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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Features
* 2-Output Step-Down Converters: Channel 1 Step-Down: VIN1 = 6V to 24V * VOUT1 Adjustable from 1.5V to 5.5V * IOUT1 up to 4.5A * High Switching Frequency * Voltage Mode Control * PWM Fixed Frequency for Low-Ripple Channel 2 (LDO): VIN2 = 2.7V to 5.5V * IOUT2 up to 600mA * 1V Dropout Voltage at 600mA * High Accuracy 1.5% * Small Solution Size System On a Chip Ultra-small External L/C * Shutdown Current <35A * Independent Enable Pins * Programmable Over-Current Protection * Over-Temperature Protection * Internal Soft Start * 4x5mm 24-Pin TQFN Low Profile Thermally Enhanced Package * -40C to 85C Temperature Range
General Description
The AAT2687 provides two independently regulated DC outputs; consisting of a high voltage step-down regulator and a low input voltage low dropout (LDO) regulator. The PMIC is optimized for low-cost 12V adapter inputs, making the device the ideal system-on-a-chip power solution for consumer communications equipment. Channel 1 is a step-down regulator with an input voltage range 6.0 to 24V, providing up to 4.5A output current. 490kHz fixed switching frequency allows small L/C filtering components. Channel 1 utilizes voltage mode control configured for optimum performance across the entire output voltage and load range. Channel 2 is a low-dropout (LDO) regulator providing up to 600mA output current. The device provides extremely low output noise, low quiescent current and excellent transient response. The controller includes integrated cycle-by-cycle overcurrent protection, soft-start and over-temperature disable features. Independent input and enable pins provide maximum design flexibility. The AAT2687 is available in the Pb-free, 4x5mm 24-pin TQFN package. The rated operating temperature range is -40C to 85C.
Applications
* * * * DSL and Cable Modems Notebook Computers Satellite Settop Box Wireless LAN Systems
Typical Application
C3 0.1F 2 1 D1 BAS16 C14 2.2F LX1 BST VL1 RS1 OS1 IN1 FB1 R1 3.92K C5 2.2nF D2 R2 C4 2k 220nF L1 4.7H 5.3A C10 2.2nF R5 450 R3 8.87k C8 C9 C7 22F 22F 22F VOUT1 3.3V/4.5A
J1 VIN1 6.0V - 24.0V
COMP1
AAT2687
EN1 VIN2 + C1 220F 25V C13 1F 25V C2 2.2F IN2 OUT2
C12 2.2F EN2 GND TQFN45-24
C6 R4 VOUT2 150pF 1.8V/600mA 1.96k
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Pin Descriptions
Pin #
1 2 3, 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 EP
Symbol
LX1 LX1 N/C BST1 EN1 GND2 EN2 N/C IN2 OUT2 N/C RS1 OS1 COMP1 FB1 GND1 VL1 VL1 N/C N/C IN1 LX1 LX1 EP
Function
Channel 1 step-down converter switching pin. Connect output inductor to this pin. Connect all four LX1 pins together. Channel 1 step-down converter switching pin. Connect output inductor to this pin. Connect all four LX1 pins together. No Connect. Can be used to route IN1 and EP. Channel 1 step-down regulator boost drive input pin. Connect the cathode of fast rectifier from this pin and connect a 100nF capacitor from this pin to the channel 1 switching node (LX1) for internal high-side MOSFET gate drive. Channel 1 step-down regulator enable input pin. Active high enables the channel 1 output. It can be tied to VIN1. Ground pin for Channel 2. Power return pin for channel 2. Connect return of channel 2 input and output capacitors close to this pin for best noise performance. Channel 2 linear low dropout (LDO) enable input pin. Active high enables the channel 2 output. It can be tied to VOUT1. No Connect. Can be used to route IN2. Input supply voltage pin for channel 2 linear low dropout (LDO) regulator. Connect 2.2F ceramic input capacitor close to this pin. Output of channel 2 of linear low dropout (LDO) regulator. Connect a 2.2F ceramic capacitor from this pin to GND pin. No Connect. Can be used to route OUT2. Channel 1 output current sense pin. Connect a small signal resistor from this pin to channel 1 switching node (LX) to enable over-current sense for step-down converter. Channel 1 output sense voltage pin. Connect to the output capacitor to enable over-current sense for stepdown converter. Compensation pin for channel 1 step-down regulator. Connect a series resistor, capacitor network to compensate the voltage mode control loop. Feedback input pin for channel 1 step-down converter. Connect an external resistor divider to this pin to program the output voltage to the desired value. Ground pin for Channel 1. Power return pin for channel 1. Connect return of channel 1 input and output capacitors close to this pin for best noise performance. Internal linear regulator for channel 1 step-down converter. Connect a 2.2F/6.3V capacitor from this pin to GND1 pin. Internal linear regulator for channel 1 step-down converter. Connect to pin 18. No Connect. Do not connect to any node on the PCB. No Connect. Can be connected to GND. Input supply voltage pin for channel 1 step-down regulator. Connect both IN1 pins together. Connect the input capacitor close to this pin for best noise performance. Channel 1 step-down converter switching pin. Connect output inductor to this pin. Connect all four LX1 pins together. Channel 1 step-down converter switching pin. Connect output inductor to this pin. Connect all four LX1 pins together. Exposed paddle tied to IN1. Connect to PCB heatsink for optimum thermal performance of internal LDO device.
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2687.2008.06.1.0
PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Pin Configuration
TQFN45-24 (Top View)
N/C N/C IN1 LX1 LX1
24 23 22 21 20
LX1 LX1 N/C N/C BST1 EN1 GND2
1 2 3 4 5 6 7
19 18 17 16 15 14 13
VL1 VL1 GND1 FB1 COMP1 OS1 RS1
10
11
12
8
9
N/C OUT2 IN2 N/C EN2
Absolute Maximum Ratings1
Symbol
VIN1, VEN1 VIN2 VBST1-LX1 VCONTROL VEN2 IIN1(PULSED) TJ TLEAD
Description
IN1, LX, EN1 to GND IN2, VL1, OUT2 to GND BST1 to LX1 FB1, COMP1, RS1, OS1, OUT2 to GND EN2 to GND IN1 to LX1 Operating Junction Temperature Range Maximum Soldering Temperature (at leads, 10 sec)
Value
-0.3 to 30.0 -0.3 to 6.0 -0.3 to 6.0 -0.3 to VIN(LO) + 0.3 -0.3 to VIN2 + 0.3 12.0 -40 to 150 300
Units
V V V V V A C C
Thermal Information
Symbol
JA PD
Description
Thermal Resistance2 Maximum Power Dissipation3
Value
33 3.0
Units
C/W W
1. Stresses above those listed in Absolute Maximum Ratings may cause permanent damage to the device. Functional operation at conditions other than the operating conditions specified is not implied. Only one Absolute Maximum Rating should be applied at any one time. 2. Mounted on an FR4 board with exposed paddle connected to ground plane. 3. Derate 30 mW/C above 25C ambient temperature.
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Electrical Characteristics1
VIN1 = 12.0V, VIN2 =3.3V; TA = -40C to 85C, unless noted otherwise. Typical values are at TA = 25C. Symbol Description Conditions Min
6.0 VIN1 Rising VIN1 Hysteresis VIN1 Falling 300 3.0 1.5 0.591 -2.5 5.5 0.609 2.5
Typ
Max
24.0 5.0
Units
V V mV V V V % %/V %/A
Channel 1: Step-Down Converter VIN1 Input Voltage VUVLO1 UVLO Threshold
VOUT1 Output Voltage Range VFB1 Feedback Pin Voltage TA = 25C VOUT Output Voltage Accuracy IOUT1 = 0A to 4.5A VOUT/VOUT Line Regulation VIN1 = 6V to 24V, VOUT1 = 3.3V, IOUT1 = 4.5A VIN VOUT/VOUT Load Regulation VIN1 = 12V, VOUT1 = 3.3V, IOUT1 = 0A to 4.5A IOUT IQ1 Quiescent Current VEN1 = High, No load ISHDN1 Shutdown Current VEN1 = Low, VL1 = 0V VOCP1 Over-Current Offset Voltage VEN1 = High, VIN1 = 6.0V to 24.0V, TA = 25C ILX1 LX Pin Leakage Current VIN1 = 24.0V, VEN1 = Low DMAX Maximum Duty Cycle TON(MIN) Minimum On-Time VIN1 = 6.0 to 24.0V High Side On-Resistance VL1 = 4.5V RDSON(H) FOSC1 Oscillator Frequency FFOLDBACK1 Short Circuit Foldback Frequency Current Limit Triggered TS1 Start-Up Time From Enable Channel 1 to Output Regulation Channel 2: 600mA Linear Low Dropout (LDO) Regulator VIN2 Input Voltage VDO2 Dropout Voltage 98% x VOUT2(NOM), IOUT2 = 600mA IQ2 Quiescent (Ground) Current No load ISHDN2 Shutdown Current VEN2 = GND IOUT2 = 1mA to 600mA, VIN2 = 2.7V to 5.5V, TA = 25C Output Voltage Tolerance VOUT2(TOL) IOUT2 = 1mA to 600mA, VIN2 = 2.7V to 5.5V, TA = -40C to 85C Output Noise BW = 300Hz to 50kHz eN 1kHz PSRR Power Supply Rejection Ratio IOUT2 = 10mA 10kHz 1MHz ILIMIT2 Current Limit Enable Start-Up Delay From Enable Channel 2 to Output Regulation TS2 Over-Temperature, EN Logic Over-Temperature Shutdown Threshold TSD1,2 Over-Temperature Shutdown Hysteresis VEN1,EN2(L) Enable Threshold Low Enable Threshold High VEN1(H) VEN2(H) Enable Threshold High IEN1,EN2 Input Low Current
0.600 0.06 0.18 0.6
80 -1.0
100 85 100 35 490 100 2.5
35.0 120 1.0
350
650
mA A mV A % ns m kHz kHz ms V mV A A % % VRMS dB mA s C C
2.7 1000 70 -2.0 -3.5 250 67 47 45 800 15 135 15
5.5 1300 125 1.0 +2.0 +3.5
700
0.6 2.5 1.4 -1.0
1.0
V V V A
1. The AAT2687 is guaranteed to meet performance specifications over the -40C to +85C operating temperature range and is assured by design, characterization and correlation with statistical process controls.
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Typical Characteristics--Channel 1
Step-Down Converter Efficiency vs. Load Current
100 90 80
Step-Down Converter Load Regulation vs. Load Current
Output Voltage Difference (%)
2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 0.001
Efficiency (%)
70 60 50 40 30 20 10 0 0.001 0.01 0.1 1
VIN1 = 6V VIN1 = 7V VIN1 = 8V VIN1 = 12V VIN1 = 18V VIN1 = 24V
10
VIN1 = 6V VIN1 = 7V VIN1 = 8V VIN1 = 12V VIN1 = 18V VIN1 = 24V
0.01 0.1 1 10
Load Current (A)
Load Current (A)
Step-Down Converter Line Regulation vs. Load Current
Output Voltage Difference (%)
2 1.5 1 0.5 0 -0.5 -1 -1.5 -2 6 9 12 15 18 21 24
No Load Step-Down Converter Input Current vs. Input Voltage
650 625
Input Current (A)
IOUT1 = 0.1A IOUT1 = 2.25A IOUT1 = 3.5A IOUT1 = 4mA IOUT1 = 4.5A
600 575 550 525 500 475 450 6 9 12 15 18 21
85C 25C -40C
24
Input Voltage (V)
Input Voltage (V)
Step-Down Converter Switching Frequency vs. Input Voltage
6 4 2 0 -2 -4 -6 -8 6 8 10 12 14 16 18 20 22 24
Step-Down Converter Output Voltage vs. Temperature
(VOUT = 3.3V; IOUT1 = 4.5A; COUT1 = 66F; L = 4.7H; VIN1 = 12V) Output Voltage Difference (%)
1 0.75 0.5 0.25 0 -0.25 -0.5 -0.75 -1 -40 -15 10 35 60 85
Frequency Variation (%)
8
(VOUT1 = 3.3V; IOUT1 = 4.5A)
85C 25C -40C
Input Voltage (V)
Temperature (C)
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Typical Characteristics--Channel 1
Step-Down Converter Load Transient
(VOUT1 = 3.3V; VIN1 = 12V; COUT1 = 66F; L = 4.7H)
4.05 16 14 12 10 8 6 4.5A 4 2 450mA 0
Step-Down Converter Load Transient
(VOUT1 = 3.3V; VIN1 = 12V; COUT1 = 66F; L = 4.7H)
3.7 16 14 12 10 8 6 4.5A 2.25A 4 2 0
Load Current (bottom) (A)
Load Current (bottom) (A)
Output Voltage (top) (V)
3.80 3.55 3.30 3.05 2.80 2.55 2.30 2.05
Output Voltage (top) (V)
3.5 3.3 3.1 2.9 2.7 2.5 2.3 2.1
Time (100s/div)
Time (100s/div)
Step-Down Converter Load Transient
(VOUT1 = 3.3V; VIN1 = 12V; COUT1 = 66F; L = 4.7H)
3.5 10 9 8 7 6 4.5A 3.375A 5 4 3 2
Step-Down Converter Line Transient
(VOUT1 = 3.3V; COUT1 = 66F; L = 4.7H)
3.36 22 20 18 16 14 12 10 8 6
Load Current (bottom) (A)
Input Voltage (bottom) (V)
Output Voltage (top) (V)
3.4 3.3 3.2 3.1 3 2.9 2.8 2.7
Output Voltage (top) (V)
3.34 3.32 3.3 3.28 3.26 3.24 3.22 3.2
Time (100s/div)
Time (100s/div)
Step-Down Converter Output Voltage Ripple
(VOUT1 = 3.3V; VIN1 = 12V; COUT1 = 66F; L = 4.7H; IOUT1 = 4.5A)
3.34 35
Step-Down Converter Output Voltage Ripple
(VOUT1 = 3.3V; VIN1 = 12V; COUT1 = 66F; L = 4.7H; IOUT1 = 1mA)
3.36 35
Output Voltage (top) (V)
Output Voltage (top) (V)
LX Voltage (bottom) (V)
LX Voltage (bottom) (V)
3.32 3.3 3.28 3.26 3.24 3.22 3.2 3.18
30 25 20 15 10 5 0 -5
3.34 3.32 3.3 3.28 3.26 3.24 3.22 3.2
30 25 20 15 10 5 0 -5
Time (1s/div)
Time (2s/div)
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Typical Characteristics--Channel 2
LDO Input Current vs. Input Voltage
(VEN1 = 0V; VEN2 = VIN2)
100 1400
LDO Dropout Voltage vs. Temperature
Dropout Voltage (mV)
Input Current (A)
80 60 40 20 0
1200 1000 800 600 400 200 0 -40 -15 10 35 60 85
IOUT2 = 600mA IOUT2 = 500mA IOUT2 = 300mA IOUT2 = 150mA IOUT2 = 50mA
85C 25C -40C
2 2.5 3 3.5 4 4.5 5
Input Voltage (V)
Temperature (C)
LDO Dropout Voltage vs. Output Current
1500
1.30 1.25
LDO VIH and VIL vs. Input Voltage
Dropout Voltage (V)
1200
VIH and VIL (V)
900 600 300 0 0 100 200 300 400 500 600
1.20 1.15 1.10 1.05 1.00
85C 25C -40C
VIH VIL
2.5 3 3.5 4 4.5 5 5.5
Output Current (mA)
Input Voltage (V)
LDO Output Voltage Error vs. Temperature
(VIN2 = 3.3V; VOUT2 = 1.8V; IOUT2 = 600mA)
3.0
LDO Dropout Characteristic
(VOUT2 = 1.8V)
1.84
Output Voltage Error (%)
Output Voltage (V)
2.0 1.0 0.0 -1.0 -2.0 -3.0 -50 -25 0 25 50
IOUT2 = 0.1mA IOUT2 = 300mA IOUT2 = 600mA
1.82 1.80 1.78 1.76 1.74 1.72 1.70 1.5 2 2.5 3 3.5
IOUT2 = 0.1mA IOUT2 = 50mA IOUT2 = 100mA IOUT2 = 300mA IOUT2 = 600mA
4 4.5
75
100
Temperature (C)
Input Voltage (V)
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Typical Characteristics--Channel 2
LDO Turn-Off Response Time
(VIN2 = 3.3V; VEN2 = 3.3V; VOUT2 = 1.8V; IOUT2 = 600mA)
4 4
LDO Turn-On Time from Enable
(VIN2 = 3.3V; VEN2 = 3.3V; VOUT2 = 1.8V; IOUT2 = 600mA) Output Voltage (bottom) (V) Output Voltage (bottom) (V) Enable Voltage (top) (V)
3 2 1 0 3 2 1 0
Enable Voltage (top) (V)
3 2 1 0 2.0 1.0 0.0 -1.0
Time (5s/div)
Time (5s/div)
LDO Line Transient Response
(VIN2 = 3V to 4V; VOUT2 = 1.8V; IOUT2 = 600mA; COUT2 = 2.2F) Output Voltage (bottom) (V) Input Voltage (top) (V)
5 4 3 2 VOUT 1.85 1.80 1.75 1.70 0.7
LDO Load Transient Response
(IOUT2 = 0.3 to 0.6A; VIN2 = 3.3V; VOUT2 = 1.8V; COUT2 = 2.2F) Output Voltage (bottom) (V) Output Current (top) (A)
0.6 0.5 0.4 0.3 1.90 1.85 1.80 1.75
VIN
Time (200s/div)
Time (40s/div)
LDO Output Voltage Noise
(IOUT2 = 10mA; Power BW: 300~50KHz)
10
LDO Power Supply Rejection Ratio, PSRR
(IOUT2 = 10mA; BW: 100KHz to 300KHz)
70 60
Magnitude (dB)
Noise (VRMS)
50 40 30 20 10
5
0 100
1000
10000
100000
0 100
1000
10000
100000
Frequency (Hz)
Frequency (Hz)
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2687.2008.06.1.0
PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Functional Block Diagram
VINT
VL1
Reg.
IN1
OSC
OT
FB1 COMP1
Error Amp
Comp. Comp.
BST1
Logic
Control Logic Voltage Ref 1 Ref
EN1
LX1 20
Comp
OT
VOCP1 = 0.1V
RS1 OS1
IN2
OCP
FB_LDO
Error Amp
OUT2 Logic
FB_LDO
Voltage Ref 2 Control Logic
EN2
GND
Functional Description
The AAT2687 provides two independently regulated DC outputs, consisting of a high voltage step-down regulator and a low input voltage linear low dropout (LDO) regulator. The PMIC is optimized for low cost 12V adapter inputs, making the device an ideal system-on-a-chip power solution for consumer communications equipment. Channel 1 is a step-down regulator with an input voltage range 6.0 to 24V, providing up to 4.5A output current. The 490kHz fixed switching frequency allows small L/C filtering components. Channel 1 utilizes voltage mode control configured for optimum performance across the entire output voltage and load range. The controller includes integrated overcurrent, soft-start and over-temperature protection. Overcurrent is sensed through the output inductor DC winding
resistance (DCR). An external resistor network adjusts the current limit according to the DCR of the desired inductor and the desired output current limit. Frequency reduction limits over-current stresses during short-circuit events. The operating frequency returns to the nominal setting when over-current conditions are removed. Channel 2 is a linear low-dropout (LDO) regulator providing up to 600mA output current at a factory set output voltage. The device provides extremely low output noise, low quiescent current and excellent transient response. The controller includes integrated over-current, softstart and over-temperature protection. Independent input and enable pins provide maximum design flexibility. The AAT2687 is available in the Pb-free, 4x5mm 24-pin TQFN package. The rated operating temperature range is -40C to 85C.
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Channel 1 Regulator Output Capacitor Selection
Three 22F ceramic output capacitors are required to filter the inductor current ripple and supply the load transient current for IOUT = 4.5A. The 1206 package with 10V minimum voltage rating is recommended for the output capacitors to maintain a minimum capacitance drop with DC bias.
Applications Information
Output 1 is a high voltage DC/DC step-down converter providing an output voltage from 1.5V to 85% of the minimum input voltage (85% * VIN). The integrated highside n-channel MOSFET device provides up to 4.5A output current. Input voltage range is 6.0V to 24.0V. The step-down converter utilizes constant frequency (PWMmode) voltage mode control to achieve high operating efficiency while maintaining extremely low output noise across the operating range. High 490kHz (nominal) switching frequency allows small external filtering components; achieving minimum cost and solution size. External compensation allows the designer to optimize the transient response while achieving stability across the operating range. Output 2 is a low voltage, low dropout (LDO) linear regulator providing 1.8V with up to 600mA output current. The input voltage range is 2.7V to 5.5V. The LDO provides very low noise output which can be derived directly from the Output 1 channel.
Channel 1 Output Inductor Selection
The step-down converter utilizes constant frequency (PWM-mode) voltage mode control. A 4.7H inductor value is selected to maintain the desired output current ripple and minimize the converter's response time to load transients. The peak switch current should not exceed the inductor saturation current, the MOSFET or the external Schottky rectifier peak current ratings.
Channel 1 Rectifier Selection
When the high-side switch is on, the input voltage will be applied to the cathode of the Shottky diode. The rectifier's rated reverse breakdown voltage must be chosen at least equal to the maximum input voltage of the stepdown regulator. When the high-side switch is off, the current will flow from the power ground to the output through the Schottky diode and the inductor. The power dissipation of the Schottky diode during the time-off can be determined by the following equation:
Channel 1 Output Voltage and Current
Output 1 is set using an external resistor divider as shown in Table 1. Minimum output voltage is 1.5V and maximum output voltage is 5.5V. Typical maximum duty cycle is 85%. VOUT (V)
1.5 1.8 1.85 2.0 2.5 3.0 3.3 5.0
R4 = 1.96k R3 (k)
2.94 3.92 4.02 4.53 6.19 7.87 8.87 14.3
PD = IOUT * VD * 1 -
VOUT VIN
Where VD is the voltage drop across the Schottky diode.
Table 1: Feedback Resistor Values. Alternatively, the feedback resistor may be calculated using the following equation:
Channel 1 Input Capacitor Selection
For low cost applications, a 220F/25V electrolytic capacitor is selected to control the voltage overshoot across the high side MOSFET. A small ceramic capacitor with voltage rating at least 1.05 times greater than the maximum input voltage is connected as close as possible to the input pin (Pin 14) for high frequency decoupling.
R3 =
(VOUT - 0.6) * R4 0.6
R3 is rounded to the nearest 1% resistor value.
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2687.2008.06.1.0
PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
stability. Equation 3, 4, 5 and 6 relate the compensation network's poles and zeros to the components R1, R3, R5, C5, C6, and C10: Eq. 3: FZ1 =
C5 R1 C10 R5 VOUT1
Channel 1 Feedback and Compensation Networks
C6
1 2 * * R1 * C5
Eq. 4: FZ1 =
COMP1 R3
1 2 * * (R3 + R5) * C10
1
C5 * C6 C5 + C6
Eq. 5: FP1 =
2 * * R1 *
FB1 R4
Eq. 6: FP2 =
1 2 * * R5 * C10
REF
Figure 1: AAT2687 Feedback and Compensation Networks for Type III Voltage-Mode Control Loop. The transfer function of the Error Amplifier is dominated by the DC Gain and the L COUT output filter of the regulator. This output filter and its equivalent series resistor (ESR) create a double pole at FLC and a zero at FESR in the following equations: Eq. 1: FLC = Eq. 2: FESR =
Components of the feedback, feed forward, compensation, and current limit networks need to be adjusted to maintain the systems stability for different input and output voltages applications as shown in Table 1.
Channel 1 Thermal Protection
The AAT2687 has an internal thermal protection circuit which will turn on when the device die temperature exceeds 135C. The internal thermal protection circuit will actively turn off the high side regulator output device to prevent the possibility of over temperature damage. The Buck regulator output will remain in a shutdown state until the internal die temperature falls back below the 135C trip point. The combination and interaction between the short circuit and thermal protection systems allows the Buck regulator to withstand indefinite short-circuit conditions without sustaining permanent damage.
1 2 * * L * COUT
1 2 * * ESR * COUT
The feedback and compensation networks provide a closed loop transfer function with the highest 0dB crossing frequency and adequate phase margin for system
Network
Feedback Feed-forward Compensation
Components
R4 R3 C10 R5 C5 C6 R1 C4 R2 R6 R7 R8
VOUT =3.3V VIN = 6V-24V
1.96k 8.87k 2.2nF 453 2.2nF 150pF 3.92k 220nF 2k Open 2k 165k
VOUT = 5.0V VIN = 6V-24V
1.96k 8.87k 2.2nF 453 2.2nF 150pF 3.92k 220nF 2k Open 2k 165k
Current Limit
Table 1: AAT2687 Feedback, Compensation, and Current Limit Components For VOUT = 3.3V and VOUT = 5.0V.
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Channel 2 Input Capacitor
Typically, a 1F or larger capacitor is recommended for CIN in most applications. A CIN capacitor is not required for basic LDO regulator operation. However, if the AAT2687 is physically located more than three centimeters from an input power source, a CIN capacitor will be needed for stable operation. CIN should be located as close to the device VIN pin as possible. CIN values greater than 1F will offer superior input line transient response and will assist in maximizing the highest possible power supply ripple rejection. Ceramic, tantalum, or aluminum electrolytic capacitors may be selected for CIN. There is no specific capacitor ESR requirement for CIN. However, for 150mA LDO regulator output operation, ceramic capacitors are recommended for CIN due to their inherent capability over tantalum capacitors to withstand input current surges from low impedance sources, such as batteries in portable devices.
Over-Current Protection
The output 1 controller provides true-load DC output current sensing which protects the load and limits component stresses. The output current is sensed through the DC resistance in the output inductor (DCR). The controller reduces the operating frequency when an over-current condition is detected; limiting stresses and preventing inductor saturation. This allows the smallest possible inductor for a given output load. A small resistor divider may be necessary to adjust the over-current threshold and compensate for variation in inductor DCR. The preset current limit threshold is triggered when the differential voltage from RS1 to OS1 exceeds 100mV (nominal).
L1 LX1 4.7H V OUT1 3.3V/4.5A
R2 2k RS1
C4 220nF R7
Channel 2 Output Capacitor
For proper load voltage regulation and operational stability, a capacitor is required between pins VOUT and GND. The COUT capacitor connection to the LDO regulator ground pin should be connected as close as possible for maximum device performance. The AAT2687 LDO has been specifically designed to function with very low ESR ceramic capacitors. For best performance, ceramic capacitors are recommended. Typical output capacitor values for maximum output current conditions range from 1F to 10F. Applications utilizing the exceptionally low output noise and optimum power supply ripple rejection characteristics of the channel 2 should use 2.2F or greater for COUT. If desired, COUT may be increased without limit. In low output current applications where output load is less than 10mA, the minimum value for COUT can be as low as 0.47F.
OS1 R8
Figure 2: Optional Resistor Network to Adjust the Current Limit Less than the Pre-Set Over-Current Threshold (Add R7 and R8).
VOUT1 3.3V/4.5A
L1 LX1 4.7H
R2 2k RS1 OS1
C4 220nF R6 R7
Channel 2 Enable Function
The AAT2687 features an LDO regulator enable/disable function. This pin (EN) is active high and is compatible with CMOS logic. To assure the LDO regulator will switch on, the EN turn-on control level must be greater than 1.5V. The LDO regulator will go into the disable shutdown mode when the voltage on the EN pin falls below 0.6V. If the enable function is not needed in a specific application, it may be tied to VIN to keep the LDO regulator in a continuously on state. When the LDO regulator is in shut-
Figure 3: Optional Resistor Network to Adjust the Current Limit Greater than the Pre-Set OverCurrent Level (Add R6 and R7).
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
highly recommended. A larger value of CIN with respect to COUT will effect a slower CIN decay rate during shutdown, thus preventing VOUT from exceeding VIN. In applications where there is a greater danger of VOUT exceeding VIN for extended periods of time, it is recommended to place a Schottky diode across VIN to VOUT (connecting the cathode to VIN and anode to VOUT). The Schottky diode forward voltage should be less than 0.45V. down mode, an internal 1.5k resistor is connected between VOUT and GND. This is intended to discharge COUT when the LDO regulator is disabled. The internal 1.5k has no adverse effect on device turn-on time.
Channel 2 Short-Circuit Protection
The AAT2687 LDO contains an internal short-circuit protection circuit that will trigger when the output load current exceeds the internal threshold limit. Under shortcircuit conditions, the output of the LDO regulator will be current limited until the short-circuit condition is removed from the output or LDO regulator package power dissipation exceeds the device thermal limit.
Thermal Calculations
There are three types of losses associated with the AAT2687 step-down converter: switching losses, conduction losses, and quiescent current losses. Conduction losses are associated with the RDS(ON) characteristics of the power output switching devices. Switching losses are dominated by the gate charge of the power output switching devices. At full load, assuming continuous conduction mode (CCM), a simplified form of the synchronous step-down converter and LDO losses is given by:
Channel 2 Thermal Protection
The AAT2687 LDO has an internal thermal protection circuit which will turn on when the device die temperature exceeds 150C. The internal thermal protection circuit will actively turn off the LDO regulator output pass device to prevent the possibility of over temperature damage. The LDO regulator output will remain in a shutdown state until the internal die temperature falls back below the 150C trip point. The combination and interaction between the short circuit and thermal protection systems allows the LDO regulator to withstand indefinite short-circuit conditions without sustaining permanent damage.
PTOTAL =
IOUT12 * (RDS(ON)H * VOUT1 + RDS(ON)L * [VIN1 - VOUT1 ]) VIN1
+ (tSW * FS * IOUT1 + IQ1 ) * VIN1 + (VIN2 - VOUT2) * IOUT2
IQ1 and IQ2 are the step-down converter and LDO quiescent currents respectively. The term tSW is used to estimate the full load step-down converter switching losses. For asynchronous Step-Down converter, the power dissipation is only in the internal high side MOSFET during the on time. When the switch is off, the power dissipates on the external Schottky diode. The total package losses for the AAT2687 reduce to the following equation:
PTOTAL = IOUT12 * RDS(ON)H * D + (tSW * FS * IOUT1 + IQ) * VIN + (VIN2 - VOUT2) * IOUT2
Channel 2 No-Load Stability
The AAT2687 is designed to maintain output voltage regulation and stability under operational no load conditions. This is an important characteristic for applications where the output current may drop to zero.
Channel 2 Reverse Output-to-Input Voltage Conditions and Protection
Under normal operating conditions, a parasitic diode exists between the output and input of the LDO regulator. The input voltage should always remain greater than the output load voltage, maintaining a reverse bias on the internal parasitic diode. Conditions where VOUT might exceed VIN should be avoided since this would forward bias the internal parasitic diode and allow excessive current flow into the VOUT pin, possibly damaging the LDO regulator. In applications where there is a possibility of VOUT exceeding VIN for brief amounts of time during normal operation, the use of a larger value CIN capacitor is
Where: D = VOUT is the duty cycle. VIN Since RDS(ON), quiescent current, and switching losses all vary with input voltage, the total losses should be investigated over the complete input voltage range. Given the total losses, the maximum junction temperature can be derived from the JA for the TQFN45-24 package, which is 33C/W.
TJ(MAX) = PTOTAL * JA + TAMB
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
5. 6. Connect unused signal pins to ground or input to avoid unwanted noise coupling. The critical small signal components include feedback components, and compensation components should be placed close to the FB1 and COMP1 pins. The feedback resistors should be located as close as possible to the FB1 pin with its ground tied straight to the signal ground plane which is separated from power ground plane. C4 should be connected close to the RS1 and OS1 pins, while R2 should be connected directly to the output pin of the inductor. For the best current limit performance, C4 and R2 should be placed at the bottom layer to avoid noise coupling from the inductor. R7 should be connected directly to the output pin of inductor L1 to sense precisely its DCR. For good thermal coupling, a 4-layer PCB layout is recommended and PCB vias are required from the exposed pad (EP) for the TQFN45-24 paddle to the middle plans and bottom plane. The EP is internally connected to IN.
Layout Considerations
The suggested PCB layout for the AAT2687 is shown in Figures 5, 6, 7, and 8. The following guidelines should be used to help ensure a proper layout. 1. The power input capacitors (C1 and C15) should be connected as close as possible to high voltage input pin (IN1) and power ground. C1, L1, D2, C7, C8, and C9 should be place as close as possible to minimize any parasitic inductance in the switched current path which generates a large voltage spike during the switching interval. The connection of inductor to switching node should be as short as possible. The feedback trace or FB1 pin should be separated from any power trace and connected as close as possible to the load point. Sensing along a high-current load trace will degrade DC load regulation. The resistance of the trace from the load returns to PGND should be kept to a minimum. This will help to minimize any error in DC regulation due to differences in the potential of the internal signal ground and the power ground.
2.
7.
3.
8. 9.
4.
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AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
VOUT1 U1
1
L1 LX1 LX1 N/C FB1 RS1 OS1
24
3.3V/4.5A C10 2.2nF R6 open R5 453
C3 0.1F
LX1 LX1 BST VL1 VL1 IN1 EP N/C N/C EN1 IN2 N/C EN2
2
23
R2 D2 2k
4.7H 5.3A C4 220nF
5 2 1
R3 8.87k
20
J1 VIN1 6.0V - 24.0V
D1 BAS16
18
16
19
13
22
AAT2687
14
R7 2k
C6 150pF
C7 R1 22F 3.92K R4 1.96k C5 2.2nF
C8 C9 22F 22F
C14 2.2F
3 2 1
3 4
COMP1 OUT2 N/C GND GND2 N/C
15
11
VOUT2 1.8V/0.6A
6
12
EN1 C1 220F 25V + C15 open C13 1F 25V VIN2
10
17
R8 165k C12 2.2F C11 open
9
7
C2 2.2F EN2
3 2 1 8 21
TQFN45-24
U1 C1 C2, C12, C14 C3 C4 C5, C6, C10 C7, C8, C9 C13 D1 D2 L1 R1 - R5
AAT2687 Analogic Technologies, Hi-Voltage Buck/LDO, TQFN45-24 Cap, MLC, 220F/25V, Electrolytic cap Cap, MLC, 2.2F, 6.3V, 0805 Cap, MLC, 0.1F/6.3V, 0603 Cap, MLC, 220nF/6.3V, 0402 Cap, MLC, misc, 0603 Cap, MLC, 22F/10V, 1206 Cap, MLC, 1F, 25V, 0805 BAS16, Generic, Rectifier, 0.2A/85V, Ultrafast, SOT23 B540C, Generic, Schottky Rectifier, 5A/40V, SMC RCH108NP-4R7M, Sumida, 4.7H, ISAT = 5.7A, DCR = 11.7m or Wurth 744 771 004, 4.7H, ISAT = 6.8A, DCR = 11m Carbon film resistor, 0402
Figure 4: AAT2687IFK Evaluation Board Schematic For VIN = 6V - 24V and VOUT = 3.3V.
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Figure 5: AAT2687IFK Evaluation Board Top Layer.
Figure 6: AAT2687IFK Evaluation Board MID1 Layer.
Figure 7: AAT2687IFK Evaluation Board MID2 Layer.
Figure 8: AAT2687IFK Evaluation Board Bottom Layer.
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AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
AAT2687 Design Example
Specifications
VO1 = 3.3V @ 4.5A, Pulsed Load ILOAD = 4.5A VO2 = 1.8V @ 600mA VIN1 = 12V FS = 490kHz TAMB = 85C in TQFN45-24 Package
Channel 1 Output Inductor
For Sumida inductor RCH108NP-4R7M, 4.7H, DCR = 11.7m max.
I =
VOUT1 VOUT1 3.3V 3.3V * 1= * 1= 1A L1 * FS VIN1 4.7H * 490kHz 12V I = 4.5A + 1A = 5.5A 2
IPK1 = IOUT1 +
PL1 = IOUT12 * DCR = 5.5A2 * 11.7m = 354mW
Channel 1 Output Capacitor
VDROOP = 0.4V
COUT =
3 * ILOAD 3 * 4.5A = = 69F; use 3x22F 0.4V * 490kHz VDROOP * FS 1 2* 3
IRMS(MAX) =
*
VOUT1 * (VIN(MAX) - VOUT1) 1 3.3V * (24V - 3.3V) * = 357mARMS = L * FS * VIN1(MAX) 2 * 3 4.7H * 490kHz * 24V
PRMS = ESR * IRMS2 = 5m * (357mA)2 = 0.6W
Channel 1 Input Capacitor
Input Ripple VPP = 33mV
CIN1 =
1 VPP - ESR * 4 * FS IOUT1
=
1 5.5mV - 5m * 4 * 490kHz 4.5A
= 219F
For low cost applications, a 220F/25V electrolytic capacitor in parallel with a 1F/25V ceramic capacitor is used to reduce ESR.
IRMS =
IOUT1 = 2.25A 2
P = ESR * (IRMS)2 = 5m * (2.25A)2 = 25.3mW
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Channel 1 Current Limit
Voltage sense VS = 100mV Total trace parasitic resister and inductor DCR is 10m ILIMIT = 5A.
IPRESET =
VS 100mV = = 10A > ILIMIT 10m DCR
R8 =
VOUT * R2 3.3V * 2k = 165k = 0.1V - 6A * 10m VS - ILIMIT * DCR R2 * R 8 2k * 165k = = 2k 165k - 2k R8 - R 2
R7 =
AAT2687 Losses
All values assume 25C ambient temperature and thermal resistor of 50C/W in the TQFN45-24 package.
PTOTAL = IOUT12 * RDS(ON)H * D + (tSW * FS * IOUT1 + IQ) * VIN1 + (VIN2 - VOUT2) * IOUT2
2 PTOTAL = 4.5A * 70m * 3.3V + (5ns * 490kHz * 4.5A + 70A) * 12V + (3.3 - 1.8) * 600mA 12V
PTOTAL = 1.42W TJ(MAX) = TAMB + JA * PLOSS = 85C + (33C/W) * 1.42W = 131C
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Ordering Information
Voltage Package
TQFN45-24 Channel 1 Adj (0.6) Channel 2 1.8
Marking1
3PXYY
Part Number (Tape and Reel)2
AAT2687IFK-AI-T1
All AnalogicTech products are offered in Pb-free packaging. The term "Pb-free" means semiconductor products that are in compliance with current RoHS standards, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. For more information, please visit our website at http://www.analogictech.com/about/quality.aspx.
Legend Voltage
Adjustable (0.6) 1.8
Code
A I
1. XYY = assembly and date code. 2. Sample stock is generally held on part numbers listed in BOLD.
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PRODUCT DATASHEET
AAT2687
SystemPowerTM PMIC Solution for 24V Systems with 2 High Performance Step-Down Converters
Package Information1
TQFN45-24
Pin 1 Dot by Marking 4.000 0.050 0.400 0.050 2.800 0.050 Pin 1 Identification Chamfer 0.400 x 45
5.000 0.050
3.800 0.050
3.000 REF
0.750 0.050
0.203 REF 0.000 - 0.050
Side View
0.500 BSC 2.000 REF
0.250 0.050
Top View
All dimensions in millimeters.
Bottom View
1. The leadless package family, which includes QFN, TQFN, DFN, TDFN and STDFN, has exposed copper (unplated) at the end of the lead terminals due to the manufacturing process. A solder fillet at the exposed copper edge cannot be guaranteed and is not required to ensure a proper bottom solder connection.
Advanced Analogic Technologies, Inc. 3230 Scott Boulevard, Santa Clara, CA 95054 Phone (408) 737-4600 Fax (408) 737-4611
(c) Advanced Analogic Technologies, Inc. AnalogicTech cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in an AnalogicTech product. No circuit patent licenses, copyrights, mask work rights, or other intellectual property rights are implied. AnalogicTech reserves the right to make changes to their products or specifications or to discontinue any product or service without notice. Except as provided in AnalogicTech's terms and conditions of sale, AnalogicTech assumes no liability whatsoever, and AnalogicTech disclaims any express or implied warranty relating to the sale and/or use of AnalogicTech products including liability or warranties relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right. In order to minimize risks associated with the customer's applications, adequate design and operating safeguards must be provided by the customer to minimize inherent or procedural hazards. Testing and other quality control techniques are utilized to the extent AnalogicTech deems necessary to support this warranty. Specific testing of all parameters of each device is not necessarily performed. AnalogicTech and the AnalogicTech logo are trademarks of Advanced Analogic Technologies Incorporated. All other brand and product names appearing in this document are registered trademarks or trademarks of their respective holders.
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