iru3073 International Rectifier Corp., iru3073 Datasheet - Page 8

• Manufacturer Part Number: iru3073
• Description: Synchronous Pwm Controller With Over-current Protection / Ldo Controller -
• Manufacturer: International Rectifier Corp.

IRU3073
APPLICATION INFORMATION
Design Example:
The following example is a typical application for IRU3073,
the schematic is Figure 17 on page 16.
Supply Voltage
V
Output Voltage Programming
Output voltage is programmed by reference voltage and
external voltage divider. The Fb 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 (V
The output voltage is defined by using the following equa-
tion:
When an external resistor divider is connected to the
output as shown in Figure 8.
Equation (7) can be rewritten as:
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:
8
CC
=V
Figure 8 - Typical application of the IRU3039 for
Choose R
V
V
R
Css
Where
OUT
P
6
C
L=V
= R
= V
= V
5
C
REF
203
programming the output voltage.
t
3
H=12V
START
P
5
P
= 0.8V
V
V
(
3 1 +
= 1K. This will result to R
) is connected to reference voltage (V
t
REF
P
IRU3073
START
(
V
is the desirable start-up time (s)
V
OUT
P
Switcher
V
V
I
DV
f
OUT
S
R
R
(mF)
IN
OUT
- 1
= 200KHz
6
5
OUT
)
= 5V
= 8A
Fb
)
= 2.5V
= 50mV
---(8)
---(7)
Linear Regulator
V
V
I
OUT
V
IN
OUT
6
OUT
= 2.15K
= 2.5V
= 2A
R
R
= 1.6V
6
5
www.irf.com
REF
).
For a start-up time of 5ms, the soft-start capacitor will
be 0.1mF. Choose a ceramic capacitor at 0.1mF.
Supply VcL and VcH
To drive the high side switch, it is necessary to supply a
gate voltage at least 4V greater than the Bus voltage.
For this application, VcL and VcH are biased with a sepa-
rate 12V supply.
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 upper
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. Choose two Poscap from Sanyo 6TPB47M
(16V, 47mF) with a max allowable ripple current of 5.2A.
Inductor Selection
The inductor is selected based on operating frequency,
transient performance and allowable output voltage ripple.
Low inductor value results to faster response to step
load (high di/dt) and smaller size but will cause larger
output ripple due to increase of inductor ripple current.
As a rule of thumb, select an inductor that produces a
ripple current of 10-40% of full load DC.
For the buck converter, the inductor value for desired
operating ripple current can be determined using the fol-
lowing relation:
V
L = (V
Where:
V
V
f
D = Duty Cycle
I
Where:
D is the Duty Cycle, D=V
I
I
For V
S
RMS
RMS
OUT
i = Inductor Ripple Current
t = Turn On Time
IN
IN
OUT
= Switching Frequency
- V
= Maximum Input Voltage
= I
is the RMS value of the input capacitor current.
is the output current for each channel.
= Output Voltage
IN
IN
OUT
OUT
=5V, I
- V
= L3
OUT
D3(1-D)
OUT
)3
=8A and D=0.5, the I
Dt
Di
V
IN
; Dt = D3
3Di3f
V
OUT
OUT
S
---(9)
/V
IN.
f
1
S
; D =
---(11)
RMS
=4A
V
V
OUT
IN
09/17/03
Rev. 1.0