APU3146 Datasheet, PDF (10/28 Pages) Advanced Power Electronics Corp. – DUAL SYNCHRONOUS PWM CONTROLLER WITH CURRENT SHARING CIRCUITRY AND AUTO-RESTART

APU3146
APPLICATION INFORMATION
Design Example:
The following example is a typical application for APU3146,
the schematic is Figure18 on page17.
VIN = 12V
VOUT(2.5V) = 2.5V @ 10A
VOUT(1.8V) = 1.8V @ 10A
∆VOUT = Output voltage ripple ≅ 3% of VOUT
FS = 300KHz
Output Voltage Programming
Output voltage is programmed by reference voltage and
external voltage divider. The Fb1 pin is the inverting input
of the error amplifier, which is referenced to the voltage
on non-inverting pin of error amplifier. For this applica-
tion, this pin (VP) is connected to reference voltage (VREF).
The output voltage is defined by using the following equa-
tion:
( ) ^{VOUT = VP ×}
1+
R6
R5
---(4)
VP2 = VREF = 0.8V
When an external resistor divider is connected to the
output as shown in Figure 11.
VOUT
APU3146
R6
VREF
Fb
VP
R5
Figure 11 - Typical application of the APU3146 for
programming the output voltage.
Soft-Start Programming
The soft-start timing can be programmed by selecting
the soft-start capacitance value. The start-up time of
the converter can be calculated by using:
Css ≅ 25×tSTART (µF) ---(5)
Where tSTART is the desired start-up time (ms)
For a start-up time of 4ms for both output, the soft-start
capacitor will be 0.1µF. Connect ceramic capacitors at
0.1µF from SS1 pin and SS2 pin to GND.
Supply VCH1 and VCH2
To drive the high side switch, it is necessary to supply
a gate voltage at least 4V grater than the bus voltage.
This is achieved by using a charge pump configuration
as shown in Figure 12. This method is simple and inex-
pensive. The operation of the circuit is as follows: when
the lower MOSFET is turned on, the capacitor (C1)
charges up to VOUT3, through the diode (D1). The bus
voltage will be added to this voltage when upper
MOSFET turns on in next cycle, and providing supply
voltage (VCH1) through diode (D2). Vc is approximately:
VCH1 ≅ VOUT3 + VBUS - (VD1 + VD2)
Capacitors in the range of 0.1µF and 1µF are generally
adequate for most applications. The diode must be a
fast recovery device to minimize the amount of charge
fed back from the charge pump capacitor into VOUT3.
The diodes need to be able to block the full power rail
voltage, which is seen when the high side MOSFET is
switched on. For low voltage application, schottky di-
odes can be used to minimize forward drop across the
diodes at start up.
Equation (4) can be rewritten as:
( ) ^{R6 = R5 ×}
VOUT
VP
-1
Will result to:
VOUT(2.5V) = 2.5V
VREF = 0.8V
R9= 2.14K, R5= 1K
VOUT(1.8V) = 1.8V
VREF = 0.8
R7= 1.24K, R8 = 1K
If the high value feedback resistors are used, the input
bias current of the Fb pin could cause a slight increase
in output voltage. The output voltage can be set more
accurately by using low value, precision resistors.
VOUT3
D1
C3
D2
Regulator
VCH1
VBUS
C2 C1
Q1
L2
APU3146
HDrv
Q2
Figure 12 - Charge pump circuit.
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