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850kHz 1A Synchronous Buck DC/DC Converter General Description
The AAT1152 SwitchRegTM is a member of AnalogicTechTM's Total Power ManagementTM IC product family. The Step-down switching converter is ideal for applications where high efficiency, small size, and low ripple are critical. Able to deliver 1A with internal Power MOSFETs, the current-mode controlled IC provides high efficiency using synchronous rectification. Fully internally compensated, the AAT1152 simplifies system design and lowers external part count. The AAT1152 features a Power Good (POK) function which monitors the output, alerting the system if the output voltage falls out of regulation. The AAT1152 is available in MSOP-8 package, rated over -40 to 85C.
AAT1152
Features
* * * * * * * * * * * * * * * * *
SwitchRegTM
5.5V max supply input Fixed output voltage: 1.1V-4.2V with 100 mV increment 1A output current Integrated low on resistance power switches Synchronous rectification Up to 95% efficiency Power Good signal Internally compensated current mode control High initial accuracy: 1% 850kHz switching frequency Fixed or adjustable VOUT 1.0 to 4.2V Constant PWM mode Low output ripple with light load Internal softstart Current limit protection Over-Temperature protection MSOP-8 package
Applications
* * * * * Computer Peripherals Set Top Boxes Network Cards Cable/DSL Modems High efficiency conversion from 5V or 3.3V supply
Typical Application
INPUT
100k
10F
VP
AAT1152
POK
FB
4.1H LX
ENABLE 100 VCC OUTPUT SGND 0.1F PGND 2x 22F
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850kHz 1A Synchronous Buck DC/DC Converter Pin Descriptions
Pin #
1 2 3 4 5 6 7 8
AAT1152
Symbol
FB SGND EN VCC VP LX POK PGND
Function
Feedback input pin Signal Ground Converter enable pin Small Signal Filtered Bias Supply Input supply for converter power stage Inductor connection pin Power Good indicator. Open-drain output is low when VOUT falls out of regulation. Power ground return for output stage
Pin Configuration
MSOP-8
FB SGND EN VCC
1
8
PGND POK LX VP
1
2 2
7
3
6
4
5
2
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850kHz 1A Synchronous Buck DC/DC Converter Absolute Maximum Ratings
Symbol
VCC, VP VLX VFB VEN, VPOK TJ TLEAD VESD
AAT1152
(TA=25C unless otherwise noted) Value
6 -0.3 to VP+0.3 -0.3 to VCC+0.3 -0.3 to 6 -40 to 150 300 3000
Description
VCC, VP to GND LX to GND FB to GND POK, EN to GND Operating Junction Temperature Range Maximum Soldering Temperature (at leads, 10 sec) ESD Rating 1 - HBM
Units
V V V V C C V
Note: 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. Note 1: Human body model is a 100pF capacitor discharged through a 1.5K resistor into each pin.
Thermal Characteristics
Symbol
JA PD
Description
Maximum Thermal Resistance (MSOP-8) Maximum Power Dissipation (MSOP-8) 2
2
Value
150 833
Units
C/W mW
Note 2: Mounted on a demo board.
Recommended Operating Conditions
Symbol
T
Description
Ambient Temperature Range
Rating
-40 to +85
Units
C
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850kHz 1A Synchronous Buck DC/DC Converter Electrical Characteristics
values are at TA = 25C) Symbol
VIN VOUT ILIM IQ VOUT (VOUT*VIN) VOUT/VOUT FOSC RDSON(H) RDSON(L) VEN(H) VEN(L) IEN VUVLO VUVLO(hys) TSD THYS ISHDN VTH(POK) RPOK
AAT1152
(VIN = VCC = VP = 5V, TA= -40 to 85C unless otherwise noted. Typical
Description
Operation Voltage DC Output Voltage Tolerance Current Limit Quiescent Supply Current Load Regulation Line Regulation Oscillator frequency High-side Switch On-resistance Low-side Switch On-resistance Enable input high voltage Enable input low voltage Enable Pin Leakage Current Undervoltage Lockout Undervoltage Lockout Hysteresis Over Temp Shutdown Threshold Over Temp Shutdown Hysteresis Shutdown current Power Good Threshold Power Good Pull-Down On-Resistance
Conditions
IOUT = 500mA TA = 25C Full temp
Min
2.7 -1.0 -2.0 1.2
Typ
Max
5.5 +1.0 +2.0
Units
V % A A % %/V kHz m m V V A V mV C C A % of VFB
TA = 25C No load, VFB = 0 VIN = 4.2V, ILOAD = 0 to 1A VIN = 2.7 to 5.5V TA = 25C TA = 25C TA = 25C VIN = 2.7 to 5.5V VIN = 2.7 to 5.5V VEN = 5.5V VIN rising VIN falling
700
160 3 0.2 850 110 100
300
1000 150 150 0.6 1 2.5
1.4
1.2 250 140 15
VEN = 0, VIN = 5.5V VFB Ramping Up VFB Ramping Down
1 90 88 4
4
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850kHz 1A Synchronous Buck DC/DC Converter Typical Characteristics
High Side RDS(ON) vs. Temperature
170 150 170
AAT1152
Low Side RDS(ON) vs. Temperature
3.6V 2.7V RDS(ON) (m)
150 130 110 90 70 -20
RDS(ON) (m)
130 110 90 70 -20
3.6V 2.7V 5.5V 4.2V
0 20 40 60 80 100 120
4.2V
5.5V
0
20
40
60
80
100
120
Temperature (C)
Temperature (C)
RDS(ON) vs. Input Voltage
130 1.2 120
Enable Threshold vs. Input Voltage
Enable Threshold (V)
High Side RDS(ON) (m)
110 100
1.1
VEN(H)
1
0.9 0.8
90 80 2.5 3 3.5 4
Low Side
VEN(L)
4.5
5
5.5
0.7 2.5 3 3.5 4 4.5 5 5.5
Input Voltage (V)
Input Voltage (V)
Oscillator Frequency Variation vs. Supply Voltage
3.5 2.5
Oscillator Frequency Variation vs. Temperature VIN=3.6V
10 6
Variation (%)
1.5 0.5 -0.5 -1.5 2.5 3 3.5 4 4.5 5 5.5
Variation (%)
2 -2 -6 -10 -20
0
20
40
60
80
100
Supply Voltage (V)
Temperature (C)
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850kHz 1A Synchronous Buck DC/DC Converter Typical Characteristics
Output Voltage vs. Temperature IOUT=900mA, VOUT=1.5V
Output Voltage Error (%)
1.0 0.6 0.2 -0.2 -0.6 -1.0 -20
AAT1152
Line Regulation VOUT=1.5V
0.25 0.15
VIN = 2.7V
Accuracy (%)
IOUT = 1.0A
0.05 -0.05 -0.15 -0.25
VIN = 3.6V
IOUT = 0.4A
0
20
40
60
80
100
2.5
3
3.5
4
4.5
5
5.5
Temperature (C)
Input Voltage (V)
Load Regulation VOUT= 1.5V, VIN=3.6V
0 -1 0 -1
Load Regulation VOUT=3.3V, VIN=5.0V
VOUT Error (%)
Error (%)
-2 -3 -4
-2 -3 -4 -5
-5 0 150 300 450 600 750 900
0
150
300
450
600
750
900
1050
IOUT (mA)
Output Current (A)
Efficiency vs. Input Voltage VOUT=1.5V
100
Loop Gain and Phase vs. Output Capacitor VIN = 3.6V, IOUT = 0.3A, CO = 22F
40 225 180 135 32 24
16
IO = 1A
90
Efficiency (%)
Gain (dB)
80 70 60 50 2.5 3 3.5
IO = 0.4A
4x
Phase
3x 2x
Phase (degrees)
90 45 0 -45 -90 -135
8 0 -8
-16 2x
-24 -32 -40
4 4.5 5 5.5
Gain
3x 4x
-180 -225 1000
10
100
Input Voltage (V)
Frequency (kHz)
6
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850kHz 1A Synchronous Buck DC/DC Converter Typical Characteristics
No Load Input Current vs. Temperature VCC = VP
12
AAT1152
Non-Switching IQ vs. Temperature FB = 0V, VP = VCC
Operating Current (A)
200 190 180 170 160 150 140 130 120 110 100 -20 -5
VCC = 5.5V
VCC = 5.0V
Input Current (mA)
10 8 6 4 2 0 -20 -5 10 25 40 55 70 85
VCC = 5.5V VCC = 5.0V VCC = 4.2V VCC = 2.7V
10 25
VCC = 4.2V
VCC = 3.6V
VCC = 2.7V
VCC = 3.6V
40
55
70
85
Temperature (C)
Temperature (C)
Switching Waveform
VOUT 50mV/div V(LX) 2V/div Inductor Current 500mA/div
Transient Response
IL 500mA/div
VIN=3.6V VOUT=1.5V IOUT=1.2A
VIN=3.6V VOUT=1.5V ILOAD=0.25 to 1.2A 500nsec/div 20s/div
Output Ripple 1.5V, No Load
VOUT 5mV/div BW=20MHz VIN=3.6V VOUT=1.5V IOUT=0A LX 2V/div 500nsec/div VOUT 5mV/div BW=20MHz VIN=3.6V VOUT=1.5V IOUT=1A LX 2V/div
Output Ripple 1.5V, 1A Load
500nsec/div
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850kHz 1A Synchronous Buck DC/DC Converter Typical Characteristics
Output Ripple 3.3V, No Load
VOUT 5mV/div BW=20MHz VIN=5.0V VOUT=3.3V IOUT=0A LX 2V/div 500nsec/div VOUT 5mV/div BW=20MHz VIN=5.0V VOUT=3.3V IOUT=1A LX 2V/div 500nsec/div
AAT1152
Output Ripple 3.3V, 1A Load
8
1152.2004.08.1.2
850kHz 1A Synchronous Buck DC/DC Converter Functional Block Diagram
VCC VP= 2.7V- 5.5V
AAT1152
1.0V REF
FB
OP. AMP
CMP
DH
LOGIC OSC
LX
DL Temp. Sensing Power Good
SGND
POK
EN
PGND
Applications Information
850 kHz 1 Amp DC-DC Synchronous Buck Converter Control Loop
The AAT1152 is a peak current mode buck converter. The inner, wide bandwidth loop controls the peak current of the output inductor. The output inductor current is sensed through the P-Channel MOSFET (high side) and is also used for short circuit and overload protection. A fixed slope compensation signal is added to the sensed current to maintain stability. The 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 does not have a large DC gain typical of most error amplifiers. This eliminates the need for external compensation components while still providing sufficient DC loop gain for load regulation. The crossover frequency and phase margin are set by the output capacitor value only.
Soft-Start/Enable
Soft start increases the inductor current limit point in discrete steps when the input voltage or enable input is applied. It limits the current surge seen at the input and eliminates output voltage overshoot. The enable input, when pulled low, forces the AAT1152 into a low power non-switching state. The total input current during shutdown is less that 1A.
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850kHz 1A Synchronous Buck DC/DC Converter
AAT1152
Enable 2V/div
VOUT 1V/div IL 0.5A/div VIN=3.6V VOUT=1.5V IL=1A
200sec/div
Figure 1: Inrush Limit
Power and Signal Source
Separate small signal ground and power supply pins isolate the internal control circuitry from the noise associated with the output MOSFET switching. The low pass filter R1 and C3 in schematic figures 3 and 4 filters the noise associated with the power switching.
Current Limit and Over-temperature protection
For overload conditions the peak input current is limited. Figure 2 displays the VI current limit characteristics. As load impedance decreases and the output voltage falls closer to zero, more power is dissipated internally, raising the device temperature. Thermal protection completely disables switching when internal dissipation becomes excessive, protecting the device from damage. The junction over-temperature threshold is 140C with 15C of hysteresis.
Current Limit Characteristic
3.5 3 2.5
VCC =VP = 5.0V VO = 3.3V
VOUT (V)
2 1.5 1 0.5 0 0 0.5 1 1.5 2 2.5
VCC =VP =3.6V VO = 1.5V
IOUT (A)
Figure 2.
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850kHz 1A Synchronous Buck DC/DC Converter
Power Good
The AAT 1152 features an integrated Power Good (POK) comparator and open-drain output signal. The POK pin goes low when the converter's output is 12% or more below its nominal regulation voltage or when the device is in shutdown. Connect a pull-up resistor from POK to the converter's input or output. Typical resistor pull-up values range from 100k to 10k.
AAT1152
The factor "k" is the fraction of full load selected for the ripple current at the maximum input voltage. The corresponding inductor rms current is: IRMS = 2 I2 Io = 1.0A I+ o 12
Inductor
The output inductor is selected to limit the ripple current to some predetermined value, typically 2040% 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 saturation characteristics. The inductor should not show any appreciable saturation under all normal load conditions. During over load and short circuit conditions, the average current in the inductor can meet or exceed the ILIMIT point of the AAT1152 without effecting the converter performance. Some inductors may have sufficient peak and average current ratings yet result in excessive losses due to a high DCR. Always consider the losses associated with the DCR and its effect on the total converter efficiency when selecting an inductor. For a 1 Amp load and the ripple set to 30% at the maximum input voltage, the maximum peak to peak ripple current is 300 mA. The inductance value required is 3.9H. V VOUT 1 - OUT IO k F VIN 1.5V 1.5V 11.0A 0.3 830kHz 4.2V
I is the peak to peak ripple current which is fixed by the inductor selection above. For a peak to peak current of 30% of the full load current the peak current at full load will be 115% of the full load. The 4.1H inductor selected from the Sumida CDRH5D18 series has a 57 m DCR and a 1.95 Amp DC current rating. At full load the inductor DC loss is 57mW which amounts to a 3.8% loss in efficiency.
Input Capacitor
The primary function of the input capacitor is to provide a low impedance loop for the edges of pulsed current drawn by the AAT1152. A low ESR/ESL ceramic capacitor is ideal for this function. To minimize the stray inductance the capacitor should be placed as close as possible to the IC. This keeps the high frequency content of the input current localized, minimizing radiated and conducted EMI while facilitating optimum performance of the AAT1152. Ceramic X5R or X7R capacitors are ideal for this function. The size required will vary depending on the load, output voltage and input voltage source impedance characteristics. A typical value is around 10F. The input capacitor RMS current varies with the input voltage and the output voltage. The equation for the RMS current in the input capacitor is:
L= L=
IRMS = IO
VO VO 1VIN VIN
L = 3.9H
The input capacitor RMS ripple current reaches a maximum when VIN is two times the output voltage where it is approximately one half of the load current. Losses associated with the input ceramic capacitor are typically minimal and not an issue. The proper placement of the input capacitor can be seen in the reference design layout in figures 5 and 6.
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850kHz 1A Synchronous Buck DC/DC Converter
R5 100k U1 AAT1152-1.0 R1 100 R2 C1 10F 100k C3 0.1F EN Vp Vcc EN FB Pok LX LX R4 10k 1% L1 2.7H C2, C4 2x 22F
AAT1152
Pok
Vin+ 3.3V
R3 2.55k 1% Vo+ 1.25V1A
Sgnd Pgnd
VC1 Murata 10F 6.3V X5R GRM42-6X 5R106K6.3 C2, C4 MuRata 22F 6.3V GRM21BR60J226ME39L X5R 0805 L1 Sumida CDRH4D28-2R 7H
Figure 3: 3.3V to 1.25V converter
Output Capacitor
Since there are no external compensation components, the output capacitor has a strong effect on loop stability. Larger output capacitance will reduce the crossover frequency with greater phase margin. For the 1.5V 1A design using the 4.1 H inductor, two 22F capacitors provide a stable output. In addition to assisting stability, the output capacitor limits the output ripple and provides holdup during large load transitions.
The output capacitor RMS ripple current is given by:
1 2 3 VOUT (VIN - VOUT) L F VIN
IRMS =
For a ceramic capacitor the dissipation due to the RMS current of the capacitor is not a concern. Tantalum capacitors, with sufficiently low ESR to meet output voltage ripple requirements, also have an RMS current rating much greater than that actually seen in this application.
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1152.2004.08.1.2
850kHz 1A Synchronous Buck DC/DC Converter
R5 100k U1 AAT1152-1.5 Vp EN Vcc EN FB Pok LX LX C2, C4 2x 22F L1 4.1H Pok
AAT1152
Vin+ 2.7V-5.5V R1 100 R2 C1 10F 100k C3 0.1F
Vo+ 1.5V 1A
Sgnd Pgnd
VC1 Murata 10F 6.3V X5R GRM42-6X5R106K6.3 C2, C4 MuRata 22F 6.3V GRM21BR60J226ME39L X5R 0805 L1 Sumida CDRH5D 18-4R 1H
1.5V Efficiency vs. IOUT
100 80
2.7V
Efficiency (%)
60
4.2V
40 20 0 10 100 1000
3.6V
Iout (mA)
Figure 4: Lithium-Ion to 1.5V Output Converter
Figure 5: AAT1152 Layout Top Layer
Figure 6: AAT1152 Layout Bottom Layer
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850kHz 1A Synchronous Buck 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 3 force the output to regulate higher than 1 Volt. R4 should be 100 times less than the internal 1 MegOhm resistance of the FB pin. Once R4 is selected R3 can be calculated. For a 1.25V output with R4 set to 10.0k, R3 is 2.55k.
AAT1152
Layout Considerations
Figures 5 and 6 display the suggested PCB layout for the AAT1152. The most critical aspect of the layout is the placement of the input capacitor C1. For proper operation C1 must be placed as close as possible to the AAT1152.
Thermal Calculations
There are two types of losses associated with the AAT1152 output switching MOSFET, switching losses and conduction losses. The conduction losses are associated with the Rds(on) characteristics of the output switching device. At full load, assuming continuous conduction mode (CCM), a simplified form of the total losses is:
R3 = (VO - 1) R4 = 0.25 10.0k = 2.55k
PLOSS =
IO2 (RDSON(H) VO + RDSON(L) (VIN - VO)) + tsw F IO VIN + IQ VIN VIN
Once the total losses have been determined the junction temperature can be derived from the JA for the MSOP-8 package.
Design Example
Specifications IOUT = 1.0A IRIPPLE = 30% of full load at max VIN VOUT = 1.5V VIN = 2.7 - 4.2 V (3.6V nominal) Fs = 830 kHz
Maximum Input Capacitor Ripple:
IRMS = IO VO VO IO 1= = 0.5ARMS, VIN = 2 x VO VIN VIN 2
P = ESRCOUT IRMS2 = 5m 0.52 A = 1.25mW
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850kHz 1A Synchronous Buck DC/DC Converter
Inductor Selection:
L=
V VOUT 1.5V 1.5V 1 - OUT = 1= 3.9H IO k F VIN 1.0A 0.3 830kHz 4.2V
AAT1152
Select Sumida inductor CDRH5D18 4.1H 57m 2.0 mm height.
I =
1.5V VO V 1.5V 1- O = 1= 280mA L F VIN 4.1H 830kHz 4.2V
IPK = IOUT +
I = 1.0A + 0.14A = 1.14A 2
P = IO2 DCR =57mW
Output Capacitor Dissipation:
IRMS = VOUT (VIN - VOUT) 1.5V (4.2V - 1.5V) 1 1 = =82mARMS L F VIN 2 3 2 3 4.1H 830kHz 4.2V
PESR = ESRCOUT IRMS2 = 5m .0822 A = 33W
AAT1152 Dissipation:
P= IO2 * (RDSON(H) * VO + RDSON(L) * (VIN -VO)) VIN + (tsw * F * IO + IQ) * VIN
=
(0.14 * 1.5V + 0.145 * (3.6V - 1.5V)) 3.6V
+ (20nsec * 830kHz * 1.0A + 0.3mA) * 3.6V = 0.203W
TJ(MAX) = TAMB + JA * PLOSS = 85C + 150C/W * 0.203W = 115C
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850kHz 1A Synchronous Buck DC/DC Converter
Table 1: Surface Mount Inductors
Manufacturer
TaiyoYuden Toko Sumida Sumida MuRata MuRata
AAT1152
Part Number
NPO5DB4R7M A914BYW-3R5M-D52LC CDRH5D28-4R2 CDRH5D18-4R1 LQH55DN4R7M03 LQH66SN4R7M03
Value
4.7H 3.5H 4.2H 4.1H 4.7H 4.7H
Max DC Current
1.4A 1.34A 2.2A 1.95A 2.7A 2.2A
DCR
.038 .073 .031 .057 .041 .025
Size (mm) LxWxH 5.9 x 6.1 x 2.8 5.0 x 5.0 x 2.0 5.7 x 5.7 x 3.0 5.7 x 5.7 x 2.0 5.0 x 5.0 x 4.7 6.3 x 6.3 x 4.7
Type
Shielded Shielded Shielded Shielded Non-shielded Shielded
Table 2: Surface Mount Capacitors
Manufacturer
MuRata MuRata MuRata MuRata
Part Number
GRM40 X5R 106K 6.3 GRM42-6 X5R 106K 6.3 GRM21BR60J226ME39L GRM21BR60J106ME39L
Value
10F 10F 22F 10F
Voltage
6.3V 6.3V 6.3V 6.3V
Temp. Co.
X5R X5R X5R X5R
Case
0805 1206 0805 0805
16
1152.2004.08.1.2
850kHz 1A Synchronous Buck DC/DC Converter Ordering Information
Output Voltage 1.0V (Adj. VOUT 1.0V) 1.8V 2.5V 3.3V Package MSOP-8 MSOP-8 MSOP-8 MSOP-8 Marking1 LTXYY MLXYY MMXYY IAXYY Part Number (Tape and Reel) AAT1152IKS-1.0-T1 AAT1152IKS-1.8-T1 AAT1152IKS-2.5-T1 AAT1152IKS-3.3-T1
AAT1152
Note: Sample stock is held on part numbers listed in bold. Note 1: XYY = assembly and date code.
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.
GAUGE PLANE
0.155 0.075
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850kHz 1A Synchronous Buck DC/DC Converter
AAT1152
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, and advise customers 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.
Advanced Analogic Technologies, Inc.
830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737-4600 Fax (408) 737-4611 18
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