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1MHz 2.5A Step-Down DC/DC Converter General Description
The AAT1155 SwitchReg is a step-down switching converter ideal for applications where high efficiency, small size, and low ripple are critical. Able to deliver 2.5A with an internal power MOSFET, the current-mode controlled IC provides high efficiency. Fully internally compensated, the AAT1155 simplifies system design and lowers external parts count. The AAT1155 is available in a Pb-free MSOP-8 package and is rated over the -40C to +85C temperature range.
AAT1155
Features
* * * * * * * * * * * * * *
SwitchRegTM
5.5V Max Supply Input Fixed or Adjustable VOUT: 1.0V to 4.2V 2.5A Output Current Up to 95% Efficiency Integrated Low On Resistance Power Switches Internally Compensated Current Mode Control 1MHz Switching Frequency Constant Pulse Width Modulation (PWM) Mode Low Output Ripple with Light Load Internal Soft Start Current Limit Protection Over-Temperature Protection MSOP-8 Package -40C to +85C Temperature Range
Applications
* * * * * Cable/DSL Modems Computer Peripherals High Efficiency Conversion from 5V or 3.3V Supply Network Cards Set-Top Boxes
Typical Application
INPUT
10F
VP FB
AAT1155
EN 100 VCC
LX LX
1.5H
OUTPUT GND 0.1F 120F
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1MHz 2.5A Step-Down DC/DC Converter Pin Descriptions
Pin #
1 2 3 4 5, 8 6, 7
AAT1155
Symbol
FB GND EN VCC VP LX
Function
Feedback input pin. Signal ground. Converter enable pin. Small signal filtered bias supply. Input supply for converter power stage. Inductor connection pin.
Pin Configuration
MSOP-8
FB GND EN VCC
1
8
VP LX LX VP
1
2
2
7
3
6
4
5
2
1155.2006.09.1.7
1MHz 2.5A Step-Down DC/DC Converter Absolute Maximum Ratings1
TA = 25C, unless otherwise noted. Symbol
VCC, VP VLX VFB VEN TJ TLEAD VESD
AAT1155
Description
VCC, VP to GND LX to GND FB to GND EN to GND Operating Junction Temperature Range Maximum Soldering Temperature (at leads, 10 sec) ESD Rating2 - HBM
Value
6 -0.3 to VP+0.3 -0.3 to VCC+0.3 -0.3 to VCC+0.3 -40 to 150 300 3000
Units
V V V V C C V
Thermal Characteristics3
Symbol
JA PD
Description
Maximum Thermal Resistance Maximum Power Dissipation
Value
150 833
Units
C/W mW
Recommended Operating Conditions
Symbol
T
Description
Ambient Temperature Range
Rating
-40 to +85
Units
C
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. Human body model is a 100pF capacitor discharged through a 1.5k resistor into each pin. 3. Mounted on a demo board (FR4, in still air). 1155.2006.09.1.7
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1MHz 2.5A Step-Down DC/DC Converter Electrical Characteristics
VIN = VCC = VP = 5V, TA = -40C to +85C, unless otherwise noted. Typical values are TA = 25C. Symbol
VIN VOUT VIL VIH VUVLO VUVLO(HYS) IQ ISHDN ILIM RDS(ON)H VOUT (VOUT*VIN) VOUT/VOUT FOSC TSD THYS
AAT1155
Description
Input Voltage Range Output Voltage Tolerance Input Low Voltage Input High Voltage Under-Voltage Lockout Under-Voltage Lockout Hysteresis Quiescent Supply Current Shutdown Current Current Limit High Side Switch On Resistance Efficiency Load Regulation Line Regulation Oscillator Frequency Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis
Conditions
VIN = VOUT + 0.2 to 5.5V, IOUT = 0.5A
Min Typ Max Units
2.7 -2.5 5.5 2.5 0.6 1.4 V % V V V mV A A A m % % %/V MHz C C
VIN Rising VIN Falling No Load, VFB = 0V VEN = 0V, VIN = 5.5V TA = 25C TA = 25C IOUT = 1.0A ILOAD = 0A to 2.5A VIN = 2.7V to 5.5V TA = 25C
2.5 1.2 250 630 4.4 60 92 2.3 0.75 1 140 15 1000 1.0
4
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1MHz 2.5A Step-Down DC/DC Converter Typical Characteristics
Efficiency vs. Load Current
(VIN = 5.0V; VOUT = 3.3V)
100 95 90 80
AAT1155
RDS(ON) vs. Temperature
2.7V
3.6V
4.2V
Efficiency (%)
RDS(ON) (m)
90 85 80 75 70 65 60 0.01 0.1 1 10
70 60
5.0V
50 40 -20 0 20 40 60 80
5.5V
100
120
Output Current (A)
Temperature (C)
Oscillator Frequency Variation vs. Input Voltage
(VOUT = 3.3V)
0.5
80 75
RDS(ON) vs. Input Voltage
(IDS = 1A)
Variation (%)
0.25
RDS(ON) (m)
70 65 60 55 50 45 40 2.5 3 3.5 4 4.5 5 5.5
0
-0.25
-0.5 3.5
4
4.5
5
5.5
Input Voltage (V)
Input Voltage (V)
Oscillator Frequency Variation vs. Temperature
(VIN = 5V)
1
1.2
Enable Threshold vs. Input Voltage
Enable Threshold (V)
1.1 1 0.9 0.8 0.7 0.6
0
EN(H)
Variation (%)
-1 -2 -3 -4 -20 0 20 40 60 80 100
EN(L)
2.5 3 3.5 4 4.5 5 5.5
Temperature (C)
Input Voltage (V)
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1MHz 2.5A Step-Down DC/DC Converter Typical Characteristics
Output Voltage Variation vs. Temperature
(IOUT = 2A; VO = 3.3V)
Output Voltage Error (%)
0.4 0.2 1 0 -1 -2 -3 -4 -5 3.4
AAT1155
Line Regulation
(VOUT = 3.3V)
Variation (%)
IO = 0.3A
0 -0.2 -0.4 -0.6 -0.8 -20 0 20 40 60 80 100
IO = 3.0A
3.7 4 4.3 4.6 4.9 5.2 5.5
Temperature (C)
Input Voltage (V)
AAT1155 Evaluation Board Over-Temperature Current vs. Input Voltage
(VOUT = 3.3V)
3.6 0.0 3.4 3.2 3 2.8 2.6 2.4 2.2 2 1.8 1.6 3.5 3.75 4 4.25 4.5 4.75 5 5.25 5.5 -1.0
Load Regulation
(VIN = 5.0V; VOUT = 3.3V)
Output Current (A)
Output Error (%)
55C 70C 85C 100C
-2.0 -3.0 -4.0 -5.0 -6.0 -7.0 -8.0 -9.0 -10.0 0.01 0.1 1 10
Input Voltage (V)
Load Current (A)
Non-Switching Operating Current vs. Temperature
Operating Current (mA)
750 700 650 600 550 500 450 -20 0 20 40 60 80 100 120
Over-Temperature Shutdown Current vs. Temperature
(VOUT = 3.3V; VIN = 5.0V)
5 4.5 4 3.5 3 2.5 2 -20 -10 0 10 20 30 40 50 60 70 80 90 100
(FB = 0V)
2.7V
3.6V
4.2V
5.0V
5.5V
Output Current (A)
Temperature (C)
Temperature (C)
6
1155.2006.09.1.7
1MHz 2.5A Step-Down DC/DC Converter Typical Characteristics
Enalbe and Output Voltage (V) (top trace)
AAT1155
Inrush and Output Overshoot Characteristics
8 6 4 2 0 -2 -4 -6 -8 0 0.4 0.8 1.2 1.6 2 14 12 10 8 6 4 2 0 -2 0.04 0.02
Tantalum Output Ripple
(IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V)
7 6
Inductor Current (A) (bottom trace)
Inductor Current (bottom) (A)
Output Ripple (top) (mV)
0.00 -0.02 -0.04 -0.06 -0.08 -0.10 -0.12 0 120F 6.3V Tantalum Vishay P/N 594D127X96R3C2T 1 2 3 4 5
5 4 3 2 1 0 -1
Time (ms)
Time (s)
Loop Crossover Gain and Phase
16 12 8 Phase L = 1.5Hy 180 135 4
Output Ripple
(IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V)
7 6
AC Output Ripple (top) (mV)
90 45 0
2 0 -2 -4 -6 200F 6.3V Ceramic -10 TDK P/N C3325X5R0J107M Vishay GRM43SR60J107ME20L -12 0 1 2 3 -8
Phase (Degrees)
Inductor Current (bottom) (A)
Gain (dB)
4 0 -4 -8 -12 300F gain 100F 6.3 Ceramic TDK P/N C3225X5R0J107M Vishay GRM43SR60J107ME20L
5 4 3 2 1 0 -1 4 5
200F gain
-45 -90 -135
-16 10000
-180 100000
Frequency (Hz)
Time (s)
Loop Crossover Gain and Phase
16 12 8 180 135 90 4
Output Ripple
(IOUT = 3.0A; VOUT = 3.3V; VIN = 5.0V)
7 6
AC Output Ripple (top) (mV)
2 0 -2 -4 -6 -8 300F 6.3VCeramic TDK P/N C3325X5R0J107M Vishay GRM43SR60J107ME20L 0 1 2 3 4 5
Inductor Current (bottom) (A)
5 4 3 2 1 0 -1
Phase (Degrees)
Gain (dB)
4 0 -4 -8 -12
Gain
45 0 -45 -90
120F 6.3V Tantalum Vishay P/N 594D127X96R3C2T 100000
-135 -180 1000000
-10 -12
-16 10000
Frequency (Hz)
Time (s)
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1MHz 2.5A Step-Down DC/DC Converter Typical Characteristics
Tantalum Transient Response
(IOUT = 0 to 3.0A; VOUT = 3.3V; VIN = 5.0V)
3.40 3.30 7 6 5 4 3 2 1 120F 6.3V Tantalum Vishay P/N 594D127X96R3C2T 0 100 200 300 400 500 0 -1
3.40 3.30
AAT1155
Transient Response
(IOUT = 0 to 3.0A; VOUT = 3.3V; VIN = 5.0V)
7 6
2x 100F 6.3V Ceramic TDK P/N C3325X5R0J107M Vishay GRM43SR60J107ME20L
Inductor Current (bottom) (A)
Inductor Current (bottom) (A)
Output Voltage (top) (mV)
Output Voltage (top) (mV)
3.20 3.10 3.00 2.90 2.80 2.70 2.60
3.20 3.10 3.00 2.90 2.80 2.70 2.60 0 100 200
5 4 3 2 1 0 -1
300
400
500
Time (s)
Time (s)
Transient Response
(IOUT = 0 to 3.0A; VOUT = 3.3V; VIN = 5.0V)
3.40 3.30 7 6
3x 100F 6.3V Ceramic TDK P/N C3325X5R0J107M Vishay GRM43SR60J107ME20L
Inductor Current (bottom) (A)
Output Voltage (top) (mV)
3.20 3.10 3.00 2.90 2.80 2.70 2.60 0 100 200
5 4 3 2 1 0
300
400
-1 500
Time (s)
8
1155.2006.09.1.7
1MHz 2.5A Step-Down DC/DC Converter Functional Block Diagram
VCC VP = 2.5V to 5.5V
AAT1155
REF
FB
OP. AMP
CMP
DH
LOGIC
LX
OSC
Temp. Sensing
EN
Applications Information
Main Control Loop
The AAT1155 is a peak current mode step-down converter. The inner wide bandwidth loop controls the inductor peak current. The inductor current is sensed as it flows through the internal P-channel MOSFET. A fixed slope compensation signal is then added to the sensed current to maintain stability for duty cycles greater than 50%. The inner loop appears as a voltage-programmed current source in parallel with the output capacitor. The voltage error amplifier output programs the current loop for the necessary inductor current to force a constant output voltage for all load and line conditions. The feedback resistive divider is internal, dividing the output voltage to the error amplifier reference voltage of 1.0V. The error amplifier has a limited DC gain. This eliminates the need for external compensation components, while still providing sufficient DC loop gain for good load regulation. The crossover frequency and phase margin are set by the output capacitor value.
Duty cycle extends to 100% as the input voltage approaches the output voltage. Thermal shutdown protection disables the device in the event of a short-circuit or overload condition.
Soft Start/Enable
Soft start controls the current limit when the input voltage or enable is applied. It limits the current surge seen at the input and eliminates output voltage overshoot. When pulled low, the enable input forces the device into a low-power, non-switching state. The total input current during shutdown is less than 1A.
Power and Signal Source
Separate small signal ground and power supply pins isolate the internal control circuitry from switching noise. In addition, the low pass filter R1 and C3 (shown in Figure 1) filters noise associated with the power switching.
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1MHz 2.5A Step-Down DC/DC Converter
Vin 2.7V-5.5V VOUT 1.25V @ 2.5A R3 2.55k U1 AAT1155-1.0 FB VP L1 1.5H
AAT1155
C4 100F
R1 100
R2 100k
GND LX EN C1 10F C3 0.1F R4 10.0k LX
VCC VP
D1 B340LA
C2 120F
rtn
C1 Murata 10F 6.3V X5R GRM42-6X5R106K6.3 C2 Vishay 120F 6.3V 594D127X96R6R3C2T C3 0. F 0603ZD104M AVX 1 C4 Vishay Sprague 100F 16V 595D107X0016C 100F 16V D1 B340LA Diodes Inc. L1 CDRH6D28-1.5H Sumida Options C2 MuRata 100uF 6.3V GRM43-2 X5R 107M 100F 6.3V (two or three in parallel) C2 TDK 100F 6.3V C3325X5R0J107M 100F 6.3V (two or three in parallel)
Figure 1: AAT1155 Evaluation Board with Adjustable Output.
Current Limit and Over-Temperature Protection
Over-temperature and current limit circuitry protects the AAT1155 and the external Schottky diode during overload, short-circuit, and excessive ambient temperature conditions. The junction over-temperature threshold is 140C nominal and has 15C of hysteresis. Typical graphs of the over-temperature load current vs. input voltage and ambient temperature are shown in the Typical Characteristics section of this document.
peak ripple current is 1A. Assuming a 5V 5% input voltage and 40% ripple, the output inductance required is:
VOUT VOUT L= I * k * FS * 1 - VIN(MAX) OUT = 3.3V * 1 - 3.3V 2.5A * 0.4 * 1MHz 5.25V
= 1.23H
The factor "k" is the fraction of the full load (40%) selected for the ripple current at the maximum input voltage. The corresponding inductor RMS current is:
IRMS = 2 I 2 I+ I O = 2.5A O 12
Inductor
The output inductor is selected to limit the ripple current to 20% to 40% of the full load current at the maximum input voltage. Manufacturer's specifications list both the inductor DC current rating, which is a thermal limitation, and the peak current rating, which is determined by the inductor saturation characteristics. The inductor should not show any appreciable saturation under normal load conditions. During overload and short-circuit conditions, the inductor can exceed its peak current rating without affecting the converter performance. Some inductors may have sufficient peak and average current ratings yet result in excessive losses due to a high DCR. The losses associated with the DCR and its effect on the total converter efficiency must be considered. For a 2.5A load and the ripple current set to 40% at the maximum input voltage, the maximum peak-to10
I is the peak-to-peak ripple current which is fixed by the inductor selection above. For a peak-topeak current of 40% of the full load current, the peak current at full load will be 120% of the full load. The 1.5H inductor selected from the Sumida CDRH6D38 series has a 11m DCR and a 4.0A DC current rating with a height of 4mm. At full load, the inductor DC loss is 70mW for a 0.84% loss in efficiency.
1155.2006.09.1.7
1MHz 2.5A Step-Down DC/DC Converter
Schottky Freewheeling Diode
The Schottky average current is the load current multiplied by one minus the duty cycle.
VO 1 VIN
AAT1155
3A Surface Mount Schottky Diodes
Diodes Inc. ROHM Micro Semi B340LA RB050L-40 5820SM 0.45V @ 3A 0.45 @ 3A 0.46V @ 3A
Input Capacitor Selection
The primary function of the input capacitor is to provide a low impedance loop for the edges of pulsed current drawn by the AAT1155. A low ESR/ESL ceramic capacitor is ideal for this function. To minimize stray inductance, the capacitor should be placed as closely as possible to the IC. This also keeps the high frequency content of the input current localized, minimizing the radiated and conducted EMI while facilitating optimum performance of the AAT1155. Proper placement of the input capacitor C1 is shown in the layout in Figure 2. Ceramic X5R or X7R capacitors are ideal. The size required will vary depending on the load, output voltage, and input voltage source impedance characteristics. Typical values range from 1F to 10F. The input capacitor RMS current varies with the input voltage and the output voltage. It is highest when the input voltage is double the output voltage where it is one half of the load current.
IRMS = IO * VO V * 1- O VIN VIN
For VIN at 5V and VOUT at 3.3V, the average diode current is:
V 3.3V = 0.85A IAVG = IO * 1 - O = 2.5A * 1 VIN 5.0V
With a 125C maximum junction temperature and a 120C/W thermal resistance, the maximum average current is:
IAVG = T J(MAX)- T AMB = 125C - 70C = 1.14A 120 C / W * 0.4V
JA * VF
For overload, short-circuit, and excessive ambient temperature conditions, the AAT1155 enters overtemperature shutdown mode protecting the AAT1155 as well as the output Schottky. In this mode, the output current is limited internally until the junction temperature reaches the temperature limit (see over-temperature characteristics graphs). The diode reverse voltage must be rated to withstand the input voltage.
Vin 3.5V-5.5V
Vout 3.3V @ 2.5A U1 AAT1155-3.3 FB VP L1 1.5H
+ -
C4 100F
R1 100
R2 100k
GND LX EN C1 10F C3 0.1F LX
VCC VP
D1 B340LA
C2 120F
rtn
C1 Murata 10F 6.3V X5R GRM42-6X5R106K6.3 C2 Vishay120F 6.3V 594D127X96R6R3C2T C3 0. F 0603ZD104M AVX 1 C4 Vishay Sprague 100F 16V 595D107X0016C 100F 16V D1 B340LA Diodes Inc. L1 CDRH6D28-1.5H Sumida Options C2 MuRata 100F 6.3V GRM43-2 X5R 107M 100F 6.3V (two or three in parallel) C2 TDK 100F 6.3V C3325X5R0J107M 100F 6.3V (two or three in parallel)
Figure 2: 3.3V, 3A Output Efficiency.
1155.2006.09.1.7
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1MHz 2.5A Step-Down DC/DC Converter
A high ESR tantalum capacitor with a value about 10 times the input ceramic capacitor may also be required when using a 10F or smaller ceramic input bypass capacitor. This dampens any input oscillations that may occur due to the source inductance resonating with the converter input impedance. Due to the ESR zero associated with the tantalum capacitor, smaller values than those required with ceramic capacitors provide more phase margin with a greater loop crossover frequency.
AAT1155
Layout
Figures 3 and 4 display the suggested PCB layout for the AAT1155. The following guidelines should be used to help ensure a proper layout. 1. The connection from the input capacitor to the Schottky anode should be as short as possible. 2. The input capacitor should connect as closely as possible to VP (Pins 5 and 8) and GND (Pin 2). 3. C1, L1, and CR1 should be connected as closely as possible. The connection from the cathode of the Schottky to the LX node should be as short as possible. 4. The feedback trace (Pin 1) should be separate from any power trace and connect as closely as possible to the load point. Sensing along a high-current load trace can degrade DC load regulation. 5. The resistance of the trace from the load return to GND (Pin 2) 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 reference ground and the load return. 6. R1 and C3 are required in order to provide a cleaner power source for the AAT1155 control circuitry.
Output Capacitor
With no external compensation components, the output capacitor has a strong effect on the loop stability. Larger output capacitance will reduce the crossover frequency with greater phase margin. A 200F ceramic capacitor provides sufficient bulk capacitance to stabilize the output during large load transitions and has ESR and ESL characteristics necessary for very low output ripple. The RMS ripple current is given by:
IRMS = (V = VF) * (VIN - VOUT) 1 * OUT L * FS * VIN 2* 3
For a ceramic output capacitor, the dissipation due to the RMS current and associated output ripple are negligible. Tantalum capacitors with sufficiently low ESR to meet output ripple requirements generally have an RMS current rating much greater than that actually seen in this application. The maximum tantalum output capacitor ESR is:
ESR
VRIPPLE I
where I is the peak-to-peak inductor ripple current.
Figure 3: Evaluation Board Top Side. 12
Figure 4: Evaluation Board Bottom Side.
1155.2006.09.1.7
1MHz 2.5A Step-Down DC/DC Converter
Thermal
Losses associated with the AAT1155 output switching MOSFET are due to switching losses and conduction losses. The conduction losses are associated with the RDS(ON) characteristics of the output switching device. At the full load condition, assuming continuous conduction mode (CCM), an accurate calculation of the RDS(ON) losses can be derived from the following equations:
P = I RMS2 * RDS(ON) ON
AAT1155
Design Example
(see Figures 2 and 5 for reference) IOUT IRIPPLE VOUT VIN FS TMAX 2.5A 40% of Full Load at Max VIN 2.5V 5V 5% 1MHz 70C
RDS(ON) losses
Inductor Selection
I 2 *D IRMS = I O2 + 12
Internal switch RMS current D is the duty cycle and VF is the forward voltage drop of the Schottky diode.
L= =
VOUT VOUT * 1I O * k * FS VIN 3.3V 3.3V * 1= 1.23H 2.5A * 0.4 * 1MHz 5.25V
Use standard value of 1.5H Sumida inductor Series CDRH6D38.
D=
VO + VF VIN + VF
I = =
I is the peak-to-peak inductor ripple current. A simplified form of calculating the RDS(ON) and switching losses is given by:
IO2 * RDS(ON) VO P= + tSW * FS * IO + IQ * VIN VIN
VO V 1- O L * FS VIN 3.3V 3.3V = 0.82A 11.5H * 1MHz 5.25V
I 2
I PK = IOUT +
= 2.5A + 0.41 = 2.91A
Efficiency vs. Load Current
(VIN = 5.0V; VOUT = 3.3V)
100 95
where IQ is the AAT1155 quiescent current. Once the total losses have been determined, the junction temperature can be derived. The thermal resistance (JA) for the MSOP-8 package mounted on an FR4 printed circuit board in still air is 150C/W. TJ = P JA + TAMB TAMB is the maximum ambient temperature and TJ is the resultant maximum junction temperature.
Efficiency (%)
90 85 80 75 70 65 60 0.01 0.1 1 10
Output Current (A)
Figure 5: 5V Input, 3.3V Output.
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1MHz 2.5A Step-Down DC/DC Converter
AAT1155 Junction Temperature
P= ON = IO2 * RDS(ON) * VO tSW * FS * IO + IQ * VIN + 2 VIN 2.52 * 70m * 3.3V 20ns * 1MHz * 2.5A + + 690 A * 5V 2
AAT1155
The 120F Vishay 594D tantalum capacitor has an ESR of 85m and a ripple current rating of 1.48Arms in a C case size. Although smaller case sizes are sufficiently rated for this ripple current, their ESR level would result in excessive output ripple. The ESR requirement for a tantalum capacitor can be estimated by :
0.42 Watts
TJ(MAX) = TAMB + JA * P = 70C + 150C/W * 0.42W = 133C
ESR
IRMS =
VRIPPLE 100 mV = = 111m I 0.9A
1 *
(VOUT + VF) * (VIN + VOUT) L * FS * VIN 2* 3
1 3.65V *1.7 V * = 240mArms 2 * 3 1.5H * 1MHz * 5V
Diode
V IDIODE = IO * 1 - O VIN 3.3V = 0.93A = 2.5A * 1 5.25V
VFW = 0.35V
=
PDIODE = VFW * IDIODE
= 0.35V * 0.93A = 0.33A
Given an ambient thermal resistance of 120C/W from the manufacturer's datasheet, TJ(MAX) of the diode is:
Two or three 1812 X5R 100F 6.3V ceramic capacitors in parallel also provide sufficient phase margin. The low ESR and ESL associated with ceramic capacitors also reduces output ripple significantly over that seen with tantalum capacitors. Temperature rise due to ESR ripple current dissipation is also reduced.
Input Capacitor
The input capacitor ripple is:
T J(MAX) = T AMB + JA * P = 70C + 120C / W * 0.33W = 109C
IRMS = I O *
V V O * 1 - O = 1.82Arms VIN VIN
Output Capacitor
The output capacitor value required for sufficient loop phase margin depends on the type of capacitor selected. For a low ESR ceramic capacitor, a minimum value of 200F is required. For a low ESR tantalum capacitor, lower values are acceptable. While the relatively higher ESR associated with the tantalum capacitor will give more phase margin and a more dampened transient response, the output voltage ripple will be higher.
In the examples shown, C1 is a ceramic capacitor located as closely to the IC as possible. C1 provides the low impedance path for the sharp edges associated with the input current. C4 may or may not be required, depending upon the impedance characteristics looking back into the source. It serves to dampen any input oscillations that may arise from a source that is highly inductive. For most applications, where the source has sufficient bulk capacitance and is fed directly to the AAT1155 through large PCB traces or planes, it is not required. When operating the AAT1155 evaluation board on the bench, C4 is required due to the inductance of the wires running from the laboratory power supply to the evaluation board.
14
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1MHz 2.5A Step-Down DC/DC Converter
Adjustable Output
For applications requiring an output other than the fixed outputs available, the 1V version can be programmed externally. Resistors R3 and R4 of Figure 1 force the output to regulate higher than 1 volt. For accurate results (less than 1% error for all outputs), select R4 to be 10k. Once R4 has been selected, R3 can be calculated. For a 1.25 volt output with R4 set to 10k, R3 is 2.5k. R3 = (VO - 1) * R4 = 0.25 * 10k = 2.5k
AAT1155
Capacitors Part Number
C4532X5ROJ107M GRM43-2 X5R 107M 6.3 GRM43-2 X5R 476K 6.3 GRM42-6 X5R 106K 6.3 594D127X_6R3C2T 595D107X0016C
Manufacturer
TDK MuRata MuRata MuRata Vishay Vishay
Capacitance (F)
100 100 47 10 120 100
Voltage (V)
6.3 6.3 6.3 6.3 6.3 16
Temp Co.
X5R X5R X5R X5R
Case
1812 1812 1812 1206 C C
Inductors Part Number
CDRH6D38-4763-T055 N05D B1R5M NP06DB B1R5M LQH55DN1R5M03 LQH66SN1R5M03
Manufacturer
Sumida Taiyo Yuden Taiyo Yuden MuRata MuRata
Inductance (H)
1.5 1.5 1.5 1.5 1.5
I (Amps)
4.0 3.2 3.0 3.7 3.8
DCR ()
0.014 0.025 0.022 0.022 0.016
Height (mm)
4.0 2.8 3.2 4.7 4.7
Type
Shielded Non-Shielded Shielded Non-Shielded Shielded
Diodes Manufacturer
Diodes Inc. ROHM Micro Semi
Part Number
B340LA RB050L-40 5820SM
VF
0.45V @ 3A 0.45 @ 3A 0.46V @ 3A
1155.2006.09.1.7
15
1MHz 2.5A Step-Down DC/DC Converter Ordering Information
Output Voltage
1.0V (Adj. VOUT 1.0V) 1.8V 2.5V 3.3V
AAT1155
Package
MSOP-8 MSOP-8 MSOP-8 MSOP-8
Marking1
KXXYY KYXYY ILXYY IKXYY
Part Number (Tape and Reel)2
AAT1155IKS-1.0-T1 AAT1155IKS-1.8-T1 AAT1155IKS-2.5-T1 AAT1155IKS-3.3-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/pbfree.
Package Information
MSOP-8
4 4 1.95 BSC
3.00 0.10
4.90 0.10
0.60 0.20 PIN 1 0.254 BSC
0.95 REF
3.00 0.10 10 5
0.95 0.15 0.85 0.10
0.075 0.075 0.65 BSC 0.30 0.08
All dimensions in millimeters.
1. XYY = assembly and date code. 2. Sample stock is generally held on part numbers listed in BOLD.
16
GAUGE PLANE
0.155 0.075
1155.2006.09.1.7
1MHz 2.5A Step-Down DC/DC Converter
AAT1155
(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. Customers are advised to obtain the latest version of relevant information to verify, before placing orders, that information being relied on is current and complete. All products are sold subject to the terms and conditions of sale supplied at the time of order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability. AnalogicTech warrants performance of its semiconductor products to the specifications applicable at the time of sale in accordance with AnalogicTech's standard warranty. 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.
Advanced Analogic Technologies, Inc.
830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737-4600 Fax (408) 737-4611
1155.2006.09.1.7
17

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