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600mA Step-Down Converter General Description Features

AAT1106

600mA Step-Down Converter General Description Features
AAT1106
SwitchRegTM
Applications
Cellular Phones, Smartphones Digital Still Cameras Digital Video Cameras Microprocessor and DSP Core Supplies MP3 and Portable Media Players PDAs Wireless and DSL Modems
Typical Application
VIN 2.5V to 5.5V L1 2.2µH VOUT 1.8V
C1 4.7µF
AAT1106-1.8
C3 10µF
600mA Step-Down Converter Pin Descriptions
AAT1106
Symbol
EN GND LX IN FB / OUT
Function
Pin Configuration
TSOT23-5 (Top View)
EN GND LX
EN GND
Adjustable Output Version (AAT1106ICB-0.6)
Fixed Output Versions (AAT1106ICB-1.5, AAT1106ICB-1.8)
600mA Step-Down Converter Absolute Maximum Ratings
VIN VEN, VFB VLX, VOUT TJ TLEAD
AAT1106
Symbol
Description
Input Supply Voltage EN, FB Voltages LX, OUT Voltages Operating Temperature Range Storage Temperature Range Lead Temperature (soldering, 10s)
-0.3 to 6.0 -0.3 to VIN + 0.3 -0.3 to VIN + 0.3 -40 to +85 -65 to +150 300
Value
Units
Recommended Operating Conditions
Symbol
Description
Value
Units
600mA Step-Down Converter Electrical Characteristics
Step-Down Converter VIN Input Voltage Range IQ
AAT1106
Units
Input DC Supply Current Regulated Feedback Voltage FB Input Bias Current Regulated Output Voltage Output Voltage Line Regulation Output Voltage Load Regulation Maximum Output Current Oscillator Frequency Startup Time P-Channel MOSFET N-Channel MOSFET
VFB VOUT IFB
VOUT / VOUT / VIN VOUT / VOUT / IOUT ILIM FOSC TS RDS(ON)
Peak Inductor Current VEN(L) VEN(H) IEN TSD THYS
Output Over-Voltage Lockout Enable Threshold Low Enable Threshold High Input Low Current Over-Temperature Shutdown Threshold Over-Temperature Shutdown Hysteresis
600mA Step-Down Converter Typical Characteristics
Efficiency vs. Output Current
AAT1106
Efficiency vs. Output Current
Output Current (mA)
Efficiency vs. Output Current
Output Current (mA)
Efficiency vs. Output Current
Output Current (mA)
600mA Step-Down Converter Typical Characteristics
Efficiency vs. Input Voltage
AAT1106
Output Voltage vs. Output Current
Input Voltage (V)
Load Current (mA)
Frequency vs. Input Voltage
RDS(ON) vs. Input Voltage
Frequency (MHz)
RDS(ON) ()
P-Channel MOSFET
N-Channel MOSFET
Input Voltage (V)
Feedback Voltage vs. Temperature
RDS(ON) vs. Temperature
Feedback Voltage (V)
P-Channel
RDS(ON) ()
N-Channel
Temperature (°C) °
Temperature (°C)
600mA Step-Down Converter Typical Characteristics
AAT1106
Frequency VS. Temperature
Input Supply Current (µA)
Input Supply Current vs. Temperature
OSC Frequency (MHz)
Temperature (°C)
Temperature (°C) °
Load Transient Response
600mA Step-Down Converter Typical Characteristics
AAT1106
Startup Waveform
Input Current (bottom) (A)
Time (20µs / div)
Output Voltage (top) (V)
Startup Waveform
Input Current (bottom) (A)
Time (20µs / div)
600mA Step-Down Converter Functional Block Diagram
OSC SLOPE COMP + 4 R VIN
AAT1106
BLANKING COMP + + OVDET S Q
ISENSE COMP -
VIN 2.7 - 5.5V
RS LATCH
PWM LOGIC
NON-OV ERLA P CONTROL
0.65V V IN 0.6V
IZERO COMP -
COUT GND
SHUTDOWN
For adjustable output R1 + R2 are external
Functional Description
Current Mode PWM Control
600mA Step-Down Converter
Control Loop
AAT1106
Dropout Operation
is 150°C with 15°C of hysteresis. Once an overtemperature or over-current fault conditions is removed, the output voltage automatically recovers.
Enable
The enable pin is active high. When pulled low, the enable input forces the AAT1106 into a low-power, non-switching state. The total input current during shutdown is less than 1µA.
Current Limit and Over-Temperature Protection.
The output voltage then is the input voltage minus the voltage drop across the main switch and the inductor. At low input supply voltage, the RDS(ON) of the P-channel MOSFET increases and the efficiency of the converter decreases. Caution must be exercised to ensure the heat dissipated does not exceed the maximum junction temperature of the IC.
Where TON is the main switch on time and FOSC is the oscillator frequency (1.5MHz).
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
Maximum Load Current
600mA Step-Down Converter
Figure 1 shows the basic application circuit with AAT1106 fixed output versions.
VIN 2.5V to 5.5V IN LX L1 2.2µH VOUT 1.8V
AAT1106
Applications Information
C1 4.7µF
AAT1106-1.8
C3 10µF
Figure 1: Basic Application Circuit with Fixed Output Versions.
VIN 2.5V to 5.5V IN LX FB L1 2.2µH C2 22pF R2 634K R1 316K VOUT 1.8V
VOUT (V)
C1 4.7µF
AAT1106-0.6
C3 10µF
For applications requiring an adjustable output voltage, the AAT1106-0.6 adjustable version can be externally programmed. Resistors R1 and R2 of Figure 2 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 R1 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 R1 set to either 59k for good noise immunity or 316k for reduced no load input current. The adjustable version of the AAT1106, combined with an external feed forward capacitor (C2 in Figure 2), delivers enhanced transient response for extreme pulsed load applications. The addition of the feed forward capacitor typically requires a larger output capacitor C3 for stability. The external resistor sets the output voltage according to the following equation:
Setting the Output Voltage
Figure 2: Basic Application Circuit with Adjustable Output Version.
For most designs, the AAT1106 operates with inductor values of 1µH to 4.7µH. Low inductance values are physically smaller, but require faster switching, which results in some efficiency loss. The inductor value can be derived from the following equation:
Inductor Selection
600mA Step-Down Converter
Sumida CR43
AAT1106
L (µH)
Max DCR (m)
Rated DC Current (A)
Size WxLxH (mm)
4.5x4.0x3.5
Sumida CDRH4D18
4.7x4.7x2.0
Toko D312C
3.6x3.6x1.2
VFB VREF Error Amp
For output voltages above 2.0V, when light-load efficiency is important, the minimum recommended inductor size is 2.2µH. For optimum voltage-positioning load transients, choose an inductor with DC series resistance in the 50m to 150m range. For higher efficiency at heavy loads (above 200mA), or minimal load regulation (with some transient overshoot), the resistance should be kept below 100m. The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation (600mA + 105mA). Table 2 lists some typical surface mount inductors that meet target applications for the AAT1106.
Slope Compensation
600mA Step-Down Converter
AAT1106
Therefore:
Always examine the ceramic capacitor DC voltage coefficient characteristics when selecting the proper value. For example, the capacitance of a 10µF, 6.3V, X5R ceramic capacitor with 5.0V DC applied is actually about 6µF. The maximum input capacitor RMS current is: VO V · 1- O VIN VIN
Therefore:
Input Capacitor Selection
With these adaptive settings, a 2.2µH inductor can be used for all output voltages from 0.6V to 5V.
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.
The input capacitor reduces the surge current drawn from the input and switching noise from the device. The input capacitor impedance at the switching frequency shall be less than the input source impedance to prevent high frequency switching current passing to the input. A low ESR input capacitor sized for maximum RMS current must be used. Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their low ESR and small temperature coefficients. A 4.7µF ceramic capacitor is sufficient for most applications.
600mA Step-Down Converter
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 Figure 3. 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. The output capacitor is required to keep the output voltage ripple small and to ensure regulation loop stability. The output capacitor must have low impedance at the switching frequency. Ceramic capacitors with X5R or X7R dielectrics are recommended due to their low ESR and high ripple current. The output ripple VOUT is determined by:
AAT1106
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
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 4.7µF. 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 · F · VIN(MAX) 2· 3 · 1
Output Capacitor Selection
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. There are three types of losses associated with the AAT1106 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
Thermal Calculations
The output capacitor limits the output ripple and provides holdup during large load transitions. A 4.7µF to 10µF 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 14
600mA Step-Down Converter
IQ is the step-down converter quiescent current. The term tsw is used to estimate the full load stepdown converter switching losses.
AAT1106
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 TSOT23-5 package which is 150°C / W.
Layout Guidance
When laying out the PC board, the following steps should be taken to ensure proper operation of the AAT1106. These items are also illustrated graphically in Figure 3.
1. The power traces (GND, LX, IN) should be kept short, direct, and wide to allow large current flow. Place sufficient multiply-layer pads when needed to change the trace layer. 2. The input capacitor (C1) should connect as closely as possible to IN (Pin 4) and GND (Pin 2). 3. The output capacitor C3 and L1 should be connected as closely as possible. The connection of L1 to the LX pin should be as short as possible and there should not be any signal lines under the inductor. 4. The feedback FB trace or OUT pin (Pin 5) 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 5) to minimize the length of the high impedance feedback trace. 5. The resistance of the trace from the load return to the GND (Pin 2) should be kept to a minimum. This will help to minimize any error in DC regulation due to differences in the potential of the internal signal ground and the power ground.
600mA Step-Down Converter
VIN 2.5V to 5.5V IN LX FB L1 2.2µH C2 22pF R2 634K VOUT 1.8V
AAT1106
C1 4.7µF
AAT1106-0.6
C3 10µF
R1 316K
a: Top Layer
b: Internal GND Plane
c: Bottom Layer
d: Middle Layer
Figure 3: AAT1106 Four-Layer Layout Example with the Internal GND Plane.
600mA Step-Down Converter Ordering Information
Output Voltage
Adj. 0.6 to VIN Fixed 1.5V Fixed 1.8V
AAT1106
TSOT23-5 TSOT23-5 TSOT23-5
Package
Marking1
VVXYY VXXYY VYXYY
Part Number (Tape & Reel)2
AAT1106ICB-0.6-T1 AAT1106ICB-1.5-T1 AAT1106ICB-1.8-T1
Package Information3
TSOT23-5
1.90 BSC
Top View
0.95 BSC
End View
All dimensions in millimeters.
Side View
Detail "A"
600mA Step-Down Converter
AAT1106
830 E. Arques Avenue, Sunnyvale, CA 94085 Phone (408) 737-4600 Fax (408) 737-4611 18
Advanced Analogic Technologies, Inc.