IRU3048CF IRF [International Rectifier], IRU3048CF Datasheet - Page 6

• Manufacturer Part Number: IRU3048CF
• Description: DUAL SYNCHRONOUS PWM CONTROLLER CIRCUITRY AND LDO CONTROLLER
• Manufacturer: IRF [International Rectifier]

IRU3048
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 set point can be
more accurate by using precision resistor.
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:
For a start-up time of 7.5ms, the soft-start capacitor will
be 0.1mF. Choose a ceramic capacitor at 0.1mF.
Boost Supply Vc
To drive the high-side switch it is necessary to supply a
gate voltage at least 4V greater than the bus voltage.
This is achieved by using a charge pump configuration
as shown in Figure 9. The capacitor is charged up to
approximately twice the bus voltage. A capacitor in the
range of 0.1mF to 1mF is generally adequate for most
applications.
Input Capacitor selection
The input filter capacitor should be based on how much
ripple the supply can tolerate on the DC input line. The
ripple current generated during the on time of control
MOSFET should be provided by input capacitor. The RMS
value of this ripple is expressed by:
For higher efficiency, a low ESR capacitor is recom-
mended.
6
I
t
Where:
C
Where:
D is the Duty Cycle, simply D=V
I
I
For V
Results to: I
And for V
Results to: I
For V
16TPB47M (16V, 47mF, 70mV, 1.4A)
For V
40mV, 1.9A).
RMS
RMS
OUT
START
SS
= I
is the soft-start capacitor (mF)
is the output current for each channel.
is the RMS value of the input capacitor current.
IN1
IN2
= 75 3 Css (ms)
I N 1
OUT
=12V, I
=5V, choose one 6TPC150M (6.3V, 150mF,
=12V, choose one Poscap from Sanyo
IN2
=5V, I
RMS1
RMS2
D3(1-D)
OUT1
=1.78A
=1.92A
OUT2
=4A and D1=0.275
=4A and D2=0.36
---(3)
---(2)
OUT
/V
IN.
www.irf.com
Output Capacitor Selection
The criteria to select the output capacitor is normally
based on the value of the Effective Series Resistance
(ESR). In general, the output capacitor must have low
enough ESR to meet output ripple and load transient
requirements, yet have high enough ESR to satisfy sta-
bility requirements. The ESR of the output capacitor is
calculated by the following relationship:
The Sanyo TPC series, PosCap capacitor is a good
choice. The 6TPC150M 150mF, 6.3V has an ESR 40mV.
Selecting two of these capacitors in parallel for each
output, results to an ESR of
low ESR goal.
The capacitor value must be high enough to absorb the
inductor's ripple current. The larger the value of capaci-
tor, the lower will be the output ripple voltage.
The resulting output ripple current is smaller then each
channel ripple current due to the 1808 phase shift. These
currents cancel each other. The cancellation is not the
maximum because of the different duty cycle for each
channel.
Inductor Selection
The inductor is selected based on output power, operat-
ing frequency and efficiency requirements. Low induc-
tor value causes large ripple current, resulting in the
smaller size, but poor efficiency and high output noise.
Generally, the selection of inductor value can be reduced
to desired maximum ripple current in the inductor ( i);
the optimum point is usually found between 20% and
50% ripple of the output current.
For the buck converter, the inductor value for desired
operating ripple current can be determined using the fol-
lowing relation:
Where:
V
V
f
ESR [
Where:
DV
DI
DV
V
L = (V
S
i = Inductor Ripple Current
IN
OUT
IN
= Switching Frequency
O
O
O
= Max Input Voltage
- V
= Output Current
=75mV and DI
= Output Voltage
= Output Voltage Ripple
IN
OUT
DV
DI
- V
= L3
O
O
OUT
)3
Di
Dt
V
O
IN
=3A, results to: ESR=25mV
; Dt = D3
3Di3f
V
---(4)
OUT
20mV which achieves our
S
f
1
S
D = Duty Cycle
t = Turn On Time
; D =
---(5)
V
V
OUT
IN
09/12/02
Rev. 1.7