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APU3048 View Datasheet(PDF) - Advanced Power Electronics Corp

Part Name
Description
Manufacturer
APU3048
A-POWER
Advanced Power Electronics Corp A-POWER
APU3048 Datasheet PDF : 17 Pages
1 2 3 4 5 6 7 8 9 10 Next Last
APU3048
Cross Over Frequency:
FO1
=
R73C103
VIN
VOSC
3
1
2p3Lo3Co
Where:
VIN = Maximum Input Voltage
VOSC = Oscillator Ramp Voltage
Lo = Output Inductor
Co = Total Output Capacitors
---(15)
The stability requirement will be satisfied by placing the
poles and zeros of the compensation network according
to following design rules. The consideration has been
taken to satisfy condition 14 regarding transconduc-
tance error amplifier.
1) Select the crossover frequency:
Fo < FESR and Fo [ (1/10 ~ 1/6)3 fS
2
2) Select R7, so that R7 >> gm
3) Place first zero before LC’s resonant frequency pole.
FZ1 75% FLC
C11 =
1
2p 3 FZ1 3 R7
4) Place third pole at the half of the switching frequency.
FP3 =
fS
2
C12 =
1
2p 3 R7 3 FP3
C12 > 50pF
If not, change R7 selection.
5) Place R7 in (15) and calculate C10:
8) Use equation (1) to calculate R5:
R5 =
VREF
VOUT - VREF
3 R6
These design rules will give a crossover frequency ap-
proximately one-tenth of the switching frequency. The
higher the band width, the potentially faster the load tran-
sient speed. The gain margin will be large enough to
provide high DC-regulation accuracy (typically -5dB to -
12dB). The phase margin should be greater than 458 for
overall stability.
LDO Section
Output Voltage Programming
Output voltage for LDO is programmed by reference volt-
age and external voltage divider. The Fb3 pin is the in-
verting input of the error amplifier, which is internally ref-
erenced to 1.25V. The divider is ratioed to provide 1.25V
at the Fb3 pin when the output is at its desired value.
The output voltage is defined by using the following equa-
tion:
( ) VOUT2 = VREF3
1+
RHIGH
RLOW
For:
VOUT2 = 2.5V
VREF = 1.25V
RLOW = 1K
Results to: RHIGH=1K
LDO Power MOSFET Selection
The first step in selecting the power MOSFET for the
linear regulator is to select the maximum RDS(ON) based
on the input to the dropout voltage and the maximum
load current.
C10 [
2p 3 Lo 3 FO 3 Co
R7
3
VOSC
VIN
6) Place second pole at ESR zero.
FP2 = FESR
R8
=
2p
3
1
C10
3
FP2
1
Check if R8 > gm
If R8 is too small, increase R7 and start from step 2.
7) Place second zero around the resonant frequency.
FZ2 = FLC
RDS(ON) =
VIN3 - VOUT2
IOUT2
For:
VIN3 = 3.3V
VOUT2 = 2.5V
IOUT2 = 2A
Results to: RDS(ON)(MAX) = 0.4V
Note that since the MOSFET RDS(ON) increases with tem-
perature, this number must be divided by ~1.5 in order
to find the RDS(ON)(MAX) at room temperature. The AP20T03GH
has a maximum of 0.05V RDS(ON) at room temperature,
which meets our requirements.
R6 =
1
2p 3 C10 3 FZ2
-
R8
10
 

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