RILIM = 400k 16 INDUCTOR CURRENT (A)
MAX1917 toc17
18 16 14 CURRENT LIMIT (A) 12 10 8 6 4 2 0 -40 -20 0 20 40 60 80 RILIM = 200k RILIM = 402k
18
14
IPEAK
12 IVALLEY 10 CIRCUIT OF FIGURE 3 8
100
1
3
5
7
9
11
13
15
TEMPERATURE (C)
VIN (V)
Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 NAME EN/HSD DDR POK VTT ILIM FSEL REF GND VTTR V+ VL DL PGND BST DH LX FUNCTION Enable/High-Side Drain. Connect to the high-side N-FET drain for normal operation. Leave unconnected or connect to GND for low-power shutdown. DDR Reference Input. An applied voltage at DDR sets VVTT and VVTTR to 1/2VDDR. DDR voltage range is from 0 to 3.6V. Power-OK Output. POK is an open-drain output and is logic high when both VTT and VTTR are within 12% of regulation. POK is pulled low in shutdown. VTT Feedback Input. Connect to VTT output. Current-Limit Threshold Adjustment. Connect a resistor from ILIM to GND to set the current-limit threshold, or connect ILIM to VL for default setting. See the Setting the Current Limit section. Frequency Select. Selects the switching frequency of the MAX1917. See Table 1 for configuration of FSEL. Reference Bypass. Connect a 0.22F or larger capacitor from REF to GND. Ground VTTR Reference Output. Connect a 1F or larger capacitor from VTTR to GND. VTTR is capable of sourcing and sinking up to 25mA. Input Supply Voltage. Supply input for the VL regulator and the VTTR regulator. Bypass with a 0.22F or larger capacitor. Internal Regulator Output. Connect a 2.2F or larger capacitor from VL to GND. VL can be connected to V+ if the operating range is 4.5V to 5.5V. Low-Side MOSFET Gate Drive. Connect to the gate of the low-side N-channel MOSFET. Power Ground Bootstrapped Supply to Drive High-Side N-Channel MOSFET. Connect a 0.47F or larger capacitor from BST to LX. High-Side MOSFET Gate Drive. Connect to the high-side N-channel MOSFET gate. Inductor Switching Node
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies
Functional Diagram
ILIM
MAX1917
1-SHOT FSEL ON TIME COMPUTE VTT Q TRIG 5A BST
TON TRIG POK 1-SHOT Q
S Q R
DH
LX
DDR
VL
DL EN/HSD PGND VTT
V+ VL
5V REF 2V REF
REF GND
MAX1917
VTTR
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies MAX1917
Detailed Description
Internal Linear Regulator (VL)
An internal regulator produces the 5V supply (VL) that powers the PWM controller, MOSFET driver, logic, reference, and other blocks within the IC. This 5V lowdropout (LDO) linear regulator supplies up to 35mA for MOSFET gate-drive and external loads. For supply voltages between 4.5V and 5.5V, connect VL to V+. This bypasses the VL regulator, which improves efficiency, and allows the IC to function at lower input voltages. fS = 0.5 x VDDR + IO (RDSONL + RDC ) x 103 kHz t ON x VIN + IO (RDSONL - RDSONH )
(
)
On-Time One-Shot and Switching Frequency
The heart of the PWM is the one-shot that sets the highside switch on time. This fast, low-jitter, adjustable oneshot includes circuitry that varies the on time in response to both input and output voltages. The high-side switch on time is inversely proportional to the input voltage as measured by the EN/HSD input, and is directly proportional to the VTT output voltage. This algorithm results in a nearly constant switching frequency despite the lack of a fixed-frequency clock generator. The switching frequency can be selected to avoid noise-sensitive regions such as the 455kHz IF band. Also, with a constant switching frequency, the inductor ripple-current operating point remains relatively constant, resulting in easy design methodology and predictable output voltage ripple. The general formula for on time (tON) is: t ON = K x N x V x DDR s VHSD 2 1
where IO is the output current, RDSONH is the on-resistance of the high-side MOSFET, R DSONL is the onresistance of the low-side MOSFET, and RDC is the DC resistance of the output inductor. The above equation is valid only when FSEL is connected to ground. The ideal switching frequency for VDDR = 2.5V is about 550kHz. The switching frequency, which is almost constant, results in relatively constant inductor ripple current regardless of input voltage and predictable output voltage ripple. This feature eases design methodology. Switching frequency increases for positive (sourcing) load current and decreases for negative (sinking) load current, due to the changing voltage drop across the low-side MOSFET, which changes the inductor-current discharge ramp rate. The on times guaranteed in the Electrical Characteristics tables are also influenced by switching delays caused by the loading effect of the external power MOSFETs.
VTTR Reference
The MAX1917 VTTR output is capable of sourcing or sinking up to 25mA of current. The VTTR output voltage is one half of the voltage applied to the DDR input. Bypass VTTR with at least a 1.0F capacitor.
EN/HSD Function
In order to reduce pin count and package size, the MAX1917 features a dual-function input pin, EN/HSD. When EN/HSD is connected to ground, the internal circuitry powers off, reducing current consumption to less than 5A typical (circuit of Figure 6). To enable normal operation, connect EN/HSD to the drain of the high-side MOSFET. If EN/HSD is not grounded, it becomes an input that monitors the high-side MOSFET drain voltage (converter input voltage) and uses that measurement to calculate the appropriate on time for the converter. Therefore, EN/HSD must be connected to this node in order for the controller to operate properly.
where VHSD and VDDR are the voltages measured at EN/HSD and DDR, respectively, and K = 1.7s. The value of N depends on the configuration of FSEL and is listed in Table 1. The actual switching frequency, which is given by the following equation, varies slightly due to voltage drop across the on-resistance of the MOSFETs and the DC resistance of the output inductor:
Table 1. Configuration of FSEL
FSEL CONNECTED TO Ground REF Floating VL N 1.00 1.33 2.00 3.00 tON (s) 0.91 1.25 1.66 2.50 FREQUENCY (kHz) 550 400 300 200 CONDITION 0.5VDDR / VHSD = 0.5 0.5VDDR / VHSD = 0.5 0.5VDDR / VHSD = 0.5 0.5VDDR / VHSD = 0.5
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies
Voltage Reference
The voltage at REF is nominally 2.00V. Connect a 0.22F ceramic bypass capacitor between REF and GND. various MOSFETs. A capacitor in parallel with RILIM can provide a variable soft-start function. Carefully observe the PC board layout guidelines to ensure that noise and DC errors do not corrupt the current-sense signals seen by LX and PGND. The IC must be mounted close to the low-side MOSFET with short, direct traces making a Kelvin-sense connection to the source and drain terminals. See the PC Board Layout section.
MAX1917
Overcurrent Protection
The current-limit circuit employs a unique "valley" current-sensing algorithm that uses the on-state resistance of the low-side MOSFET as a current-sensing element. If the current-sense signal is greater than the currentlimit threshold, the PWM is not allowed to initiate a new cycle. The actual peak current is greater than the current-limit threshold by an amount equal to the inductor ripple current. Therefore, the exact current-limit characteristic and maximum load capability are a function of the MOSFET on-resistance, inductor value, and input voltage. The reward for this uncertainty is robust, lossless overcurrent sensing. There is also a negative current limit that prevents excessive reverse inductor currents when VOUT is sinking current. The negative current-limit threshold is set to approximately 110% of the positive current limit, and tracks the positive current limit when ILIM is adjusted. The current-limit threshold can be adjusted with an external resistor (RILIM) at ILIM. A precision 5A pullup current source at ILIM sets a voltage drop on this resistor, adjusting the current-limit threshold from <50mV to >200mV. In the adjustable mode, the current-limit threshold voltage is precisely 1/10th the voltage seen at ILIM. Therefore, choose RILIM equal to 2k/mV of the current-limit threshold. The threshold defaults to 100mV when ILIM is connected to VL. The logic threshold for switchover to the 100mV default value is approximately VL - 1V. The adjustable current limit can accommodate
Voltage Positioning
The quick-PWM control architecture responds virtually instantaneously to transient load changes and eliminates the control loop delay of conventional PWM controllers. As a result, a large portion of the voltage deviation during a step load change is from the equivalent series resistance (ESR) of the output capacitors. For DDR termination applications, the maximum allowed voltage deviation is 40mV for any output load transition from sourcing current to sinking current. Passive voltage positioning adjusts the converter's output voltage based on its load current to optimize transient response and minimize the required output capacitance. Voltage positioning is implemented by connecting a 2m resistor as shown in Figure 1.
MOSFET Drivers
The DH and DL drivers are optimized for driving moderate-size, high-side and larger, low-side power MOSFETs and are optimized for 2.5V and 5V input voltages. The drivers are sized to drive MOSFETs that can deliver up to 25A output current. An adaptive deadtime circuit monitors the DL output and prevents the
VIN POK V+ DDR MAX1917 VL
POK V+
VL RBST BST MAX1917 DH LX DL PGND VTT VTTR
BST
DDR
DH LX DL PGND VTT VTTR VTTR RDRP 2m VOUT
EN/HSD REF ILIM FSEL GND
EN/HSD REF
4 x 270F 2V
ILIM FSEL GND
Figure 1. Using a Resistor for Voltage Positioning
Figure 2. Increasing the On Time of the High-Side MOSFET 11
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies
high-side FET from turning on until DL is fully off. There must be a low-resistance, low-inductance path from the DL driver to the MOSFET gate in order for the adaptive dead-time circuit to work properly. Otherwise, the sense circuitry in the MAX1917 interprets the MOSFET gate as off while there is actually still charge left on the gate. Use very short, wide traces measuring 10 squares to 20 squares (50mils to 100mils wide if the MOSFET is 1in from the MAX1917). The dead time at the other edge (DH turning off) is determined by a fixed 35ns (typ) internal delay. The internal pulldown transistor that drives DL low is robust, with a 0.5 (typ) onresistance. This helps prevent DL from being pulled up during the fast rise time of the inductor node, due to capacitive coupling from the drain to the gate of the massive low-side synchronous-rectifier MOSFET. Some combinations of high- and low-side FETs may be encountered that cause excessive gate-drain coupling, which can lead to efficiency-killing, EMI-producing shoot-through currents. This can often be remedied by adding a resistor (R BST ) in series with BST, which increases the turn-on time of the high-side FET without degrading the turn-off time (Figure 2).
MAX1917
Typical Application Circuits
VL R3 20k POK 5.5V TO 14V V+ VDDR 3 10 2 C9 0.47F/25V C8 0.47F/10V 1 7 5 6 8 POK V+ DDR EN/HSD REF ILIM FSEL GND MAX1917 VL BST DH LX DL PGND VTT VTTR 14 15 16 2.5V 12 13 4 9 C7 1F/6.3V VTTR Q2 IRF7463 C6 6 x 270F 2V L1 0.68H/9A R2 5.1k VL
2.5V 11 D1 CMPSH-3 C4 0.47F 10V C3 4.7F 10V C2 2 x 330F 6V C1 1F 6.3V
VIN
Q1 IRF7463 1.25V AT 7A VOUT
SHDN Q3 2N7002K
VL
C5 2 x 10F 6.3V PGND
OUTPUT CAPACITORS ARE SELECTED TO COMPLY WITH JEDEC SPECIFICATIONS.
Figure 3. Typical Application Circuit for 1.25V at 7A Output
12
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies
Typical Application Circuits (continued)
VL R3 20k POK 5.5V TO 14V V+ VDDR 3 10 2 C9 0.47F/25V C8 0.47F/10V 1 7 5 6 8 POK V+ DDR EN/HSD REF ILIM FSEL GND MAX1917 VL BST DH LX DL PGND VTT VTTR 11 14 15 16 2.5V 12 13 4 9 C7 1F/6.3V VTTR Q2 IRF7463 L1 0.68H/9A D1 CMPSH-3 C4 0.47F 10V R2 5.1k VL
MAX1917
2.5V C3 4.7F 10V C2 2 x 330F 6V C1 1F 6.3V
VIN
Q1 IRF7463 2m 1.25V AT 7A VOUT C6 4 x 270F 2V PGND
SHDN Q3 2N7002K
VL
OUTPUT CAPACITORS ARE SELECTED TO COMPLY WITH JEDEC SPECIFICATIONS
Figure 4. Typical Application Circuit for 1.25V at 7A Output Using Voltage Positioning
VL
VL R3 20k POK 5.5V TO 14V V+ VDDR 3 10 2 C9 0.47F/25V C8 0.47F/10V 1 7 5 6 8 POK V+ DDR EN/HSD REF ILIM FSEL GND
R2 5.1k
2.5V VL BST MAX1917 DH LX DL PGND VTT VTTR 11 14 15 16 2.5V 12 13 4 9 C7 1F/6.3V Q2 IRF7811W C6 3 x 270F 2V L1 1.0H/5A D1 CMPSH-3 C4 0.22F 10V C3 2.2F 10V C2 330F 6V C1 1F 6.3V
VIN
Q1 IRF7811W 1.25V AT 3.5A VOUT C5 10F 6.3V PGND
SHDN Q3 2N7002K
VL
VTTR
OUTPUT CAPACITORS ARE SELECTED TO COMPLY WITH JEDEC SPECIFICATIONS.
Figure 5. Typical Application Circuit for 1.25V at 3.5A Output ______________________________________________________________________________________ 13
Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies MAX1917
Typical Application Circuits (continued)
R2 10k
VL SHDN Q3 Si1029X VL 2.5V 3 5.5V TO 14V 10 2 C9 0.47F/25V 1 7 C8 0.47F/10V 5 POK V+ DDR EN/HSD REF ILIM MAX1917 VL BST DH LX DL PGND VTT VTTR 11 14 15 16 2.5V 12 13 4 9 C7 1F/10V VTTR Q2 IRF7811W C6 3 x 270F 2V L1 1.0H/5A D1 CMPSH-3 C4 0.22F 10V C3 2.2F 10V C2 330F 6V C1 1F 6.3V VIN
R3 20k POK V+ VDDR
Q1 IRF7811W 1.25V AT 3.5A VOUT
C5 10F 6.3V PGND
VL
6 FSEL 8 GND
OUTPUT CAPACITORS ARE SELECTED TO COMPLY WITH JEDEC SPECIFICATIONS.
Figure 6. Typical Application Circuit Using P/N-Channel MOSFETs for EN to Minimize the Supply Current from VIN in Shutdown Mode
5V R3 20k POK 3 10 2 1 7 5 6 8 POK V+ DDR EN/HSD REF ILIM FSEL GND MAX1917 R2 5.1k C3 4.7F 10V C2 4 x 330F 6V C1 1F 25V 4.5V TO 15V VIN
VL BST DH LX DL PGND VTT VTTR
11 14 15 16 12 13 4
D1 CMPSH-3 C4 0.47F 10V
C9 0.22F 25V C8 0.47F/10V
Q1 IRF7822 2.5V L1 0.75H/24A Q2 IRF7822 R4 15k 0.1% R5 10k 0.1% PGND 2.5V AT 12A VOUT C6 3 x 560F 4V C5 1F 6.3V
SHDN Q3 2N7002K
R6
VL
9
OUTPUT CAPACITORS ARE SELECTED TO COMPLY WITH JEDEC SPECIFICATIONS
Figure 7. Circuit to Generate a Fixed 2.5V at 12A Output with a Wide Input Voltage Range 14 ______________________________________________________________________________________
Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies
Design Procedure
Firmly establish the input voltage range and maximum load current before choosing a switching frequency and inductor operating point (ripple current ratio). The primary design trade-off is in choosing a good switching frequency and inductor operating point, and the following four factors dictate the rest of the design: 1) Input Voltage Range. The maximum value (VIN(MAX)) must accommodate the worst-case high input voltage. The minimum value (VIN(MIN)) must account for the lowest input voltage after drops due to connectors, fuses, and battery selector switches. If there is a choice at all, lower input voltages result in better efficiency. 2) Maximum Load Current. There are two values to consider. The peak load current (I LOAD(MAX) ) determines the instantaneous component stresses and filtering requirements, and thus drives output capacitor selection, inductor saturation rating, and the design of the current-limit circuit. The continuous load current (ILOAD) determines the thermal stresses and thus drives the selection of input capacitors, MOSFETs, and other critical heat-contributing components. 3) Switching Frequency. This determines the basic trade-off between size and efficiency. The optimal frequency is largely a function of maximum input voltage, due to MOSFET switching losses that are proportional to frequency and VIN2. The optimum frequency is also a moving target, due to rapid improvements in MOSFET technology that are making higher frequencies more practical. 4) Inductor Operating Point. This provides trade-offs between size and efficiency. Low inductor values cause large ripple currents, resulting in the smallest size but poor efficiency and high output noise. The minimum practical inductor value is one that causes the circuit to operate at the edge of critical conduction (where the inductor current just touches zero with every cycle at maximum load). Inductor values lower than this grant no further size-reduction benefit. The inductor ripple current also impacts transientresponse performance, especially at low VIN - VOUT differentials. Low inductor values allow the inductor current to slew faster, replenishing charge removed from the output filter capacitors by a sudden load step. The amount of output sag is also a function of the maximum duty factor, which can be calculated from the on time and minimum off time:
(ILOAD(MAX) ) x L VSAG = kHz 2 x Cf x DUTY x (VIN(MIN) - VOUT )
2
MAX1917
Output Inductor Selection
The switching frequency (on time) and operating point (% ripple or LIR) determine the inductor value as follows: VOUT L= f x LIR x ILOAD(MAX ) Example: ILOAD(MAX) = 7A, VOUT = 1.25V, f = 550kHz, 50% ripple current or LIR = 0.5: L= 1.25V = 0.65H(0.68H) 550kHz x 0.5 x 7A
Find a low-loss inductor having the lowest possible DC resistance that fits in the allotted dimensions. Ferrite cores are often the best choice, although powdered iron is inexpensive and can work well at 200kHz. The core must be large enough not to saturate at the peak inductor current: (IPEAK): IPEAK = ILOAD(MAX) + (LIR / 2) (ILOAD(MAX))
Output Capacitor Selection
The output filter capacitor must have low enough ESR to meet output ripple and load-transient requirements, yet have high enough ESR to satisfy stability requirements. Also, the capacitance value must be high enough to absorb the inductor energy going from a positive full-load to negative full-load condition or vice versa without incurring significant over/undershoot. In DDR termination applications where the output is subject to violent load transients, the output capacitor's size depends on how much ESR is needed to prevent the output from dipping too low under a load transient. Ignoring the sag due to finite capacitance: RESR VDIP ILOAD(MAX ) = 40mV = 2.85m 14A
In DDR applications, VDIP = 40mV, the output capacitor's size depends on how much ESR is needed to maintain an acceptable level of output voltage ripple: RESR VP-P 9mV = = 2.57m LIR x ILOAD(MAX ) 0.5 x 7A
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15
Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies MAX1917
The actual microfarad capacitance value required relates to the physical size needed to achieve low ESR, as well as to the chemistry of the capacitor technology. As a result, the capacitor is usually selected by ESR and voltage rating rather than by capacitance value (this is true of tantalums, OS-CONs, POSCAPs, and other electrolytics). R DRP introduces some power dissipation, which is given by: PD(DRP) = RDRP IOUT(MAX)2 RDRP should be chosen to handle this power dissipation.
MOSFET Power Dissipation
Worst-case conduction losses occur at the duty-factor extremes. For the high-side MOSFET, the worst-case power dissipation due to resistance occurs at minimum input voltage: PD(Q1) = (VOUT / VIN(MIN)) (ILOAD2) (RDS(ON)) Generally, a small high-side MOSFET is desired in order to reduce switching losses at high input voltages. However, the RDS(ON) required to stay within package power-dissipation limits often limits how small the MOSFET can be. Again, the optimum occurs when the switching (AC) losses equal the conduction (RDS(ON)) losses. Calculating the power dissipation in Q1 due to switching losses is challenging because it must allow for difficult-to-quantify factors that influence the turn-on and turn-off times. These factors include the internal gate resistance, gate charge, threshold voltage, source inductance, and PC board layout characteristics. The following switching loss calculation provides only a very rough estimate and is no substitute for breadboard evaluation, preferably including a check using a thermocouple mounted on Q1: PD(SWITCHING) = CRSS x VIN(MAX )2 x f x ILOAD IGATE
Input Capacitor Selection
The input capacitor must meet the ripple current requirement (IRMS) imposed by the switching currents. Nontantalum chemistries (ceramic, aluminum, or OSCON) are preferred due to their superior surge current capacity: V OUT x (VIN - VOUT ) IRMS = ILOAD x VIN
Setting the Current Limit
The minimum current-limit threshold must be great enough to support the maximum load current when the current limit is at the minimum tolerance value. The valley of the inductor current occurs at ILOAD(MAX) minus half of the ripple current. For example: ILIMIT(LOW) > ILOAD(MAX) - (LIR / 2) ILOAD(MAX) where ILIMIT(LOW) = minimum current-limit threshold voltage divided by the R DS(ON) of Q2. For the MAX1917, the minimum current-limit threshold (100mV default setting) is 50mV. Use the worst-case maximum value for RDS(ON) from the MOSFET Q2 data sheet, and add some margin for the rise in RDS(ON) with temperature. A good general rule is to allow 0.5% additional resistance for each C of temperature rise. When adjusting the current limit, use a 1% tolerance RILIM resistor to prevent a significant increase of errors in the current-limit tolerance.
where CRSS is the reverse transfer capacitance of Q1 and IGATE is the peak gate-drive source/sink current. For the low-side MOSFET, Q2, the worst-case power dissipation always occurs at maximum input voltage: PD(Q2) = (1 - VOUT / VIN(MAX)) ILOAD2 RDS(ON)
Setting the Voltage Positioning
The droop resistor, R DRP , in series with the output inductor before the output capacitor, sets the droop voltage, VDRP. Choose RDRP such that the output voltage at the maximum load current, including ripple, is just above the lower limit of the output tolerance: RDRP < VOUT(TYP) - VOUT(MIN) - VRIPPLE /2 IOUT(MAX )
16
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Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies
The absolute worst case for MOSFET power dissipation occurs under heavy overloads that are greater than or equal to ILOAD(MAX). To protect against this condition, design the circuit to tolerate: ILOAD = ILIMIT(HIGH) + (LIR / 2) (ILOAD(MAX)) where I LIMIT(HIGH) is the maximum valley current allowed by the current-limit circuit, including threshold tolerance and on-resistance variation. If short-circuit protection without overload protection is enough, a normal ILOAD value can be used for calculating component stresses. 3) Keep the power traces and load connections short. This practice is essential for high efficiency. The use of thick copper PC boards (2oz vs. 1oz) can enhance full-load efficiency by 1% or more. Correctly routing PC board traces is a difficult task that must be approached in terms of fractions of centimeters, where a single m of excess trace resistance causes a measurable efficiency penalty. 4) LX and PGND connections to Q2 for current limiting must be made using Kelvin-sense connections in order to guarantee the current-limit accuracy. With 8-pin SO MOSFETs, this is best done by routing power to the MOSFETs from outside using the top copper layer, while tying in PGND and LX inside (underneath) the 8-pin SO package. 5) When trade-offs in trace lengths must be made, it is preferable to allow the inductor charging path to be made longer than the discharge path. For example, it is better to allow some extra distance between the input capacitors and the high-side MOSFET than to allow distance between the inductor and the lowside MOSFET or between the inductor and the output filter capacitor. 6) Ensure that the VTT feedback connection to COUT is short and direct. In some cases, it may be desirable to deliberately introduce some trace length (droop resistance) between the FB inductor node and the output filter capacitor. 7) VTT feedback sense point should also be as close as possible to the load connection. 8) Route high-speed switching nodes away from sensitive analog nodes (DDR, EN/HSD, REF, ILIM). 9) Make all pin-strap control input connections (ILIM, etc.) to GND or VL close to the chip, and do not connect to PGND.
MAX1917
Control IC Power Dissipation
MAX1917 has on-chip MOSFETs drivers (DH and DL) that dissipate the power loss due to driving the external MOSFETs. Power dissipation due to a MOSFET driver is given by: PDR = (V + ) x fS x (QGH + QGL ) + IVTTR
(
)
where QGH and QGL are the total gate charge of the high-side and low-side MOSFETs, respectively. Select the switching frequency and V+ correctly to ensure the power dissipation does not exceed the package power dissipation requirement.
Applications Information
PC Board Layout
Careful PC board layout is critical to achieving low switching losses and clean, stable operation. The switching power stage requires particular attention. If possible, mount all of the power components on the top side of the board with their ground terminals flush against one another. Follow these guidelines for good PC board layout: 1) Keep the high-current paths short, especially at the ground terminals. This practice is essential for stable, jitter-free operation. 2) Connect GND and PGND together as close to the IC as possible.
Chip Information
TRANSISTOR COUNT: 2708 PROCESS: BiCMOS
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17
Tracking, Sinking and Sourcing, Synchronous Buck Controller for DDR Memory and Termination Supplies MAX1917
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)
QSOP.EPS
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
18 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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