iru3137 International Rectifier Corp., iru3137 Datasheet - Page 7

• Manufacturer Part Number: iru3137
• Description: 8-pin Synchronous Pwm Controller
• Manufacturer: International Rectifier Corp.

APPLICATION INFORMATION
Design Example:
The following example is a typical application for IRU3137,
the schematic is Figure 13 on page 15.
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. The output voltage
is defined by using the following equation:
When an external resistor divider is connected to the
output as shown in Figure 5.
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:
Rev. 1.0
06/22/04
Figure 5 - Typical application of the IRU3137 for
V
V
I
(output voltage ripple ≅ 3% of V
f
V
V
R
Choose R
This will result to R
OUT
S
IN
OUT
V
OUT
REF
6
= 200KHz
OUT
= R
= 5V
= 15A
= 0.8V
= 2.5V
= V
= 75mV
5
programming the output voltage.
REF
IRU3137
5
(
= 1K
V
V
(
OUT
REF
1 +
6
R
R
Fb
- 1
= 2.125K
6
5
)
)
Supply Voltage
V
CC
---(7)
= V
OUT
V
OUT
)
C
R
R
= 12V
5
6
www.irf.com
For a start-up time of 5ms, the soft-start capacitor will
be 0.1 F. Choose a ceramic capacitor at 0.1 F.
Boost Supply Vc
To drive the high side switch, it is necessary to supply a
gate voltage at least 4V grater than the bus voltage. For
single supply applications, this is achieved by using a
charge pump configuration as shown in Figure 6. This
method is simple and inexpensive. The operation of the
circuit is as follows: when the lower MOSFET is turned
on, the capacitor (C1) is pulled down to ground and
charges, up to V
bus voltage will be added to this voltage when upper
MOSFET turns on in next cycle, and providing supply
voltage (Vc) through diode (D2). Vc is approximately:
Capacitors in the range of 0.1 F and 1 F 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 Vc. The
diodes need to be able to block the full power rail volt-
age, 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. For this application, Vc is biased by
an external 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:
Css = 20
V
Where t
C
IRU3137
≅ 2
Figure 6 - Charge pump circuit.
START
V
t
BUS
START
BUS
is the desired start-up time (ms)
- (V
value, through the diode (D1). The
Vc
HDrv
( F)
D1
C2
+ V
D2
D2
)
---(8)
C1
D1
IRU3137
V
BUS
Q2
Q1
L2
7