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IRU3048CF Datasheet(PDF) 10 Page - International Rectifier

Part # IRU3048CF
Description  DUAL SYNCHRONOUS PWM CONTROLLER CIRCUITRY AND LDO CONTROLLER
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Manufacturer  IRF [International Rectifier]
Direct Link  http://www.irf.com
Logo IRF - International Rectifier

IRU3048CF Datasheet(HTML) 10 Page - International Rectifier

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10
Rev. 1.7
09/12/02
IRU3048
www.irf.com
Cross Over Frequency:
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:
2)
Select R7, so that R7 >>
3)
Place first zero before LC’s resonant frequency pole.
4)
Place third pole at the half of the switching frequency.
C12 > 50pF
If not, change R7 selection.
5)
Place R7 in (15) and calculate C10:
6)
Place second pole at ESR zero.
FP2 = FESR
Check if R8 >
If R8 is too small, increase R7 and start from step 2.
7)
Place second zero around the resonant frequency.
FZ2 = FLC
FP3 =
fS
2
8)
Use equation (1) to calculate R5:
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 45
8 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:
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.
Results to: RDS(ON)(MAX) = 0.4
V
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 IRLR2703
has a maximum of 0.065
V RDS(ON) at room temperature,
which meets our requirements.
1
gm
C12 =
1
2
p 3 R7 3 FP3
C10
[
3
2
p 3 Lo 3 FO 3 Co
R7
VOSC
VIN
R8 =
1
2
p 3 C10 3 FP2
R6 =
- R8
1
2
p 3 C10 3 FZ2
R5 =
3 R6
VREF
VOUT - VREF
RDS(ON) =
VIN3 - VOUT2
IOUT2
For:
VIN3 = 3.3V
VOUT2 = 2.5V
IOUT2 = 2A
Where:
VIN = Maximum Input Voltage
VOSC = Oscillator Ramp Voltage
Lo = Output Inductor
Co = Total Output Capacitors
FO1 = R7
3C103
3
VIN
VOSC
1
2
p3Lo3Co
---(15)
Fo < FESR and Fo
[ (1/10 ~ 1/6)3 fS
2
gm
FZ1
≅ 75% FLC
C11 =
1
2
p 3 FZ1 3 R7
For:
VOUT2 = 2.5V
VREF = 1.25V
RLOW = 1K
(
)
RHIGH
RLOW
VOUT2 = VREF
3 1+


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