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1.5A, 1.4MHz Step-Down Converter General Description
The AAT1112 SwitchReg is a 1.5A step-down converter with an input voltage range of 2.4V to 5.5V and an adjustable output voltage from 0.6V to VIN. The 1.4MHz switching frequency enables the use of small external components. The small footprint and high efficiency make the AAT1112 an ideal choice for portable applications. The AAT1112 delivers 1.5A maximum output current while consuming only 42A of no-load quiescent current. Ultra-low RDS(ON) integrated MOSFETs and 100% duty cycle operation make the AAT1112 an ideal choice for high output voltage, high current applications which require a low dropout threshold. The AAT1112 provides excellent transient response and high output accuracy across the operating range. No external compensation components are required. The AAT1112 is designed to maintain high efficiency throughout the load range. Pulling the MODE/ SYNC pin high enables "PWM Only" mode, maintaining constant frequency and low output ripple across the operating range. Alternatively, the converter may be synchronized to an external clock input via the MODE/SYNC pin. Over-temperature and short-circuit protection safeguard the AAT1112 and system components from damage. The AAT1112 is available in a Pb-free, space-saving TDFN33-12 or 2.75x3mm TSOPJW-12 package. The product is rated over an operating temperature range of -40C to +85C.
AAT1112
Features
* * * * * * * * * * * * * * *
SwitchRegTM
1.5A Maximum Output Current Input Voltage: 2.4V to 5.5V Output Voltage: 0.6V to VIN Up to 95% Efficiency 42A No Load Quiescent Current No External Compensation Required 1.4MHz Switching Frequency Synchronizable to External Clock Optional "PWM Only" Low Noise Mode 100% Duty Cycle Low-Dropout Operation Internal Soft Start Over-Temperature and Current Limit Protection <1A Shutdown Current TSOPJW-12 or TDFN33-12 Package Temperature Range: -40C to +85C
Applications
* * * * * * * Cellular Phones Digital Cameras Hard Disk Drives MP3 Players PDAs and Handheld Computers Portable Media Players USB Devices
Typical Application
VIN
VP LX
L1 3.3H
VOUT = 3.3V
AAT1112
VIN
R1 267k
C2 10F
EN MODE/SYNC GND
FB PGND
R2 59k
C1 22F
1112.2007.01.1.1
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1.5A, 1.4MHz Step-Down Converter Pin Descriptions
Pin #
TSOPJW-12 TDFN33-12
AAT1112
Symbol
LX
Function
Switching node. Connect the output inductor to this pin. The switching node is internally connected to the drain of both high- and low-side MOSFETs. Input voltage for the power switches. Not connected. Connect to ground for PFM/PWM mode and optimized efficiency throughout the load range. Connect high for low noise PWM operation under all operating conditions. Connect to an external clock for synchronization (PWM only). Enable pin. A logic low disables the converter and it consumes less than 1A of current. When connected high, it resumes normal operation. Power supply. Supplies power for the internal circuitry. Feedback input pin. This pin is connected either directly to the converter output or to an external resistive divider for an adjustable output. Non-power signal ground pin. Main power ground return pin. Connect to the output and input capacitor return. Exposed paddle (bottom); connect to ground as closely as possible to the device.
1
12
2 3 4
11 10 9
VP N/C MODE/SYNC
5
8
EN
6 7
7 6
VIN FB
8, 9, 10, 11 12 N/A
4, 5 1, 2, 3 EP
GND PGND
Pin Configuration
TSOPJW-12 (Top View) TDFN33-12 (Top View)
LX VP N/C MODE/SYNC EN VIN
1 2 3 4 5 6
12 11 10 9 8 7
PGND GND GND GND GND FB
PGND PGND PGND GND GND FB
1 2 3 4 5 6
12 11 10 9 8 7
LX VP N/C MODE/SYNC EN VIN
2
1112.2007.01.1.1
1.5A, 1.4MHz Step-Down Converter Absolute Maximum Ratings1
Symbol
VIN VLX VFB VN TJ TLEAD
AAT1112
Description
VIN, VP to GND LX Pin to GND FB Pin to GND MODE/SYNC, EN to GND Operating Junction Temperature Range Maximum Soldering Temperature (at leads, 10 sec)
Value
6.0 -0.3 to VIN + 0.3 -0.3 to VIN + 0.3 -0.3 to 6.0 -40 to 150 300
Units
V V V V C C
Thermal Information
Symbol
PD JA
Description
Maximum Power Dissipation Thermal Resistance2 TSOPJW-12 TDFN33-12 TSOPJW-12 TDFN33-12
Value
0.625 2.0 160 50
Units
W C/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. 1112.2007.01.1.1
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1.5A, 1.4MHz Step-Down Converter Electrical Characteristics1
VIN = 3.6V; TA = -40C to +85C, unless otherwise noted. Typical values are TA = 25C. Symbol
VIN VOUT VUVLO VOUT IQ ISHDN ILIM RDS(ON)H RDS(ON)L ILXLEAK ILXLK, R VLOADREG VLINEREG/VIN VFB IFB FOSC TS TSD THYS EN VIL VIH IEN MODE/SYNC VMODE/SYNC(L) VMODE/SYNC(H) IMODE/SYNC Enable Threshold Low Enable Threshold High Enable Leakage Current Enable Threshold Low Enable Threshold High Enable Leakage Current 0.6 VIN = VEN = 5.5V 1.4 -1.0 1.0 0.6 VIN = VEN = 5.5V 1.4 -1.0 1.0 V V A V V A
AAT1112
Description
Input Voltage Output Voltage Range UVLO Threshold Output Voltage Tolerance Quiescent Current Shutdown Current Current Limit High Side Switch On-Resistance Low Side Switch On-Resistance LX Leakage Current LX Reverse Leakage Current Load Regulation Line Regulation Feedback Threshold Voltage Accuracy (Adjustable Version) FB Leakage Current Internal Oscillator Frequency Synchronous Clock Start-Up Time Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis
Conditions
Min
2.4 0.6
Typ
Max Units
5.5 VIN 2.4 V V V mV V % A A A A A % %/V 0.609 0.2 1.68 3.0 V A MHz s C C
VIN Rising Hysteresis VIN Falling IOUT = 0A to 1.5A, VIN = 2.4V to 5.5V No Load VEN = GND
250 1.8 -3.0 42 1.8 0.120 0.085 3.0 90 1.0
VIN = 5.5V, VLX = 0 to VIN VIN Unconnected, VLX = 5.5V, VEN = GND ILOAD = 0A to 1.5A VIN = 2.4V to 5.5V No Load, TA = 25C VOUT = 1.0V 1.12 0.60 From Enable to Output Regulation 0.591
1.0 1.0 0.5 0.2 0.60 1.4 150 140 15
1. The AAT1112 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.
4
1112.2007.01.1.1
1.5A, 1.4MHz Step-Down Converter Typical Characteristics
Efficiency vs. Output Current
(PFM Mode; VOUT = 3.3V)
100 90
AAT1112
Load Regulation
(PFM Mode; VOUT = 3.3V)
0.50
VIN = 3.6V VIN = 4.2V VIN = 5.0V
VOUT Error (%)
Efficiency (%)
0.25
VIN = 3.6V
VIN = 4.2V
80 70 60 50 40 0.1
0.00
-0.25
VIN = 5.0V
1
10
100
1000
10000
-0.50 0.1
1
10
100
1000
10000
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(PWM Mode; VOUT = 3.3V)
100 0.50
Load Regulation
(PWM Mode; VOUT = 3.3V)
VIN = 3.6V
80
Efficiency (%)
60 40 20 0 1.0
VIN = 5.0V VIN = 4.2V
VOUT Error (%)
0.25
VIN = 3.6V
VIN = 5.0V
0.00
VIN = 4.2V
-0.25
10
100
1000
10000
-0.50 0.1
1
10
100
1000
10000
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(PFM Mode; VOUT = 2.5V)
100 90
Load Regulation
(PFM Mode; VOUT = 2.5V)
0.50
VIN = 2.7V
VOUT Error (%)
Efficiency (%)
0.25
VIN = 2.7V
VIN = 3.6V
80 70 60 50
VIN = 3.6V
VIN = 4.2V
0.00
-0.25
VIN = 4.2V
0.1
1
10
100
1000
10000
-0.50 0.1
1
10
100
1000
10000
Output Current (mA)
Output Current (mA)
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1.5A, 1.4MHz Step-Down Converter Typical Characteristics
Efficiency vs. Output Current
(PWM Mode; VOUT = 2.5V)
100 90 80 0.50
AAT1112
Load Regulation
(PWM Mode; VOUT = 2.5V) VIN = 3.6V
VIN = 2.7V
Efficiency (%)
70 60 50 40 30 20 10 0 1
VIN = 5.0V VIN = 4.2V VIN = 3.6V
10 100 1000 10000
VOUT Error (%)
0.25
VIN = 2.7V
VIN = 5.0V
0.00
-0.25
VIN = 4.2V
-0.50 0.1
1
10
100
1000
10000
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(PFM Mode; VOUT = 1.8V)
100 90 0.50
Load Regulation
(PFM Mode; VOUT = 1.8V)
VIN = 2.7V
VOUT Error (%)
0.25
Efficiency (%)
80 70 60 50 40 0.1
VIN = 3.6V
VIN = 2.7V
VIN = 3.6V
VIN = 4.2V
0.00
-0.25
VIN = 4.2V
1 10 100 1000 10000 -0.50 0.1 1 10 100 1000
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(PWM Mode; VOUT = 1.8V)
100 90 80 0.50
Load Regulation
(PWM Mode; VOUT = 1.8V)
Efficiency (%)
70 60 50 40 30 20 10 0 1 10
VIN = 4.2V
VOUT Error (%)
VIN = 2.7V
0.25
VIN = 2.7V
VIN = 3.6V
0.00
VIN = 3.6V
-0.25
VIN = 4.2V
100
1000
10000
-0.50 0.1
1
10
100
1000
10000
Output Current (mA)
Output Current (mA)
6
1112.2007.01.1.1
1.5A, 1.4MHz Step-Down Converter Typical Characteristics
Efficiency vs. Output Current
(PFM Mode; VOUT = 1.2V)
100 90 0.50
AAT1112
Load Regulation
(PFM Mode; VOUT = 1.2V)
VIN = 2.7V
VOUT Error (%)
0.25
Efficiency (%)
80 70 60 50 40 30 0.1 1 10 100 1000 10000
VIN = 2.7V
VIN = 3.6V
VIN = 3.6V
VIN = 4.2V
0.00
-0.25
VIN = 4.2V
-0.50 0.1
1
10
100
1000
10000
Output Current (mA)
Output Current (mA)
Efficiency vs. Output Current
(PWM Mode; VOUT = 1.2V)
100 90 80 0.50
Load Regulation
(PWM Mode; VOUT = 1.2V)
VIN = 2.7V VIN = 4.2V VIN = 3.6V
Efficiency (%)
70 60 50 40 30 20 10 0 1 10 100
VOUT Error (%)
0.25
VIN = 2.7V
VIN = 3.6V
0.00
-0.25
VIN = 4.2V
1000
10000
-0.50
0.1
1
10
100
1000
10000
Output Current (mA)
Output Current (mA)
Output Voltage vs. Temperature
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1A)
Supply Current vs. Supply Voltage
(VOUT = 1.8V; No Load; PFM Mode)
70 65 60 55 50 45 40 35 30 2.7 3.1 3.5 3.9 4.3 4.7
Output Voltage Change (%)
1.0 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 -40 -20 0 20 40 60 80
Supply Current (A)
0.8
85C
25C
-40C
5.1 5.5
Temperature (C)
Supply Voltage (V)
1112.2007.01.1.1
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1.5A, 1.4MHz Step-Down Converter Typical Characteristics
Switching Frequency vs. Temperature
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1A)
AAT1112
Line Regulation
(VOUT = 1.8V; IOUT = 1A)
0.12
Switching Frequency (MHz)
Output Voltage Error (%)
-20 0 20 40 60 80
1.40 1.38 1.36 1.34 1.32 1.30 1.28 1.26 1.24 -40
0.10 0.08 0.06 0.04 0.02 0.00 -0.02 -0.04 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
Temperature (C)
Supply Voltage (V)
Switching Frequency vs. Input Voltage
(IOUT = 1A)
Enable Soft Start
(VOUT = 3.6V; IOUT = 1.5A)
Switching Frequency (MHz)
1.40 1.39 1.38 1.37 1.36 1.35 1.34 1.33 1.32 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
VOUT = 1.8V VOUT = 2.5V
EN (2V/div) VOUT (1V/div) IIN (500mA/div) Time (100s/div)
VOUT = 3.3V
Input Voltage (V)
P-Channel RDS(ON) vs. Input Voltage
180 170 160 150
N-Channel RDS(ON) vs. Input Voltage
120C
140 130
120C
RDS(ON) (m)
150 140 130 120 110 100 90 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
RDS(ON) (m)
120 110 100 90 80 70 60 2.7 2.9 3.1 3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3 5.5
85C 25C
85C 25C
Input Voltage (V)
Input Voltage (V)
8
1112.2007.01.1.1
1.5A, 1.4MHz Step-Down Converter Typical Characteristics
Heavy Load Switching Waveform
(PWM Mode; VIN = 3.6V; VOUT = 1.8V; 1.5A Load)
Output Voltage (AC coupled) (top) (mV)
4.0 2.0 0.0 -2.0 -4.0 -6.0 -8.0 -10.0 -12.0 2.6
AAT1112
Light Load Switching Waveform
(PWM Mode; VIN = 3.6V; VOUT = 1.8V; 1mA Load)
Output Voltage (AC coupled) (top) (mV)
4.0 2.0 0.0 -2.0 -4.0 -6.0 -8.0 -10.0 -12.0
1200
Inductor Ripple Current (bottom) (mA)
Inductor Ripple Current (bottom) (mA)
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0
1000 800 600 400 200 0 -200 -400
Time (2.5s/div)
Time (2.5s/div)
Light Load Switching Waveform
(PFM Mode; VIN = 3.6V; VOUT = 1.8V; 1mA Load)
Output Voltage (AC coupled) (top) (mV)
8.0 4.0 0.0 -4.0 -8.0 -12.0 -16.0 -20.0 -24.0 700 2.0
Load Transient Response
(VIN = 3.6V; VOUT = 1.8V; CFF = 100pF)
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8
Inductor Ripple Current (bottom) (mA)
600 500 400 300 200 100 0 -100
1.9
Output Voltage (top) (V)
1.8 1.7 1.6 1.5 1.4 1.3 1.2
Load Current (bottom) (A)
Time (100s/div)
Time (20s/div)
Load Transient Response
(VIN = 3.6V; VOUT = 1.8V; No CFF)
2.0 1.9 2.4 2.2 5.0 4.5
Line Transient Response
(VOUT = 1.8V; 1.5A Load)
3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4
Output Voltage (top) (V)
1.7 1.6 1.5 1.4 1.3 1.2
1.8 1.6 1.4 1.2 1.0 0.8
Input Voltage (top) (V)
1.8
2.0
4.0 3.5 3.0 2.5 2.0 1.5 1.0
Output Voltage (bottom) (V)
Load Current (bottom) (A)
Time (20s/div)
Time (200s/div)
1112.2007.01.1.1
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1.5A, 1.4MHz Step-Down Converter Functional Block Diagram
FB
VIN VP
AAT1112
Err. Amp
DH VREF Logic LX
EN
MODE/SYNC
Input DL
PGND GND
Functional Description
The AAT1112 is a high performance 1.5A monolithic step-down converter operating at 1.4MHz switching frequency. It minimizes external component size and optimizes efficiency over the complete load range. Apart from the small bypass input capacitor, only a small L-C filter is required at the output. Typically, a 3.3H inductor and a 22F ceramic capacitor are recommended for a 3.3V output (see table of recommended values). At dropout, the converter duty cycle increases to 100% and the output voltage tracks the input voltage minus the RDS(ON) drop of the P-channel highside MOSFET (plus the DC drop of the external inductor). The device integrates extremely low RDS(ON) MOSFETs to achieve low dropout voltage
during 100% duty cycle operation. This is advantageous in applications requiring high output voltages (typically > 2.5V) at low input voltages. The integrated low-loss MOSFET switches can provide greater than 95% efficiency at full load. PFM operation maintains high efficiency under light load conditions (typically <150mA). The MODE/ SYNC pin allows optional "PWM only" mode. This maintains constant frequency and low output ripple across all load conditions. Alternatively, the IC can be synchronized to an external clock via the MODE/ SYNC input. External synchronization is maintained between 0.6MHz and 3.0MHz. In battery-powered applications, as VIN decreases, the converter dynamically adjusts the operating frequency prior to dropout to maintain the required duty cycle and provide accurate output regulation.
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1112.2007.01.1.1
1.5A, 1.4MHz Step-Down Converter
Output regulation is maintained until the dropout voltage, or minimum input voltage, is reached. At 1.5A output load, dropout voltage headroom is approximately 200mV. The AAT1112 typically achieves better than 0.5% output regulation across the input voltage and output load range. A current limit of 2.0A (typical) protects the IC and system components from short-circuit damage. Typical no load quiescent current is 42A. Thermal protection completely disables switching when the maximum junction temperature is detected. The junction over-temperature threshold is 140C with 15C of hysteresis. Once an over-temperature or over-current fault condition is removed, the output voltage automatically recovers. Peak current mode control and optimized internal compensation provide high loop bandwidth and excellent response to input voltage and fast load transient events. Soft start eliminates output voltage overshoot when the enable or the input voltage is applied. Under-voltage lockout prevents spurious start-up events. and line conditions. Internal loop compensation terminates the transconductance voltage error amplifier output. The reference voltage is internally set to program the converter output voltage greater than or equal to 0.6V.
AAT1112
Soft Start/Enable
Soft start limits the current surge seen at the input and eliminates output voltage overshoot. When pulled low, the enable input forces the AAT1112 into a low-power, non-switching state. The total input current during shutdown is less than 1A.
Current Limit and Over-Temperature Protection
For overload conditions, the peak input current is limited. To minimize power dissipation and stresses under current limit and short-circuit conditions, switching is terminated after entering current limit for a series of pulses. Switching is terminated for seven consecutive clock cycles after a current limit has been sensed for a series of four consecutive clock cycles. Thermal protection completely disables switching when internal dissipation becomes excessive. The junction over-temperature threshold is 140C with 15C of hysteresis. Once an over-temperature or over-current fault conditions is removed, the output voltage automatically recovers.
Control Loop
The AAT1112 is a peak current mode step-down converter. The current through the P-channel MOSFET (high side) is sensed for current loop control, as well as short-circuit and overload protection. A fixed slope compensation signal is added to the sensed current to maintain stability for duty cycles greater than 50%. The peak current mode loop appears as a voltage-programmed current source in parallel with the output capacitor. The output of the voltage error amplifier programs the current mode loop for the necessary peak switch current to force a constant output voltage for all load
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN input. Under-voltage lockout (UVLO) guarantees sufficient VIN bias and proper operation of all internal circuitry prior to activation.
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1.5A, 1.4MHz Step-Down Converter
VIN
AAT1112
C1 10F
11 7 3 2 1 8 9 10 3 2 1 5
U1 AAT1112 TDFN33-12 VP VCC EN LX N/C FB
12 4 6 3 2 1
3.3V L1 3.3H R2 C3 (optional) C2 22F
Enable
SYNC PGND N/C GND PGND PGND
R3 59K
SYNC
Figure 1: AAT1112 Schematic.
Component Selection
Inductor Selection
The step-down converter uses peak current mode control with slope compensation to maintain stability for duty cycles greater than 50%. The output inductor value must be selected so the inductor current down slope meets the internal slope compensation requirements. The inductor should be set equal to the output voltage numeric value in H. This guarantees that there is sufficient internal slope compensation. 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 normal load conditions. Some inductors may meet the 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. The 3.3H CDRH4D28 series Sumida inductor has a 49.2m worst case DCR and a 1.57A DC current rating. At full 1.5A load, the inductor DC loss is 97mW which gives less than 1.5% loss in efficiency for a 1.5A, 3.3V output.
Input Capacitor
Select a 10F to 22F X7R or X5R ceramic capacitor for the input. To estimate the required input capacitor size, determine the acceptable input ripple level (VPP) and solve for C. The calculated value varies with input voltage and is a maximum when VIN is double the output voltage.
CIN =
VO V * 1- O VIN VIN
VPP - ESR * FS IO
VO V 1 * 1 - O = for VIN = 2 * VO 4 VIN VIN
CIN(MIN) =
1
VPP - ESR * 4 * FS IO
Always examine the ceramic capacitor DC voltage coefficient characteristics when selecting the proper value. For example, the capacitance of a 10F, 6.3V, X5R ceramic capacitor with 5.0V DC applied is actually about 6F.
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1112.2007.01.1.1
1.5A, 1.4MHz Step-Down Converter
The maximum input capacitor RMS current is:
VO V * 1- O VIN VIN
AAT1112
IRMS = IO *
problem often becomes apparent in the form of excessive ringing in the output voltage during load transients. Errors in the loop phase and gain measurements can also result. Since the inductance of a short PCB trace feeding the input voltage is significantly lower than the power leads from the bench power supply, most applications do not exhibit this problem. In applications where the input power source lead inductance cannot be reduced to a level that does not affect the converter performance, a high ESR tantalum or aluminum electrolytic should be placed in parallel with the low ESR/ESL bypass ceramic capacitor. This dampens the high Q network and stabilizes the system.
The input capacitor RMS ripple current varies with the input and output voltage and will always be less than or equal to half of the total DC load current.
VO V * 1- O = VIN VIN
for VIN = 2 * VO
D * (1 - D) =
0.52 =
1 2
IRMS(MAX) =
IO 2
Output Capacitor
The output capacitor limits the output ripple and provides holdup during large load transitions. A 10F to 22F X5R or X7R ceramic capacitor typically provides sufficient bulk capacitance to stabilize the output during large load transitions and has the ESR and ESL characteristics necessary for low output ripple. The output voltage droop due to a load transient is dominated by the capacitance of the ceramic output capacitor. During a step increase in load current, the ceramic output capacitor alone supplies the load current until the loop responds. Within two or three switching cycles, the loop responds and the inductor current increases to match the load current demand. The relationship of the output voltage droop during the three switching cycles to the output capacitance can be estimated by:
3 * ILOAD VDROOP * FS
The term VIN VIN appears in both the input voltage ripple and input capacitor RMS current equations and is a maximum when VO is twice VIN. This is why the input voltage ripple and the input capacitor RMS current ripple are a maximum at 50% duty cycle. The input capacitor provides a low impedance loop for the edges of pulsed current drawn by the AAT1112. Low ESR/ESL X7R and X5R ceramic capacitors are ideal for this function. To minimize stray inductance, the capacitor should be placed as closely as possible to the IC. This keeps the high frequency content of the input current localized, minimizing EMI and input voltage ripple. The proper placement of the input capacitor (C1) can be seen in the evaluation board layout in the Layout section of this datasheet (see Figure 2). A laboratory test set-up typically consists of two long wires running from the bench power supply to the evaluation board input voltage pins. The inductance of these wires, along with the low-ESR ceramic input capacitor, can create a high Q network that may affect converter performance. This
VO
V * 1- O
COUT =
Once the average inductor current increases to the DC load level, the output voltage recovers. The above equation establishes a limit on the minimum value for the output capacitor with respect to load transients.
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1.5A, 1.4MHz Step-Down Converter
The internal voltage loop compensation also limits the minimum output capacitor value to 10F. This is due to its effect on the loop crossover frequency (bandwidth), phase margin, and gain margin. Increased output capacitance will reduce the crossover frequency with greater phase margin.
AAT1112
Thermal Calculations
There are three types of losses associated with the AAT1112 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 losses is given by:
IO2 * (RDS(ON)H * VO + RDS(ON)L * [VIN - VO]) VIN
Adjustable Output Resistor Selection
The output voltage on the AAT1112 is programmed with external resistors R1 and R2. To limit the bias current required for the external feedback resistor string while maintaining good noise immunity, the minimum suggested value for R2 is 59k. Although a larger value will further reduce quiescent current, it will also increase the impedance of the feedback node, making it more sensitive to external noise and interference. Table 1 summarizes the resistor values for various output voltages with R2 set to either 59k for good noise immunity or 221k for reduced no load input current. R2 = 59k VOUT (V)
0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.8 1.85 2.0 2.5 3.0 3.3
PTOTAL =
+ (tsw * FS * IO + IQ) * VIN
R2 = 221k R1 (k)
75 113 150 187 221 261 301 332 442 464 523 715 887 1000
IQ is the step-down converter quiescent current. The term tsw is used to estimate the full load stepdown converter switching losses. For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to:
PTOTAL = IO2 * RDS(ON)H + IQ * VIN
R1 (k)
19.6 29.4 39.2 49.9 59.0 68.1 78.7 88.7 118 124 137 187 237 267
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 TDFN3-12 and TSOPJW-12 packages, which is 50C/W and 160C/W respectively.
TJ(MAX) = PTOTAL * JA + TAMB
Table 1: AAT1112 Resistor Values for Various Output Voltages.
14
1112.2007.01.1.1
1.5A, 1.4MHz Step-Down Converter
Layout
The suggested PCB layout for the AAT1112 is shown in Figures 2 and 3. The following guidelines should be used to help ensure a proper layout. 1. The input capacitor (C1) should connect as closely as possible to VP and PGND. 2. C2 and L1 should be connected as closely as possible. The connection of L1 to the LX pin should be as short as possible. 3. The feedback trace or FB pin should be separate from any power trace and connect as closely as possible to the load point. Sensing along a high-current load trace will degrade DC load regulation. 4. The resistance of the trace from the load return 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. 5. Connect unused signal pins to ground to avoid unwanted noise coupling.
AAT1112
SYNC Vin LL PWM GND L1 LX C1 U1 R3 Vout C2 C3 R2 On Off Enable GND
GND
AAT1112 AnalogicTech
Figure 2: AAT1112 Evaluation Board Top Side Layout.
SYNC Vin LL PWM GND L1 LX C1
C4 GND On U1 R3 C3 R2 Off Enable
Vout
C2
GND
AAT1112 AnalogicTech
Figure 3: AAT1112 Evaluation Board Bottom Side Layout.
1112.2007.01.1.1
15
1.5A, 1.4MHz Step-Down Converter Design Example
Specifications
VO VIN FS 3.3V @ 1.5A, Pulsed Load ILOAD = 1.5A 2.7V to 4.2V (3.6V nominal) 1.2MHz
AAT1112
TAMB 85C in TDFN33-12 Package
Output Inductor
L1 = VO(H) = 3.3H; see Table 2. For Sumida inductor CDRH4D28 3.3H DCR = 49.2m max.
VO V 3.3V 3.3V 1 - O1 = 1= 179mA VIN 3.3H 1.2MHz 4.2V L1 FS
I1 =
IPK1 = IO1 +
I1 = 1.5A + 0.089A = 1.59A 2
PL1 = IO12 DCR = 1.5A2 49.2m = 110mW
Output Capacitor
VDROOP = 0.2V
3 * ILOAD 3 * 1.5A = = 18.8F; use 22F 0.2V * 1.2MHz VDROOP * FS 1 2* 3 * (VOUT) * (VIN(MAX) - VOUT) 1 3.3V * (4.2V - 3.3V) * = 52mArms = L * FS * VIN(MAX) 2 * 3 3.3H * 1.2MHz * 4.2V
COUT =
IRMS(MAX) =
Pesr = esr * IRMS2 = 5m * (52mA)2 = 13.3W
16
1112.2007.01.1.1
1.5A, 1.4MHz Step-Down Converter
Input Capacitor
Input Ripple VPP = 50mV
AAT1112
CIN =
1
VPP - ESR * 4 * FS IO1 + IO2
=
1 = 7.3F; use 10F 50mV - 5m * 4 * 1.2MHz 1.5A
IRMS(MAX) =
IO = 0.75Arms 2
P = esr * IRMS2 = 5m * (0.75A)2 = 3mW
AAT1112 Losses
Total losses can be estimated by calculating the dropout (VIN = VO) losses where the power MOSFET RDS(ON) will be at the maximum value. All values assume an 85C ambient temperature and a 120C junction temperature with the TDFN 50C/W package.
PLOSS = IO12 * RDS(ON)H = 1.5A2 * 0.16 = 0.36W
TJ(MAX) = TAMB + JA * PLOSS = 85C + (50C/W) * 360mW = 103C
The total losses are also investigated at the nominal lithium-ion battery voltage (3.6V). The simplified version of the RDS(ON) losses assumes that the N-channel and P-channel RDS(ON) are equal.
PTOTAL = IO2 * RDS(ON) + (tsw * FS * IO + IQ) * VIN
= 1.5A2 * 152m + (5ns * 1.2MHz * 1.5A + 50A) * 3.6V = 375mW
TJ(MAX) = TAMB + JA * PLOSS = 85C + (50C/W) * 375mW = 104C
1112.2007.01.1.1
17
1.5A, 1.4MHz Step-Down Converter
Inductance (H)
3.3 2.2 1.8 1.8 1.2 1.0 1.0 1.0
AAT1112
VOUT (V)
3.3 2.5 1.8 1.5 1.2 1.0 0.8 0.6
Part Number
CDRH4D28 CDRH4D28 CDRH4D28 CDRH4D28 CDRH4D28 SD3114-1.0 SD3114-1.0 SD3114-1.0
Manufacturer
Sumida Sumida Sumida Sumida Sumida Cooper Cooper Cooper
Size (mm)
5x5x3 5x5x3 5x5x3 5x5x3 5x5x3 3.1x3.1x1.45 3.1x3.1x1.45 3.1x3.1x1.45
Rated Current (A)
1.57 2.04 2.2 2.2 2.56
IRMS (A)
ISAT (A)
DCR ()
36.4 23.2 20.4 20.4 17.5 0.042 0.042 0.042
1.67 1.67 1.67
2.07 2.07 2.07
Table 2: Surface Mount Inductors.
Manufacturer
MuRata MuRata
Part Number
GRM21BR60J106KE19 GRM21BR60J226ME39
Value
10F 22F
Voltage
6.3V 6.3V
Temp. Co.
X5R X5R
Case
0805 0805
Table 3: Surface Mount Capacitors.
18
1112.2007.01.1.1
1.5A, 1.4MHz Step-Down Converter Ordering Information
Package
TSOPJW-12 TDFN33-12
AAT1112
Marking1
SBXYY
Part Number (Tape and Reel)2
AAT1112ITP-0.6-T1 AAT1112IWP-0.6-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 Information3
TSOPJW-12
+ 0.10 - 0.05
0.20
2.40 0.10
0.50 BSC 0.50 BSC 0.50 BSC 0.50 BSC 0.50 BSC
2.85 0.20
7 NOM 3.00 0.10
0.9625 0.0375
0.04 REF
0.15 0.05
+ 0.10 1.00 - 0.065
0.055 0.045
4 4
0.010
0.45 0.15 2.75 0.25
All dimensions in millimeters.
1. XYY = assembly and date code. 2. Sample stock is generally held on part numbers listed in BOLD. 3. 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. 1112.2007.01.1.1
19
1.5A, 1.4MHz Step-Down Converter
TDFN33-12
Index Area Detail "A" 0.43 0.05
AAT1112
0.1 REF
3.00 0.05 2.40 0.05
Pin 1 Indicator (optional)
3.00 0.05
1.70 0.05
Top View
Bottom View Detail "A"
0.75 0.05
0.05 0.05
Side View
All dimensions in millimeters.
(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 20
1112.2007.01.1.1
0.23 0.05
0.23 0.05
0.45 0.05

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