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3MHz Fast Transient 400mA Step-Down Converter General Description
The AAT1149 SwitchReg is a 3.0MHz step-down converter with an input voltage range of 2.7V to 5.5V and output voltage as low as 1.0V. It is optimized to react quickly to load variations and operate with a tiny 0603 inductor that is only 1mm tall. The AAT1149 output voltage is programmable via external feedback resistors. It can deliver 400mA of load current while maintaining a low 45A no load quiescent current. The 3.0MHz switching frequency minimizes the size of external components while keeping switching losses low. The AAT1149 maintains high efficiency throughout the operating range, which is critical for portable applications. The AAT1149 is available in a Pb-free, space-saving 2.0x2.1mm SC70JW-8 package and is rated over the -40C to +85C temperature range.
AAT1149
Features
* * * * * * * * * * * * * * *
SwitchRegTM
Ultra-Small 0603 Inductor (Height = 1mm) VIN Range: 2.7V to 5.5V VOUT Adjustable from 1.0V to VIN 400mA Max Output Current Up to 98% Efficiency 45A No Load Quiescent Current 3.0MHz Switching Frequency 70s 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
VIN = 3.6V U1 AAT1149 IN C2 4.7F EN AGND PGND LX FB PGND PGND R2 59k R1 118k C1 4.7F VOUT = 1.8V L1 1.8H
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3MHz Fast Transient 400mA Step-Down Converter Pin Descriptions
Pin #
1 2 3 4 5 6, 7, 8
AAT1149
Symbol
EN FB IN LX AGND PGND
Function
Enable pin. Feedback input pin. This pin is connected 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 FB IN LX
1 2 3 4
8 7 6 5
PGND PGND PGND AGND
2
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3MHz Fast Transient 400mA Step-Down Converter Absolute Maximum Ratings1
Symbol
VIN VLX VFB VEN TJ TLEAD
AAT1149
Description
Input Voltage to GND LX to GND FB 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. 1149.2006.11.1.0
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3MHz Fast Transient 400mA Step-Down Converter Electrical Characteristics1
VIN = 3.6V, TA = -40C to +85C, unless otherwise noted. Typical values are TA = 25C. Symbol Description Conditions Min
2.7 VIN Rising Hysteresis VIN Falling IOUT = 0 to 400mA, VIN = 2.7V to 5.5V No Load VEN = GND 600 0.45 0.40 VIN = 5.5V, VLX = 0 to VIN, VEN = GND VIN = 2.7V to 5.5V 0.6V Output, No Load TA = 25C 0.6V Output From Enable to Output Regulation TA = 25C 1 0.1 591 600 609 0.2 70 3.0 140 15 0.6 VIN = VOUT = 5.5V 1.4 -1.0 1.0 100 1.8 -3.0 1.0 45 3.0 VIN 70 1.0
AAT1149
Typ
Max
5.5 2.7
Units
V V mV V % V A A mA A %/V mV A 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 TS FOSC TSD THYS EN VEN(L) VEN(H) IEN Enable Threshold Low Enable Threshold High Input Low Current UVLO Threshold Output Voltage Tolerance Adjustable 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 Start-Up Time Oscillator Frequency Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis
1. The AAT1149 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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3MHz Fast Transient 400mA Step-Down Converter Typical Characteristics
Efficiency vs. Load Current
(VOUT = 3V; L = 3H)
100 1.00
AAT1149
Load Regulation
(VOUT = 3V; L = 3H)
0.75
VIN = 3.3V
Output Error (%)
90
Efficiency (%)
0.50 0.25 0.00 -0.25 -0.50 -0.75
VIN = 4.2V VIN = 5V
80 70 60 50 0.1 1 10 100 1000
VIN = 4.2V
VIN = 5V
VIN = 3.3V
0.1 1 10 100 1000
-1.00
Load Current (mA)
Load Current (mA)
Efficiency vs. Load Current
(VOUT = 1.8V; L = 2.2H)
100 90
Load Regulation
(VOUT = 1.8V; L = 2.2H)
1.00 0.75
VIN = 3V VIN = 3.6V
VIN = 2.7V
Output Error (%)
Efficiency (%)
0.50 0.25 0.00 -0.25 -0.50 -0.75
80 70 60 50
VIN = 3V
VIN = 4.2V
VIN = 5V VIN = 4.2V
VIN = 5V
VIN = 3.6V VIN = 2.7V
0.1
1
10
100
1000
-1.00 0.1
1
10
100
1000
Load Current (mA)
Load Current (mA)
No Load Quiescent Current vs. Input Voltage
70 2
Switching Frequency vs. Input Voltage
Supply Current (A)
60 50 40 30 20 10 0 2.5
85C
Frequency Variation (%)
25C
1 0 -1 -2 -3 -4
VOUT = 1.1V
-40C
VOUT = 1.8V
VOUT = 3V
3
3.5
4
4.5
5
5.5
6
2.5
3
3.5
4
4.5
5
5.5
Input Voltage (V)
Input Voltage (V)
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3MHz Fast Transient 400mA Step-Down Converter Typical Characteristics
Switching Frequency Variation vs. Temperature
10 8 6
AAT1149
Output Voltage Error vs. Temperature
(VIN = 3.6V; VO = 1.8V; IOUT = 400mA)
2.0
Output Error (%)
Variation (%)
4 2 0 -2 -4 -6 -8 -10 -40 -20 0 20 40 60 80 100 120
1.0
0.0
-1.0
-2.0 -40
-20
0
20
40
60
80
100
Temperature (C)
Temperature (C)
Line Regulation
(VOUT = 3V)
1 0.8 0.6
Line Regulation
(VOUT = 1.8V)
1 0.8 0.6
Accuracy (%)
Accuracy (%)
0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 2.5 3 3.5
1mA 400mA 300mA 600mA 100mA 0mA
0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1
400mA 0mA 100mA
600mA
4
4.5
5
5.5
2.5
3
3.5
4
4.5
5
5.5
6
Input Voltage (V)
Input Voltage (V)
Line Regulation
(VOUT = 1.1V)
1 0.6 4.50
Line Transient
(VOUT = 1.8; 400mA Load; No Feedforward Capacitor)
1.90 1.88 1.86 1.84 1.82 1.80 1.78 1.76 1.74
Output Voltage (bottom) (V)
Input Voltage (top) (V)
6
0.8
4.25 4.00 3.75 3.50 3.25 3.00 2.75 2.50
Accuracy (%)
0.4 0.2 0 -0.2 -0.4 -0.6 -0.8
1mA
0mA 400mA
600mA
-1 2.5
3
3.5
4
4.5
5
5.5
Input Voltage (V)
Time (50s/div)
6
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3MHz Fast Transient 400mA Step-Down Converter Typical Characteristics
N-Channel RDS(ON) vs. Input Voltage
750 700 650 120C
AAT1149
P-Channel RDS(ON) vs. Input Voltage
750 700
RDS(ON) (m)
600 550 500 450 400 350 300 2.5 3.0 3.5 25C 85C
RDS(ON) (m)
100C
650 600 550 500 450 400 350 300 25C
120C
100C
85C
4.0
4.5
5.0
5.5
6.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Input Voltage (V)
Input Voltage (V)
Line Transient
(VOUT = 1.8; No Load; No Feedforward Capacitor)
Input Voltage (top) (V)
4.25 4.00 3.75 3.50 3.25 3.00 2.75 2.50 1.92 1.90 1.88 1.86 1.84 1.82 1.80 1.78
Line Transient
(VOUT = 1.8; CFF = 100pF)
Output Voltage (bottom) (V)
1.86 1.85 1.84 1.83 1.82 1.81 1.80 1.79 1.78 4.50 4.25 4.00 3.75 3.50 3.25 3.00 2.75 2.50 1.94
4.50
Output Voltage (bottom) (V)
Input Voltage (top) (V)
Time (50s/div)
Time (20s/div)
Load Transient
(VOUT = 1.1V; No Feedforward Capacitor)
1.30 2.00 1.75 400mA 1.50 1.25 1.00 1mA 0.75 0.50 0.25 0.00 1.30
Load Transient
(VOUT = 1.1V; CFF = 100pF)
2.00 1.75 1.50 400mA 1mA 1.25 1.00 0.75 0.50 0.25 0.00
Load and Inductor Current (bottom) (A)
Load and Inductor Current (bottom) (A)
Output Voltage (top) (V)
Output Voltage (top) (V)
1.20 1.10 1.00 0.90 0.80 0.70 0.60 0.50
1.20 1.10 1.00 0.90 0.80 0.70 0.60 0.50
Time (50s/div)
Time (50s/div)
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3MHz Fast Transient 400mA Step-Down Converter Typical Characteristics
Load Transient
(VOUT = 1.8V; No Feedforward Capacitor)
2.25 2.00 1.75 1.50 400mA 10mA 1.25 1.00 0.75 2.00
AAT1149
Load Transient
(VOUT = 1.8V; CFF = 100pF)
2.00 1.75 1.50 400mA 10mA 1.25 1.00 0.75 0.50 0.25 0.00
Load and Inductor Current (bottom) (A)
Load and Inductor Current (bottom) (A)
Output Voltage (top) (V)
Output Voltage (top) (V)
2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25
1.90 1.80 1.70 1.60 1.50 1.40 1.30 1.20
0.25 0.00
Time (50s/div)
Time (50s/div)
Load Transient
(VOUT = 1.8V; No Feedforward Capacitor)
2.25 2.00 1.75 1.50 400mA 1mA 1.25 1.00 0.75 0.50 0.25 0.00 2.00
Load Transient
(VOUT = 1.8V; CFF = 100pF)
2.00 1.75 1.50 400mA 1.25 1.00 1mA 0.75 0.50 0.25 0.00
Load and Inductor Current (bottom) (A)
Load and Inductor Current (bottom) (A)
Output Voltage (top) (V)
Output Voltage (top) (V)
2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25
1.90 1.80 1.70 1.60 1.50 1.40 1.30 1.20
Time (50s/div)
Time (50s/div)
Soft Start
(VOUT = 1.8V; No Feedforward Capacitor)
Enable and Output Voltage (top) (V)
3.00 2.00 1.00 0.00 -1.00 -2.00 -3.00 -4.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0.00
Soft Start
(VOUT = 1.8V; CFF = 100pF)
Enable and Output Voltage (top) (V)
2.50 2.00 1.50 1.00 0.50 0.00 -0.50 -1.00 -1.50 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0.00 2.00
4.00
Inductor Current (bottom) (250mA/div)
Inductor Current (bottom) (250mA/div)
Time (50s/div)
Time (50s/div)
8
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3MHz Fast Transient 400mA Step-Down Converter Typical Characteristics
Soft Start
(VOUT = 3V; No Feedforward Capacitor)
Enable and Output Voltage (top) (V) Enable and Output Voltage (top) (V)
4.00 3.00 2.00 1.00 0.00 -1.00 -2.00 -3.00 -4.00 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 -0.50
AAT1149
Soft Start
(VOUT = 1.1V; No Feedforward Capacitor)
1.25 1.00 0.75 0.50 0.25 0.00 -0.25 -0.50 -0.75
Inductor Current (bottom) (250mA/div)
Inductor Current (bottom) (250mA/div)
Time (50s/div)
Time (20s/div)
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3MHz Fast Transient 400mA Step-Down Converter Functional Block Diagram
FB IN
AAT1149
Err Amp .
DH
Voltage Reference
Logic
LX
EN
INPUT
DL
PGND
AGND
Functional Description
The AAT1149 is a high performance 400mA 3.0MHz monolithic step-down converter. It minimizes external component size, enabling the use of a tiny 0603 inductor that is only 1mm tall, 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 1.8H inductor and a 4.7F ceramic capacitor are recommended (see table of values). Only three external power components (CIN, COUT, and L) are required. Output voltage is programmed with external feedback resistors, ranging from 1.0V to the input voltage. An additional feed-forward
capacitor can also be added to the external feedback to provide improved transient response (see Figure 4). 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. 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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3MHz Fast Transient 400mA Step-Down Converter
Control Loop
The AAT1149 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 the adjustable output, the error amplifier reference is fixed at 0.6V.
AAT1149
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. Table 1 displays suggested inductor values for various output voltages. 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 1.8H CDRH2D09 series inductor selected from Sumida has a 131m DCR and a 400mA saturation current rating. At full load, the inductor DC loss is 21mW which gives a 2.8% loss in efficiency for a 400mA, 1.8V output.
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 AAT1149 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.
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.
CIN =
VO V * 1- O VIN VIN
VPP - ESR * FS IO
VO V 1 * 1 - O = for VIN = 2 * VO VIN VIN 4
Under-Voltage Lockout
Internal bias of all circuits is controlled via the IN input. Under-voltage lockout (UVLO) guarantees sufficient VIN bias and proper operation of all internal circuitry prior to activation.
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CIN(MIN) =
1
VPP - ESR * 4 * FS IO
11
3MHz Fast Transient 400mA Step-Down Converter
AAT1149
Configuration
0.6V Adjustable With External Feedback
Output Voltage
1V, 1.2V 1.5V, 1.8V 2.5V 3.3V
Typical Inductor Value
1.0H to 1.2H 1.5H to 1.8H 2.2H to 2.7H 3.3H
Table 1: Inductor Values.
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. The maximum input capacitor RMS current is:
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 1. 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. 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.
IRMS = IO *
VO V * 1- O VIN VIN
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
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 AAT1149. 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
VO
V * 1- O
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 out-
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3MHz Fast Transient 400mA Step-Down Converter
put 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 COUT = VDROOP * FS
AAT1149
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. The maximum output capacitor RMS ripple current is given by:
VOUT * (VIN(MAX) - VOUT) L * FS * VIN(MAX) 2* 3 * 1
Once the average inductor current increases to the DC load level, the output voltage recovers. The above equation establishes a limit on the minimum
IRMS(MAX) =
Figure 1: AAT1149 Evaluation Board Top Side.
Figure 2: Exploded View of Evaluation Board Top Side.
Figure 3: AAT1149 Evaluation Board Bottom Side.
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3MHz Fast Transient 400mA Step-Down Converter
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 AAT1149, combined with an external feedforward capacitor (C3 in Figure 4), 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)
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.8 1.85 2.0 2.5 3.3
AAT1149
Feedback Resistor Selection
Resistors R1 and R2 of Figure 4 program 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
R2 = 221k R1
113K 150K 187K 221K 261K 301K 332K 442K 464K 523K 715K 1.00M
R1 (k)
29.4 39.2 49.9 59.0 68.1 78.7 88.7 118 124 137 187 267
Table 2: Feedback Resistor Values.
VIN C3
1
U1 AAT1149
EN OUT IN LX PGND PGND PGND AGND
8 7 6 5 2 3
1 2 3
Enable
R1 VOUT C1 4.7F GND GND LX
L1 R2 59k C2 4.7F
4
Figure 4: AAT1149 Evaluation Board Schematic.
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3MHz Fast Transient 400mA Step-Down Converter
Thermal Calculations
There are three types of losses associated with the AAT1149 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
AAT1149
Layout
The suggested PCB layout for the AAT1149 is shown in Figures 1, 2, and 3. The following guidelines should be used to help ensure a proper layout. 1. The input capacitor (C2) should connect as closely as possible to IN (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. 3. The feedback trace or FB 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 FB 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, as shown in Figure 5.
PTOTAL =
+ (tsw * FS * 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. 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
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.
TJ(MAX) = PTOTAL * JA + TAMB
Figure 5: Minimum Footprint Evaluation Board Using 2.0x1.25x1.0mm Inductor.
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3MHz 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) = 3.0MHz = 85C
AAT1149
1.8V Output Inductor
L1 = 1 sec sec VO = 1 1.8V = 1.8H A A
(use 2.2H; see Table 1)
For Taiyo Yuden inductor CBC2518T2R2M, 2.2H, DCR = 130m.
VO V 1.8V 1.8V 1- O = 1 = 156mA VIN 2.2H 3.0MHz 4.2V L1 FS
IL1 =
IPKL1 = IO +
IL1 = 0.4A + 0.078A = 0.478A 2
PL1 = IO2 DCR = 0.4A2 130m = 21mW
1.8V Output Capacitor
VDROOP = 0.1V
3 * ILOAD 3 * 0.3A = = 3.0F; use 4.7F 0.1V * 3.0MHz VDROOP * FS 1 2* 3 * (VO) * (VIN(MAX) - VO) 1 1.8V * (4.2V - 1.8V) * = 45mArms = L1 * FS * VIN(MAX) 2 * 3 2.2H * 3.0MHz * 4.2V
COUT = IRMS =
Pesr = esr * IRMS2 = 5m * (45mA)2 = 10W
16
1149.2006.11.1.0
3MHz Fast Transient 400mA Step-Down Converter
Input Capacitor
Input Ripple VPP = 25mV
AAT1149
CIN =
VPP IO
1 1 = = 1.45F; use 2.2F 25mV - 5m * 4 * 3.0MHz - ESR * 4 * FS 0.4A
IRMS =
IO = 0.2Arms 2
P = esr * IRMS2 = 5m * (0.2A)2 = 0.2mW
AAT1149 Losses
PTOTAL = IO2 * (RDS(ON)H * VO + RDS(ON)L * [VIN -VO]) VIN
+ (tsw * FS * IO + IQ) * VIN
=
0.42 * (0.725 * 1.8V + 0.7 * [4.2V - 1.8V])
4.2V
+ (5ns * 3MHz * 0.4A + 70A) * 4.2V = 140mW
TJ(MAX) = TAMB + JA * PLOSS = 85C + (160C/W) * 140mW = 107C
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17
3MHz Fast Transient 400mA Step-Down Converter
Adjustable Version (0.6V device) VOUT (V)
1.0 1.2 1.5 1.8 2.5 3.3
AAT1149
R2 = 59k R1 (k)
39.2 59.0 88.7 118 187 267
R2 = 221k1 R1 (k)
150 221 332 442 715 1000
L1 (H)
1.0 1.2 1.5 1.8 2.2 3.3
Table 3: Evaluation Board Component Values.
Manufacturer
Part Number/ Type
BRC1608
Inductance (H)
0.77 1.0 1.5 1.5 2.2 3.3 1.0 2.2 1.2 1.5 1.8 2.5 3.0 1.0 1.5 2.2 3.3 0.68 0.82 1.2 1.5 2.2 3.3
Rated Current (mA)
660 520 410 600 550 450 1000 890 590 520 480 440 400 485 445 425 375 980 830 720 630 510 430
DCR ()
110 180 300 200 250 350 80 130 97.5 110 131 150 195 300 400 480 600 31 54 75 104 116 139
Size (mm) LxWxH
0603 (HMAX = 1mm) 0805 (HMAX = 1mm) 2.5x1.8x1.8
Taiyo Yuden
BRL2012 CBC2518 Wire Wound Chip
Sumida
CDRH2D09 Shielded
3.2x3.2x1.0
Murata
LQH2MCN4R7M02 Unshielded
2.0x1.6x0.95
Coiltronics
SD3118 Shielded
3.15x3.15x1.2
Table 4: Typical Surface Mount Inductors.
1. For reduced quiescent current, R2 = 221k.
18
1149.2006.11.1.0
3MHz Fast Transient 400mA Step-Down Converter
Manufacturer
Murata Murata Murata
AAT1149
Part Number
GRM219R61A475KE19 GRM21BR60J106KE19 GRM185R60J475M
Value
4.7F 10F 4.7F
Voltage
10V 6.3V 6.3V
Temp. Co.
X5R X5R X58
Case
0805 0805 0603
Table 5: Surface Mount Capacitors.
1149.2006.11.1.0
19
3MHz Fast Transient 400mA Step-Down Converter Ordering Information
Output Voltage1
0.6; Adj 1.0
AAT1149
Package
SC70JW-8
Marking2
RGXYY
Part Number (Tape and Reel)3
AAT1149IJS-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. 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
1149.2006.11.1.0
0.05 0.05

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