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Fast Transient 400mA Step-Down Converter General Description
The AAT1146 SwitchReg is a member of AnalogicTech's Total Power Management ICTM (TPMICTM) product family. It is a 1.4MHz stepdown converter with an input voltage range of 2.7V to 5.5V and output voltage as low as 0.6V. It is optimized to react quickly to a load variation. The AAT1146 is available in fixed voltage versions with internal feedback and a programmable version with external feedback resistors. It can deliver 400mA of load current while maintaining a low 27A no load quiescent current. The 1.4MHz switching frequency minimizes the size of external components while keeping switching losses low. The AAT1146 is designed to maintain high efficiency throughout the operating range, which is critical for portable applications. The AAT1146 is available in a Pb-free, space-saving 2.0x2.1mm SC70JW-8 package and is rated over the -40C to +85C temperature range.
AAT1146
Features
* * * * * * * * * * * * * *
SwitchRegTM
VIN Range: 2.7V to 5.5V VOUT Fixed or Adjustable from 0.6V to VIN 27A No Load Quiescent Current Up to 98% Efficiency 400mA Max Output Current 1.4MHz Switching Frequency 120s Soft Start Fast Load Transient Over-Temperature Protection Current Limit Protection 100% Duty Cycle Low-Dropout Operation <1A Shutdown Current SC70JW-8 Package Temperature Range: -40C to +85C
Applications
* * * * * * Cellular Phones Digital Cameras Handheld Instruments Microprocessor / DSP Core / IO Power PDAs and Handheld Computers USB Devices
Typical Application (Fixed Output Voltage)
VIN
3 1
VO U1 AAT1146 VIN EN AGND PGND LX OUT PGND PGND
4 2 7 6
L1 4.7H C1 4.7F
C2 4.7F
5 8
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Fast Transient 400mA Step-Down Converter Pin Descriptions
Pin #
1 2 3 4 5 6, 7, 8
AAT1146
Symbol
EN OUT VIN LX AGND PGND
Function
Enable pin. Feedback input pin. This pin is connected either directly to the converter output or to an external resistive divider for an adjustable output. Input supply voltage for the converter. Switching node. Connect the inductor to this pin. It is internally connected to the drain of both high- and low-side MOSFETs. Non-power signal ground pin. Main power ground return pins. Connect to the output and input capacitor return.
Pin Configuration
SC70JW-8 (Top View)
EN OUT VIN LX
1 2 3 4
8 7 6 5
PGND PGND PGND AGND
2
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Fast Transient 400mA Step-Down Converter Absolute Maximum Ratings1
Symbol
VIN VLX VOUT VEN TJ TLEAD
AAT1146
Description
Input Voltage GND LX to GND OUT to GND 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
2, 3
Value
625 160
Units
mW 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. 3. Derate 6.25mW/C above 25C. 1146.2006.04.1.3
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Fast Transient 400mA Step-Down Converter Electrical Characteristics1
TA = -40C to +85C, unless otherwise noted. Typical values are TA = 25C, VIN = 3.6V. Symbol Description Conditions Min
2.7 VIN Rising Hysteresis VIN Falling IOUT = 0 to 400mA, VIN = 2.7V to 5.5V No Load, 0.6V Adjustable Version EN = AGND = PGND 600 0.45 0.40 VIN = 5.5V, VLX = 0 to VIN, EN = GND VIN = 2.7V to 5.5V 0.6V Output, No Load TA = 25C 0.6V Output >0.6V Output From Enable to Output Regulation TA = 25C 1 0.1 591 250 150 1.0 1.4 140 15 2.0 600 609 0.2 100 1.8 -3.0 0.6 27 +3.0 VIN 70 1.0
AAT1146
Typ
Max
5.5 2.7
Units
V V mV V % V A A mA A %/V mV A k s MHz C C V V A
Step-Down Converter VIN Input Voltage VUVLO VOUT VOUT IQ ISHDN ILIM RDS(ON)H RDS(ON)L ILXLEAK VLinereg VOUT IOUT ROUT TS FOSC TSD THYS EN VEN(L) VEN(H) IEN Enable Threshold Low Enable Threshold High Input Low Current UVLO Threshold Output Voltage Tolerance Output Voltage Range Quiescent Current Shutdown Current P-Channel Current Limit High Side Switch On Resistance Low Side Switch On Resistance LX Leakage Current Line Regulation Out Threshold Voltage Accuracy Out Leakage Current Out Impedance Start-Up Time Oscillator Frequency Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis
0.6 VIN = VOUT = 5.5V 1.4 -1.0 1.0
1. The AAT1146 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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Fast Transient 400mA Step-Down Converter Typical Characteristics
Efficiency vs. Load
(VOUT = 1.8V; L = 4.7H)
100 90
AAT1146
DC Regulation
(VOUT = 1.8V)
1.0
VIN = 2.7V
Output Error (%)
0.5
Efficiency (%)
80 70 60 50 0.1
VIN = 3.6V
VIN = 4.2V
VIN = 4.2V
0.0
-0.5
VIN = 3.6V VIN = 2.7V
1
10
100
1000
-1.0 0.1
1
10
100
1000
Output Current (mA)
Output Current (mA)
Efficiency vs. Load
(VOUT = 2.5V; L = 6.8H)
100
1.0
DC Regulation
(VOUT = 2.5V)
VIN = 2.7V
Output Error (%)
90
VIN = 4.2V
0.5
Efficiency (%)
80 70 60 50 0.1
VIN = 5.0V VIN = 4.2V VIN = 3.6V
VIN = 5.0V
0.0
-0.5
VIN = 3.6V VIN = 3.0V
-1.0
1
10
100
1000
0.1
1
10
100
1000
Output Current (mA)
Output Current (mA)
Efficiency vs. Load
(VOUT = 3.3V; L = 6.8H)
100
DC Regulation
(VOUT = 3.3V; L = 6.8H)
1.0
VIN = 3.6V
Output Error (%)
90
VIN = 5.0V
0.5
Efficiency (%)
80 70 60 50 0.1
VIN = 4.2V VIN = 5.0V
VIN = 4.2V
0.0
-0.5
VIN = 3.6V
-1.0
1 10 100 1000
0.1
1
10
100
1000
Output Current (mA)
Output Current (mA)
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Fast Transient 400mA Step-Down Converter Typical Characteristics
Soft Start
(VIN = 3.6V; VOUT = 1.8V; IOUT = 400mA)
AAT1146
Line Regulation
(VOUT = 1.8V)
1.6
Enable and Output Voltage (top) (V)
5.0 4.0 3.0 2.0 1.0 0.0 -1.0 -2.0 -3.0 -4.0 -5.0
0.40 0.30
VEN
VO
1.4 1.0 0.8 0.6 0.4 0.2
Accuracy (%)
1.2
0.20 0.10 0.00 -0.10 -0.20 -0.30 -0.40 2.5 3.0 3.5
IOUT = 10mA
Inductor Current (bottom) (A)
IOUT = 1mA IOUT = 400mA
IL
0.0 -0.2 -0.4
4.0
4.5
5.0
5.5
6.0
Time (100s/div)
Input Voltage (V)
Output Voltage Error vs. Temperature
(VIN = 3.6V; VO = 1.8V; IOUT = 400mA)
2.0 15.0 12.0 9.0
Switching Frequency vs. Temperature
(VIN = 3.6V; VOUT = 1.8V)
Output Error (%)
Variation (%)
1.0
6.0 3.0 0.0 -3.0 -6.0 -9.0 -12.0 -15.0 -40
0.0
-1.0
-2.0 -40
-20
0
20
40
60
80
100
-20
0
20
40
60
80
100
Temperature (C)
Temperature (C)
Frequency vs. Input Voltage
2.0
No Load Quiescent Current vs. Input Voltage
50
Frequency Variation (%)
1.0 0.0 -1.0
VOUT = 1.8V
Supply Current (A)
45 40 35 30 25 20 15 10 2.7
85C
25C
VOUT = 2.5V
-2.0 -3.0 -4.0 2.7 3.1 3.5 3.9
VOUT = 3.3V
-40C
3.1 3.5 3.9 4.3 4.7 5.1 5.5
4.3
4.7
5.1
5.5
Input Voltage (V)
Input Voltage (V)
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Fast Transient 400mA Step-Down Converter Typical Characteristics
P-Channel RDS(ON) vs. Input Voltage
750 700 650 120C 100C 750 700 650 120C
AAT1146
N-Channel RDS(ON) vs. Input Voltage
RDS(ON) (m)
RDS(ON) (m)
100C
600 550 500 450 400 350 300 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 25C 85C
600 550 500 450 400 350 300 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 25C 85C
Input Voltage (V)
Input Voltage (V)
Load Transient Response
(1mA to 300mA; VIN = 3.6V; VOUT = 1.8V; C1 = 10F; CFF = 100pF)
2.0 1.9 1.90 1.85
Load Transient Response
(300mA to 400mA; VIN = 3.6V; VOUT = 1.8V; C1 = 4.7F)
Load and Inductor Current (200mA/div) (bottom)
Load and Inductor Current (200mA/div) (bottom)
VO IO IL
VO IO
Output Voltage (top) (V)
1.7 300mA 1mA
Output Voltage (top) (V)
1.8
1.80 1.75 400mA 300mA 0.4
0
IL Time (50s/div)
0.3 0.2 0.1
Time (50s/div)
Load Transient Response
(300mA to 400mA; VIN = 3.6V; VOUT = 1.8V; C1 = 10F)
1.90 1.85
Load Transient Response
(300mA to 400mA; VIN = 3.6V; VOUT = 1.8V; C1 = 10F; C4 = 100pF)
1.850 1.825
Load and Inductor Current (200mA/div) (bottom)
Load and Inductor Current (200mA/div) (bottom)
VO IO
VO IO
400mA 300mA 0.4
Output Voltage (top) (V)
1.75
Output Voltage (top) (V)
1.80
1.800 1.775
400mA 300mA
0.4
IL
0.3 0.2 0.1
IL
0.3 0.2 0.1
Time (50s/div)
Time (50s/div)
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Fast Transient 400mA Step-Down Converter Typical Characteristics
Line Response
(VOUT = 1.8V @ 400mA)
AAT1146
Output Ripple
(VIN = 3.6V; VOUT = 1.8V; IOUT = 1mA)
Output Voltage (AC coupled) (top) (mV)
1.82 1.81
6.0 5.5
40 20 0 -20 -40 -60 -80 -100 -120
0.30
VO
0.25
Inductor Current (bottom) (A)
Output Voltage (top) (V)
0.20 0.15 0.10
Input Voltage (bottom) (V)
1.80 1.79 1.78 1.77 1.76
5.0 4.5 4.0 3.5 3.0
IL
0.05 0.00 -0.05 -0.10
Time (25s/div)
Time (10s/div)
Output Ripple
(VIN = 3.6V; VOUT = 1.8V; IOUT = 400mA)
Output Voltage (AC coupled) (top) (mV)
40 20 0 -20 -40 -60 -80 -100 -120
0.9
VO
0.8
Inductor Current (bottom) (A)
0.7 0.6 0.5 0.4 0.3
IL
0.2 0.1
Time (500ns/div)
8
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Fast Transient 400mA Step-Down Converter Functional Block Diagram
OUT VIN
AAT1146
See note
Err Amp .
DH
Voltage Reference
Logic
LX
DL
EN
INPUT
PGND AGND
Note: For adjustable version, the internal feedback divider is omitted and the OUT pin is tied directly to the internal error amplifier.
Functional Description
The AAT1146 is a high performance 400mA 1.4MHz monolithic step-down converter. It has been designed with the goal of minimizing external component size and optimizing 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 4.7H inductor and a 4.7F ceramic capacitor are recommended (see table of values). The fixed output version requires only three external power components (CIN, COUT, and L). The adjustable version can be programmed with external feedback to any voltage, ranging from 0.6V to the
input voltage. An additional feed-forward capacitor can also be added to the external feedback to provide improved transient response (see Figure 1). At dropout, the converter duty cycle increases to 100% and the output voltage tracks the input voltage minus the RDSON drop of the P-channel highside MOSFET. The input voltage range is 2.7V to 5.5V. The converter efficiency has been optimized for all load conditions, ranging from no load to 400mA. The internal error amplifier and compensation provides excellent transient response, load, and line regulation. Soft start eliminates any output voltage overshoot when the enable or the input voltage is applied.
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Fast Transient 400mA Step-Down Converter
AAT1146
1 2 3
Enable
VIN
C4 100pF
1
U1 AAT1146 R1
EN OUT VIN LX PGND PGND PGND AGND 8 7 6 5 2 3 4
VOUT =1.8V
L1 118k 4.7H C1 10F
C3 n/a
R2 59k
C2 4.7F
GND LX GND2 U1 AAT1146 SC70JW-8 L1 CDRH3D16-4R7 C2 4.7F 10V 0805 X5R C1 10F 6.3V 0805 X5R
Figure 1: Enhanced Transient Response Schematic.
Control Loop
The AAT1146 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 and line conditions. Internal loop compensation terminates the transconductance voltage error amplifier output. For fixed voltage versions, the error amplifier reference voltage is internally set to program the converter output voltage. For the adjustable output, the error amplifier reference is fixed at 0.6V.
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.
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 AAT1146 10
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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Fast Transient 400mA Step-Down Converter Applications Information
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 internal slope compensation for the adjustable and low-voltage fixed versions of the AAT1146 is 0.24A/sec. This equates to a slope compensation that is 75% of the inductor current down slope for a 1.5V output and 4.7H inductor. 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 4.7H CDRH3D16 series inductor selected from Sumida has a 105m DCR and a 900mA DC current rating. At full load, the inductor DC loss is 17mW which gives a 2.8% loss in efficiency for a 400mA, 1.5V output.
AAT1146
Input Capacitor
Select a 4.7F to 10F 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.
0.75 VO 0.75 1.5V A m= = = 0.24 L 4.7H sec
This is the internal slope compensation for the adjustable (0.6V) version or low-voltage fixed versions. When externally programming the 0.6V version to 2.5V, the calculated inductance is 7.5H.
0.75 VO L= = m
sec 0.75 VO 3 A VO A 0.24A sec
CIN =
VO V * 1- O VIN VIN
VPP - ESR * FS IO
=3
sec 2.5V = 7.5H A
VO V 1 * 1 - O = for VIN = 2 x VO VIN VIN 4
In this case, a standard 6.8H value is selected. For high-voltage fixed versions (2.5V), m = 0.48A/ sec. Table 1 displays inductor values for the AAT1146 fixed and adjustable options. 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
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.
Configuration
0.6V Adjustable With External Feedback Fixed Output
Output Voltage
1V, 1.2V 1.5V, 1.8V 2.5V, 3.3V 0.6V to 3.3V Table 1: Inductor Values.
Inductor
2.2H 4.7H 6.8H 4.7H
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Fast Transient 400mA Step-Down Converter
The maximum input capacitor RMS current is:
VO V * 1- O VIN VIN
AAT1146
IRMS = IO *
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. 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 x VO
D * (1 - D) =
0.52 =
1 2
Output Capacitor
The output capacitor limits the output ripple and provides holdup during large load transitions. A 4.7F to 10F 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
IRMS(MAX) =
IO 2
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 AAT1146. 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 (C2) can be seen in the evaluation board layout in 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 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. 12
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. The internal voltage loop compensation also limits the minimum output capacitor value to 4.7F. 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.
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Fast Transient 400mA Step-Down Converter
AAT1146
Figure 2: AAT1146 Evaluation Board Top Side.
Figure 3: Exploded View of Evaluation Board Top Side Layout.
Figure 4: AAT1146 Evaluation Board Bottom Side. The maximum output capacitor RMS ripple current is given by:
VOUT * (VIN(MAX) - VOUT) L * F * VIN(MAX) 2* 3 * 1
IRMS(MAX) =
Dissipation due to the RMS current in the ceramic output capacitor ESR is typically minimal, resulting in less than a few degrees rise in hot-spot temperature.
the output to regulate at a voltage higher than 0.6V. 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 2 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.
VOUT 1.5V R1 = V -1 * R2 = 0.6V - 1 * 59k = 88.5k REF
Adjustable Output Resistor Selection
For applications requiring an adjustable output voltage, the 0.6V version can be externally programmed. Resistors R1 and R2 of Figure 5 program
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Fast Transient 400mA Step-Down Converter
The adjustable version of the AAT1146, combined with an external feedforward capacitor (C4 in Figure 1), delivers enhanced transient response for extreme pulsed load applications. The addition of the feedforward capacitor typically requires a larger output capacitor C1 for stability. 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 3.3
AAT1146
Thermal Calculations
There are three types of losses associated with the AAT1146 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 * (RDSON(HS) * VO + RDSON(LS) * [VIN - VO]) VIN
R2 = 221k R1
75K 113K 150K 187K 221K 261K 301K 332K 442K 464K 523K 715K 1.00M
R1 (k)
19.6 29.4 39.2 49.9 59.0 68.1 78.7 88.7 118 124 137 187 267
PTOTAL =
+ (tsw * F * IO + IQ) * VIN
IQ is the step-down converter quiescent current. The term tsw is used to estimate the full load stepdown converter switching losses.
Table 2: Adjustable Resistor Values For Use With 0.6V Step-Down Converter.
1 2 3
VIN
Enable
1
U1 AAT1146
EN OUT VIN LX PGND PGND PGND AGND
8 7 6 5 2 3 4
R1 118k VOUT C1 10F L1 4.7H R2 59k
C2 4.7F
GND
GND2
LX U1 AAT1146 SC70JW-8 L1 CDRH3D16-4R7 C1 10F 10V 0805 X5R C2 4.7F 10V 0805 X5R
Figure 5: AAT1146 Adjustable Evaluation Board Schematic.
14
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Fast Transient 400mA Step-Down Converter
For the condition where the step-down converter is in dropout at 100% duty cycle, the total device dissipation reduces to:
PTOTAL = IO2 * RDSON(HS) + IQ * VIN
AAT1146
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 SC70JW-8 package which is 160C/W.
3. The feedback trace or OUT pin (Pin 2) 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. If external feedback resistors are used, they should be placed as closely as possible to the OUT pin (Pin 2) to minimize the length of the high impedance feedback trace. 4. The resistance of the trace from the load return to the PGND (Pins 6-8) 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. A high density, small footprint layout can be achieved using an inexpensive, miniature, nonshielded, high DCR inductor. An evaluation board is available with this inductor and is shown in Figure 6. The total solution footprint area is 40mm2.
TJ(MAX) = PTOTAL * JA + TAMB
Layout
The suggested PCB layout for the AAT1146 is shown in Figures 2, 3, and 4. The following guidelines should be used to help ensure a proper layout. 1. The input capacitor (C2) should connect as closely as possible to VIN (Pin 3) and PGND (Pins 6-8). 2. C1 and L1 should be connected as closely as possible. The connection of L1 to the LX pin should be as short as possible.
Figure 6: Minimum Footprint Evaluation Board Using 2.0mm x 1.6mm x 0.95mm Inductor.
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Fast Transient 400mA Step-Down Converter Step-Down Converter Design Example
Specifications
VO VIN FS TAMB = 1.8V @ 400mA (adjustable using 0.6V version), Pulsed Load ILOAD = 300mA = 2.7V to 4.2V (3.6V nominal) = 1.4MHz = 85C
AAT1146
1.8V Output Inductor
L1 = 3 sec sec VO2 = 3 1.8V = 5.4H A A
(use 4.7H; see Table 1)
For Sumida inductor CDRH3D16, 4.7H, DCR = 105m.
VO V 1.8V 1.8V 1- O = 1 = 156mA VIN 4.7H 1.4MHz 4.2V L1 F
IL1 =
IPKL1 = IO +
IL1 = 0.4A + 0.068A = 0.468A 2
PL1 = IO2 DCR = 0.4A2 105m = 17mW
1.8V Output Capacitor
VDROOP = 0.1V
3 * ILOAD 3 * 0.3A = = 6.4F; use 10F 0.1V * 1.4MHz VDROOP * FS 1 2* 3 * (VO) * (VIN(MAX) - VO) 1 1.8V * (4.2V - 1.8V) * = 45mArms = L1 * F * VIN(MAX) 2 * 3 4.7H * 1.4MHz * 4.2V
COUT =
IRMS =
Pesr = esr * IRMS2 = 5m * (45mA)2 = 10W
16
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Fast Transient 400mA Step-Down Converter
Input Capacitor
Input Ripple VPP = 25mV
AAT1146
CIN =
VPP IO
1 1 = = 3.11F; use 4.7F 25mV - 5m * 4 * 1.4MHz - ESR * 4 * FS 0.4A
IRMS =
IO = 0.2Arms 2
P = esr * IRMS2 = 5m * (0.2A)2 = 0.2mW
AAT1146 Losses
IO2 * (RDSON(HS) * VO + RDSON(LS) * [VIN -VO]) VIN
PTOTAL =
+ (tsw * F * IO + IQ) * VIN
=
0.42 * (0.725 * 1.8V + 0.7 * [4.2V - 1.8V])
4.2V
+ (5ns * 1.4MHz * 0.4A + 70A) * 4.2V = 126mW
TJ(MAX) = TAMB + JA * PLOSS = 85C + (160C/W) * 126mW = 105.1C
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17
Fast Transient 400mA Step-Down Converter
Adjustable Version (0.6V device) 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 3.3
AAT1146
R2 = 59k R1 (k)
19.6 29.4 39.2 49.9 59.0 68.1 78.7 88.7 118 124 137 187 267
R2 = 221k1 R1 (k)
75.0 113 150 187 221 261 301 332 442 464 523 715 1000
L1 (H)
2.2 2.2 2.2 2.2 2.2 2.2 4.7 4.7 4.7 4.7 6.8 6.8 6.8
Fixed Version VOUT (V)
0.6-3.3V
R2, R4 Not Used R1 (k)
0
L1 (H)
4.7
Table 3: Evaluation Board Component Values.
Manufacturer
Sumida Sumida Sumida MuRata MuRata Coilcraft Coiltronics Coiltronics Coiltronics
Part Number
CDRH3D16-2R2 CDRH3D16-4R7 CDRH3D16-6R8 LQH2MCN4R7M02 LQH32CN4R7M23 LPO3310-472 SD3118-4R7 SD3118-6R8 SDRC10-4R7
Inductance (H)
2.2 4.7 6.8 4.7 4.7 4.7 4.7 6.8 4.7
Max DC Current (A)
1.20 0.90 0.73 0.40 0.45 0.80 0.98 0.82 1.30
DCR ()
0.072 0.105 0.170 0.80 0.20 0.27 0.122 0.175 0.122
Size (mm) LxWxH
3.8x3.8x1.8 3.8x3.8x1.8 3.8x3.8x1.8 2.0x1.6x0.95 2.5x3.2x2.0 3.2x3.2x1.0 3.1x3.1x1.85 3.1x3.1x1.85 5.7x4.4x1.0
Type
Shielded Shielded Shielded Non-Shielded Non-Shielded 1mm Shielded Shielded 1mm Shielded
Table 4: Typical Surface Mount Inductors.
1. For reduced quiescent current, R2 and R4 = 221k.
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1146.2006.04.1.3
Fast Transient 400mA Step-Down Converter
Manufacturer
MuRata MuRata MuRata
AAT1146
Part Number
GRM219R61A475KE19 GRM21BR60J106KE19 GRM21BR60J226ME39
Value
4.7F 10F 22F
Voltage
10V 6.3V 6.3V
Temp. Co.
X5R X5R X5R
Case
0805 0805 0805
Table 5: Surface Mount Capacitors.
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19
Fast Transient 400mA Step-Down Converter Ordering Information
Output Voltage1
1.875 Adj 0.6
AAT1146
Package
SC70JW-8 SC70JW-8
Marking2
QMXYY OXXYY
Part Number (Tape and Reel)3
AAT1146IJS-1.875-T1 AAT1146IJS-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 Information
SC70JW-8
0.50 BSC 0.50 BSC 0.50 BSC
1.75 0.10
0.225 0.075 2.00 0.20
2.20 0.20
0.048REF
0.85 0.15
1.10 MAX
0.15 0.05
0.100
7 3
0.45 0.10 2.10 0.30
4 4
All dimensions in millimeters.
1. Contact Sales for other voltage options. 2. XYY = assembly and date code. 3. Sample stock is generally held on part numbers listed in BOLD.
(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.
Advanced Analogic Technologies, Inc.
830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737-4600 Fax (408) 737-4611 20
1146.2006.04.1.3
0.05 0.05

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