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19-3995; Rev 0; 3/06
KIT ATION EVALU ILABLE AVA
Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown
General Description
The MAX8627 step-up converter is a high-efficiency, low-quiescent current, synchronous boost converter with True ShutdownTM and inrush current limiting. The MAX8627 generates any boosted output voltage from 3V to 5V from either a 2-cell NiMH/NiCd or a single-cell Li+/Li polymer battery. Quiescent current is only 20A (typ), and at light loads the converter pulses only as needed for best efficiency. At higher loads, PWM mode maintains fixed 1MHz operation for lowest noise and ripple. The MAX8627 includes an internal soft-start to limit inrush current to a maximum of 500mA. Additional features include True Shutdown, internal compensation, and adjustable current limit. The MAX8627 is available in a tiny 3mm x 3mm TDFN package and is ideal for use in handheld devices such as DSCs, PDAs, and smartphones. 1MHz PWM Switching Frequency True Shutdown Output Up to 95% Efficiency 1.0A Guaranteed Output Current Soft-Start Eliminates Inrush Current 20A (typ) Quiescent Current 0.1A Logic-Controlled Shutdown Internal Synchronous Rectifier Internal Compensation Adjustable Current Limit Low-Noise Antiringing Feature Tiny 14-Pin, 3mm x 3mm, TDFN Package
Features
MAX8627
Applications
DSC Motors and Backup Power Microprocessor/DSP Core Power Cellphones, PDAs, MP3 Players Portable Handheld Devices
MAX8627ETD+ PART
Ordering Information
PINPACKAGE 14 TDFN-EP* 3mm x 3mm PKG CODE T1433-2 TOP MARK AAQ
True Shutdown is a trademark of Maxim Integrated Products, Inc.
Note: The device operates in the -40C to +85C extended operating temperature range. *EP = Exposed pad. +Denotes lead-free package.
Typical Operating Circuit
C2
Pin Configuration
AGND OUTS ILIM
PG
PG
LX 9 6 POUT
14
4, 5 BATT ON OFF 12 R4 3 ON LX POUT 8, 9 6,7 C3 C4 OUTPUT 3V TO 5V UP TO 1A
13
12
11
10
MAX8627
AGND ILIM OUTS FB
14
R1
MAX8627ETD+
+
13
2
R3 1 GND PGND 10, 11
R2
1 GND
2 FB
3 ON
4 BATT
5 BATT
TDFN 3mm x 3mm
________________________________________________________________ Maxim Integrated Products
POUT
LX 8 7
INPUT BATTERY 2.5V TO 4.2V C1
L1
TOP VIEW
1
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown MAX8627
ABSOLUTE MAXIMUM RATINGS
OUTS, BATT to GND ................................................-0.3V to +6V LX Current (Note 1) ...............................................................3.5A AGND, PG to GND ................................................-0.3V to +0.3V POUT to OUTS ......................................................-0.3V to +0.3V FB, ILIM, ON to GND.....0.3V to the higher of (VOUTS + 0.3V) and (VBATT + 0.3V) Continuous Power Dissipation (TA = +70C) 14-Pin TDFN 3mm x 3mm (derate 18.2mW/C above +70C) .............................1454mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C Note 1: LX has internal clamp diodes to the IC internal power node VPWR (where VPWR is the higher of BATT or POUT) and PG. Applications that forward bias these diodes should take care not to exceed the device's power-dissipation limits.
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
(VOUTS = VPOUT = 5V, VON = VBATT = 3.6V, VILIM = GND, TA = -40C to +85C, typical values are at TA = +25C, unless otherwise noted.)
PARAMETER GENERAL Operating Input Voltage Range Minimum Startup Voltage Maximum Startup Current Limit Shutdown, ON = GND Supply Current No load, no switching No load, switching OSCILLATOR Switching Frequency Startup Switching Frequency Maximum Duty Cycle Output Voltage Adjust Range FB Regulation Voltage FB Load Regulation FB Line Regulation FB Input Leakage Current ILIM Dual ModeTM Threshold Idle Mode Trip Level DC-DC SWITCHES n-Channel On-Resistance p-Channel On-Resistance Damping Switch On-Resistance n-Channel Current limit VILIM = 0V VILIM = 0.6V 3.2 0.15 0.15 17 3.5 1.0 0.25 0.25 30 3.7 A No load 0A to 1A output current load step VBATT = 2.7V to 3V, output current = 0.5A TA = +25C VFB = 1.2V, VOUTS = VPOUT = VBATT = 5.5V TA = +85C Low level High level (Note 3) 0.45 50 -50 82.5 3.0 1.005 1.015 -30 +20 -10 -10 0.25 +50 0.95 1.0 2.0 87.0 5.2 1.025 1.05 MHz MHz % V V mV/A mV nA V mA TA = +25C TA = +85C TA = 0C to +85C TA = -40C (Note 2) (Note 1) No load (Note 1) 0.9 1.2 0.5 0.1 0.2 20 20 20 30 35 A 1 5.5 1.5 V V A CONDITIONS MIN TYP MAX UNITS
Dual Mode is a trademark of Maxim Integrated Products, Inc. 2 _______________________________________________________________________________________
Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown
ELECTRICAL CHARACTERISTICS (continued)
(VOUTS = VPOUT = 5V, VON = VBATT = 3.6V, VILIM = GND, TA = -40C to +85C, typical values are at TA = +25C, unless otherwise noted.)
PARAMETER p-Channel Turn-Off Current POUT Leakage Current LX Leakage Current Soft-Start Interval Overload Protection Fault Delay LOGIC INPUTS ON Input Low Level 1.5V < VPOUT = VOUTS = VBATT 1.8V 1.8V < VPOUT = VOUTS = VBATT 5.5V 1.5V < VPOUT = VOUTS = VBATT 1.8V 1.8V < VPOUT = VOUTS + VBATT 5.5V ON, Input Leakage Current Thermal Shutdown VOUTS = VPOUT = VBATT = 5.5V, ON = 0V or ON = 5.5V TA = +25C TA = +85C VPOUT 0.2 1.6 0.01 0.02 +160 1 A C 0.2 0.5 V VLX = 0V, VPOUT = VBATT = 5.5V VLX = 0V and VPOUT = 5.5V or VLX = 5.5V and VOUTS = VPOUT = 0V Output current = 0.5A TA = +25C TA = +85C TA = +25C TA = +85C CONDITIONS MIN TYP 10 0.1 0.2 0.1 0.2 5.25 65 1 1 MAX UNITS mA A A ms ms
MAX8627
ON Input High Level
V
Note 1: The MAX8627 is powered from OUTS. Once started, the IC operates down to 0.9V. Note 2: Specifications to -40C are guaranteed by design and not production tested. Note 3: The idle-mode current threshold is the transition point between fixed-frequency PWM operation and idle-mode operation. The specification is given in terms of output load current for an inductor value of 1H. For a step-up converter, the idlemode transition varies with the input-to-output voltage ratio.
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3
Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown MAX8627
Typical Operating Characteristics
(Circuit of Figure 1, VOUTS = VPOUT = 5V, VON = VBATT = 3.6V, TA = +25C, unless otherwise noted.)
EFFICIENCY vs. LOAD CURRENT WITH 3.3V OUTPUT
MAX8627 toc01
EFFICIENCY vs. LOAD CURRENT WITH 5V OUTPUT
MAX8627 toc02
MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE
MAX8627 toc03
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0
VBATT = 3.0V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0
VBATT = 4.2V
3.5 3.0 2.5 2.0 1.5 1.0 VPOUT = 5V 0.5 0 VPOUT = 3.3V
VBATT = 2.4V VBATT = 1.8V VBATT = 1.5V
VBATT = 3.6V VBATT = 3.0V VBATT = 2.4V VBATT = 1.8V
0.01
0.1
1
10
100
1000
0.01
0.1
1
10
100
1000
MAXIMUM LOAD CURRENT (A)
1.0
2.0
3.0
4.0
5.0
LOAD CURRENT (mA)
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
OUTPUT VOLTAGE vs. LOAD CURRENT
MAX8627 toc04
NO-LOAD INPUT CURRENT vs. INPUT VOLTAGE WITH 3.3V OUTPUT
MAX8627 toc05
NO-LOAD INPUT CURRENT vs. INPUT VOLTAGE WITH 5V OUTPUT
160 140 INPUT CURRENT (mA) 120 100 80 60 40 20 0 TA = +25C TA = 40C 1 2 3 INPUT VOLTAGE (V) 4 5 R1 = 2M, R2 = 499k OUTPUT ONLY LOADED WITH THE FB RESISTOR-DIVIDER NETWORK. TA = +85C
MAX8627 toc06 MAX8627 toc09
5.12 5.10 5.08 OUTPUT VOLTAGE (V) 5.06 5.04 5.02 5.00 4.98 4.96 4.94 4.92 0.01 0.1 1 10 100 VBATT = 4.2V VBATT = 3.6V VBATT = 3.0V VBATT = 2.4V VBATT = 1.8V
90 80 70 INPUT CURRENT (mA) 60 50 40 30 20 10 0
R1 = 1.15M, R2 = 499k OUTPUT ONLY LOADED WITH THE FB RESISTOR-DIVIDER NETWORK. TA = +85C
180
TA = +25C TA = 40C 1.0 1.5 2.0 2.5 3.0 3.5
1000
LOAD CURRENT (mA)
INPUT VOLTAGE (V)
SHUTDOWN CURRENT vs. INPUT VOLTAGE
MAX8627 toc07
STARTUP VOLTAGE vs. LOAD CURRENT WITH 5V OUTPUT
MAX8627 toc08
SOFT-START TIME vs. LOAD CURRENT
6 5 SOFT-START TIME (ms) 4 3 2 1 0
700 600 SHUTDOWN CURRENT (nA) 500 400 300 200 100 0 1.5 2.5 3.5 INPUT VOLTAGE (V) 4.5 TA = 40C TA = +25C TA = +85C
3.0 2.5 STARTUP VOLTAGE (V) TA = +25C 2.0 1.5 1.0 0.5 0 TA = 40C TA = +85C
5.5
0
200
400
600
800
1000
0
0.1
0.2
0.3
0.4
0.5
LOAD CURRENT (mA)
LOAD CURRENT (A)
4
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Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown MAX8627
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VOUTS = VPOUT = 5V, VON = VBATT = 3.6V, TA = +25C, unless otherwise noted.)
SOFT-START TIME vs. INPUT VOLTAGE
MAX8627 toc10
PEAK INDUCTOR CURRENT vs. VILIM
3.5 PEAK INDUCTOR CURRENT (A) 3.0 2.5 2.0 1.5 1.0 0.5 0
MAX8627 toc11
5.0 4.5 4.0 SOFT-START TIME (ms) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0.5 1.5 2.5 3.5 4.5
4.0
5.5
0.45
0.65
0.85 VILIM (V)
1.05
1.25
INPUT VOLTAGE (V)
HEAVY LOAD SWITCHING WAVEFORMS
MAX8627 toc12
LIGHT-LOAD SWITCHING WAVEFORMS
MAX8627 toc13
VPOUT
100mV/div (AC-COUPLED)
VPOUT
100mV/div (AC-COUPLED)
2V/div VLX 0 VLX
2V/div 0
ILI 1s/div
1A/div 0
1A/div ILI 0 40s/div
LOAD TRANSIENT RESPONSE
MAX8627 toc14
LINE TRANSIENT RESPONSE
MAX8627 toc15
2V/div VPOUT 100mV/div (AC-COUPLED) VBATT 0
VPOUT ILOAD 500mA/div 0
100mV/div (AC-COUPLED)
20s/div
100s/div
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5
Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown MAX8627
Typical Operating Characteristics (continued)
(Circuit of Figure 1, VOUTS = VPOUT = 5V, VON = VBATT = 3.6V, TA = +25C, unless otherwise noted.)
STARTUP WAVEFORMS WITH NO LOAD
MAX8627 toc16
STARTUP WAVEFORMS WITH 100mA LOAD
MAX8627 toc17
VON
2V/div 0 2V/div
VON
2V/div 0 2V/div
VPOUT 0
VPOUT
0
ILX
500mV/div
ILX
500mV/div
400s/div
400s/div
SWITCHING FREQUENCY vs. TEMPERATURE
MAX8627 toc18
BODE PLOT WITH 2 x 22F CERAMIC OUTPUT CAPACITORS, 500mA LOAD
MAX8627 toc19
1006 1004 SWITCHING FREQUENCY (kHz) 1002 1000
40 30 20 GAIN (dB)
GAIN 180 150 PHASE (DEG) 7dB GAIN MARGIN 120 90 60 48-DEG PHASE MARGIN 94kHz 30 0
10 PHASE 0 -10 -20 -30
998 996 994 992 990 -40 -15 10 35 60 85
-40 1k 10k 100k 1M
TEMPERATURE (C)
FREQUENCY (Hz)
BODE PLOT WITH 2 x 47F TANTALUM OUTPUT CAPACITORS (130m ESR), 500mA LOAD
MAX8627 toc20
40 30 20 GAIN (dB) 10 PHASE GAIN 180 125 - DEG PHASE MARGIN 150 27kHz 120 13dB GAIN MARGIN 90 60 30 0
PHASE (DEG)
-10 -20 -30 -40 1k 10k 100k
1M
FREQUENCY (Hz)
6
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Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown
Pin Description
PIN 1 2 3 4, 5 6, 7 8, 9 10, 11 12 13 NAME GND FB ON BATT POUT LX PG AGND ILIM Analog Ground. Connect to PG and AGND. Voltage Feedback Input. Connect FB to the center of an external feedback network between OUTS and GND (see the Setting the Output Voltage section). FB regulates to 1.015V (typ). Active-High Enable Input. Connect ON to BATT or logic high for normal operation. Connect ON to GND or logic low for True Shutdown mode. Supply Voltage Input. Connect to the battery or a supply from 1.5V to 5.5V. Connect two 22F ceramic capacitors from BATT to PG. Power Output. Connect two 22F ceramic capacitors from POUT to PG (see the Capacitor Selection section). Inductor Connection. LX is high impedance in shutdown. Power Ground. Connect to GND and AGND. Analog Ground. Connect to GND and PG. n-Channel Current-Limit Control. For the maximum current limit of 3.5A, connect ILIM to GND. For lower current-limit settings, connect ILIM to a resistor-divider from POUT to GND (see the Setting the Current Limit section). IC Power Input. Supplied from the output. Connect OUTS to POUT. Exposed Pad. Connect EP to GND. This does not remove the requirement for a proper ground connection to GND. FUNCTION
MAX8627
14 --
OUTS EP
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7
Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown MAX8627
INPUT: 1.5V TO 5.5V C1 22F C2 22F L1 1H
BATT ON OFF ON AGND R3 ILIM GND R4 PG
LX POUT
OUTPUT: 3V TO 5V, UP TO 1A C3 22F C4 22F
MAX8627
R1 OUTS FB
R2
Figure 1. Typical Applications Circuit with an Adjustable Output Voltage and Adjustable Current Limit
ON
ILIM
2.7V OUTS
+ -
UVLO
MAX8627
POUT ON
ON
STARTUP OSCILLATOR
ON BATT CONTROL
ON 1MHz OSCILLATOR GND REFERENCE
DAMPING SWITCH
LX
PG
FB
Figure 2. Functional Diagram 8 _______________________________________________________________________________________
Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown
Detailed Description
The MAX8627 is a current-mode step-up converter that uses a fixed-frequency PWM architecture with True Shutdown. Consuming only 20A of quiescent current, the MAX8627 is highly efficient, with an internal switch and synchronous rectifier. Shutdown reduces the quiescent current to less than 1A. Low quiescent current and low noise make this device ideal for powering portable equipment. The MAX8627 step-up DC-to-DC switching converter typically generates a 3V to 5V output voltage from a 1.5V to 4.2V battery input voltage. The IC operates in bootstrapped mode with the output powering the IC once the output voltage is equal to, or exceeds, 2.7V. The default current limit is set at 3.5A to deliver 1A at 5V with an Li+ battery, or 500mA at 5V using a 2-cell NiCd/NiMH battery. The current limit may be lowered using an external resistor at ILIM to allow for smaller components in lower power applications. Internal softstart limits the inrush current to less than 500mA under no-load conditions during startup. The MAX8627 switches at an internally set frequency of 1MHz allowing for tiny external components. Internal compensation further reduces the external component count in cost and space-sensitive applications. The MAX8627 is optimized for use in DSC and other applications requiring low quiescent current for maximum battery life. Figure 1 shows the typical applications circuit. Figure 2 gives the functional diagram. The device operates in PWM when driving medium to heavy loads. As the load current decreases and crosses the low-power idle mode threshold, the PWM comparator and oscillator are disabled. In this low-power idle mode, switching occurs only as needed to service the output. This improves the efficiency for light loads and the IC consumes only 20A under no-load conditions. At light loads, the output ripple has a frequency component that varies with load current. The threshold for entering the low-power mode is determined by sensing the voltage drop across the internal switch and comparing it to an internally generated reference level. This threshold is approximately 50mA with a 3.6V input and 5V output. When switching in low-power mode, the inductor current terminates at zero for each switching cycle. When operating in this manner, the inductor current is called discontinuous. In older DC-DC converters, radiated noise may be higher when inductor current is discontinuous, because of ringing at the LX switch. The MAX8627 features an internal damping switch to minimize ringing at LX when inductor current is discontinuous. The damping switch places an impedance across the inductor and supplies a path to dissipate the resonant energy in the inductor and capacitor to damp the ringing at the LX. The damping switch has little effect on output voltage ripple but does reduce EMI. At higher loads, the MAX8627 operates in PWM mode. Regulation is achieved by modulating the MOSFET switch pulse to control the amount of power transferred per cycle. Switching harmonics generated by fixedfrequency operation are consistent and easily filtered. This is important in noise-sensitive applications.
MAX8627
DC-DC Converter
The MAX8627 uses a current-mode PWM control scheme. The voltage difference between FB and an internal 1.01V reference generates an error signal that programs the peak inductor current to regulate the output voltage. The default peak inductor current limit is typically 3.5A. Inductor current is sensed across the internal switch and summed with a slope-compensation signal. The PWM comparator compares this summed signal to the error amplifier output. At the beginning of each clock cycle, the n-channel switch turns on until the PWM comparator trips. During this time, inductor current ramps up, storing energy in its magnetic field. When the n-channel switch turns off, the internal synchronous p-channel rectifier turns on. The inductor releases the stored energy as the current ramps down and provides energy to the output.
Load-Transient Response/Voltage Positioning
The MAX8627 matches the load regulation to the voltage droop seen during load transients. This is sometimes called voltage positioning. Benefits include lower peak-to-peak output-voltage deviation for a given load step without requiring an increase in filter load capacitance. There is minimal voltage droop when transitioning from a light load to full load and minimum overshoot when going from full load to light load.
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9
Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown
The term "positioning" refers to setting the output voltage to a level that is dependent on load current (see Figure 3). At minimum load, the output voltage is set to a slightly higher than nominal level. At full load, the output voltage is slightly lower than the nominal level. With voltage positioning, the total voltage deviation during a transient is significantly improved over traditional highgain control loops. Traditional high-gain loops use integrators that maximize gain at low frequencies to provide tight DC-load regulation; however, due to the capacitive element in the feedback loop, these highgain amplifiers typically take hundreds of microseconds to respond to a load step and return to steady state. As a result, the voltage can droop by as much as 6% or more during the recovery time. In portable equipment where the output load can change frequently, and the amount of output capacitance that can fit is limited, this can result in a wide short-term output fluctuation (see Figure 4). Voltage positioning on the MAX8627 allows up to 3% (typ) of load regulation and no further transient droop (Figures 3 and 4). Thus, during load transients the voltage delivered remains within specification more effectively than other regulators that might have tighter DC accuracy. In systems with high-speed CPUs, thousands of system clock cycles can occur during the time it takes a traditional high-gain loop to respond to a load step. Consequently, 3% load regulation with no transient droop is better suited to such systems than a power supply that may spec 1% DC load regulation, but then exhibits 6% or more of transient droop during load steps (see the Load Transient Response in the Typical Operating Characteristics section).
OUTPUT VOLTAGE vs. LOAD CURRENT
5.02 5.00 4.98 OUTPUT VOLTAGE (V) 4.96 4.94 4.92 4.90 4.88 4.86 4.84 4.82 4.80 0 500 VIN = 2.8V VIN = 1.8V 1000 1500 2000
(b) VOLTAGE POSITIONING WITH DC LOAD REGULATION 3% (a) HIGH-GAIN DC LOAD REGULATION WITH POOR TRANSIENT RESPONSE 9%
MAX8627
True Shutdown
Connecting ON to GND or logic low places the MAX8627 in shutdown mode and reduces supply current to 0.1A. In shutdown, the control circuitry, internal switching MOSFET, and synchronous rectifier turn off and LX becomes high impedance. Connect ON to BATT or logic high for normal operation. The MAX8627 has an internal synchronous rectifier, which allows for conversion efficiencies as high as 95%. In conventional boost circuits, the body diode of the synchronous rectifier is forward biased in shutdown and allows current flow from the battery to the output. If the load cannot be shut down, an external switch is required to avoid depleting the battery during shutdown. A proprietary design in the MAX8627 allows the synchronous rectifier to provide True Shutdown with no additional components. This allows the output to fall to GND in shutdown and removes any connection between the input and output.
Soft-Start
The MAX8627 has internal soft-start circuitry that eliminates inrush current at startup, reducing transients on the input source. Soft-start is particularly useful for higher impedance input sources, such as Li+ and alkaline cells. The soft-start duration is proportional to the size of the output capacitor and load resistance with a typical time of 5.25ms. See the Typical Operating Characteristics section for plots of Soft-Start Time vs. Load Current and Soft-Start Time vs. Input Voltage. Inrush current is controlled during startup and initially set to 500mA. After 1000 clock cycles, if the output voltage is not within regulation, the startup current limit is
VIN = 4V
LOAD CURRENT (mA)
Figure 3. Load-Regulation Specification 10
Figure 4. Transient-Response Comparison
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Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown
incremented by 230mA. If after 13 increments, the output is still not in regulation, the MAX8627 latches off, assuming a short-circuit overload condition exists on the output. To clear the latched condition, cycle ON. which sets peak-to-peak inductor current at 1/2 the DC inductor current: L= 2 x VBATT x D x (1- D) IOUT(MAX) x fSW
MAX8627
Fault Protection
The MAX8627 has a fault-overload protection. After soft-start, the device is set to detect an out-of-regulation state that could be caused by an overload. If the output remains faulted for 65ms, then the MAX8627 latches off. Fault-detection circuitry is disabled during soft-start. If short on the output exists before the MAX8627 is turned ON, the converter completes the soft-start sequence and latches off. The converter can be reinitialized from a fault latch-off state by toggling the ON pin or by cycling the input power.
where fSW is the switching frequency (1MHz), and D is the duty factor given by D = 1 - ( VBATT / VOUT ). Using L from the equation above results in a peak-topeak inductor current ripple of 0.5 x IOUT / (1 - D), and a peak inductor current of 1.25 x IOUT / (1 - D). Ensure the peak (saturation) current rating of the inductor meets or exceeds this requirement. The recommended inductance range for the MAX8627 is 1H to 4.7H. See Table 1 for recommended inductors.
BATT/Damping Switch
The MAX8627 features an internal damping switch to minimize ringing at LX caused by the resonant circuit formed by the inductor and output capacitor in discontinuous conduction mode. This occurs at light loads. The damping switch connects across the inductor when the inductor energy is depleted and supplies a path to dissipate the resonant energy. Damping LX ringing does not change the output ripple but reduces EMI.
Capacitor Selection
Output Capacitor Output capacitors C3 and C4 in Figure 1 are required to keep the output voltage ripple small and to ensure regulation loop stability. The output capacitors must have low impedance at the switching frequency. Ceramic capacitors are highly recommended due to their small size and low ESR. Make sure the output capacitors maintain their capacitance over DC bias and the desired operating temperature range. Ceramic capacitors with X5R or X7R temperature characteristics generally perform well. Two 22F ceramic capacitors in parallel are recommended. Alternatively, two 47F tantalum capacitors with 70m or lower ESR may be used. Input Capacitor Input capacitors C1 and C2 reduce the current peaks drawn from the battery or input power source and reduce switching noise in the IC. The impedance of the input capacitors at the switching frequency should be kept very low. Ceramic capacitors are highly recommended due to their small size and low ESR. Make sure the input capacitors maintain their capacitance over DC bias and the desired operating temperature range. Ceramic capacitors with X5R or X7R temperature characteristics generally perform well. Two 22F ceramic capacitors are recommended.
Applications Information
Setting the Output Voltage
To set the output voltage to between 3V and 5V, connect FB to the center of an external resistor voltagedivider between OUTS and GND, as shown in Figure 1. Select the value of R2 less than 500k, and then calculate the value for R1 as follows: V R1 = R2 x OUT - 1 VFB where VFB is the FB regulation voltage, 1.015V (typ).
Inductor Selection
In most step-up converter designs, a reasonable inductor value can be derived from the following equation,
Table 1. Recommended Inductors
PART TOKO A918CY TOKO A997AS INDUCTANCE (H) 1.0 1.5 RATED CURRENT (mA) 3500 2150 SIZE: L (mm, typ) x W (mm, typ) x H (mm, max) 6.3 x 6.2 x 2 3.8 x 3.8 x 1.8
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11
Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown MAX8627
Setting the Current Limit
ILIM sets the current limit when the output reaches regulation. It is different from the startup current limit used during soft-start to control inrush current. For the maximum current limit of 3.5A, connect ILIM to GND. To set the current limit (ILIM) lower than 3.5A, connect ILIM to a resistor-divider from POUT to GND as shown in Figure 1. Note, however, that the idle-mode threshold does not change with voltage setting on ILIM. Set R3 between 30k and 300k, then calculate the value of R4 as follows: VPOUT R4 = R3 x -1 (ILIM + 0.64A ) x 0.2865
PC Board Layout and Routing
Good PC board layout is important to achieve optimal performance from the MAX8627. Poor design can cause excessive conducted and/or radiated noise. Conductors carrying discontinuous currents and any high-current path should be made as short and wide as possible. Keep the feedback network (R1 and R2) very close to the IC, preferably within 0.2in of the FB and GND pins. Nodes with high dV/dt (switching nodes) should be kept as small as possible and routed away from FB. Connect the input and output capacitors as close as possible to the IC. Refer to the MAX8627 evaluation kit for a PC board layout example.
Chip Information
PROCESS: BiCMOS
12
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Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown
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.)
MAX8627
PACKAGE OUTLINE, 6,8,10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm
21-0137
H
1 2
______________________________________________________________________________________
13
6, 8, &10L, DFN THIN.EPS
Low VBATT, 20A IQ, 1MHz Synchronous Boost Converter with True Shutdown MAX8627
Package Information (continued)
(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.)
COMMON DIMENSIONS SYMBOL A D E A1 L k A2 MIN. 0.70 2.90 2.90 0.00 0.20 MAX. 0.80 3.10 3.10 0.05 0.40
PACKAGE VARIATIONS PKG. CODE T633-1 T633-2 T833-1 T833-2 T833-3 T1033-1 T1033-2 T1433-1 T1433-2 N 6 6 8 8 8 10 10 14 14 D2 1.500.10 1.500.10 1.500.10 1.500.10 1.500.10 1.500.10 1.500.10 1.700.10 1.700.10 E2 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 2.300.10 e 0.95 BSC 0.95 BSC 0.65 BSC 0.65 BSC 0.65 BSC 0.50 BSC 0.50 BSC 0.40 BSC 0.40 BSC JEDEC SPEC MO229 / WEEA MO229 / WEEA MO229 / WEEC MO229 / WEEC MO229 / WEEC MO229 / WEED-3 MO229 / WEED-3 ------b 0.400.05 0.400.05 0.300.05 0.300.05 0.300.05 0.250.05 0.250.05 0.200.05 0.200.05 [(N/2)-1] x e 1.90 REF 1.90 REF 1.95 REF 1.95 REF 1.95 REF 2.00 REF 2.00 REF 2.40 REF 2.40 REF
0.25 MIN. 0.20 REF.
PACKAGE OUTLINE, 6,8,10 & 14L, TDFN, EXPOSED PAD, 3x3x0.80 mm
-DRAWING NOT TO SCALE-
21-0137
H
2 2
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.
14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2006 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. Inc.

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