ir3637s International Rectifier Corp., ir3637s Datasheet - Page 6

• Manufacturer Part Number: ir3637s
• Description: 1% Accurate Synchronous Pwm Controller
• Manufacturer: International Rectifier Corp.

IR3637SPBF
APPLICATION INFORMATION
6
Design Example:
The following example is a typical application for IR3637.
Appliaction circuit is shown in page 12.
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 internally referenced to
0.8V. The divider is ratioed to provide 0.8V at the Fb pin
when the output is at its desired value. The output volt-
age is defined by using the following equation:
When an external resistor divider is connected to the
output as shown in Figure 5.
Equation (1) 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:
Figure 5 - Typical application of the IR3637 for pro-
V
Vc=12V
V
I
∆V
F
R
V
Choose R
This will result to R
OUT
IN
OUT
S
OUT
6
= 400kHz
OUT
= R
= Vcc = 5V
= 6A
= 1.8V
= V
= 50mV
5
gramming the output voltage.
×
REF
IR3637
5
(
= 1KΩ
×
V
V
OUT
REF
(
1 +
Fb
6
- 1
= 1.25KΩ
R
R
)
6
5
)
V
OUT
---(1)
R
R
5
6
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For a start-up time of 4ms, the soft-start capacitor will
be 0.1µF. Choose a ceramic capacitor at 0.1µF.
Boost Supply for Single 5V appliaction
To drive the high side switch, it is necessary to supply a
gate voltage at least 4V grater than the bus voltage. This
is achieved by using a charge pump configuration as
shown in Figure 6. This method is simple and inexpen-
sive. 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
the diode (D1). The 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 0.1µF and 1µF 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 V
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.
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:
Vc ≅ 2V
Css ≅ 25×
Where
IR3637
t
BUS
START
Figure 6 - Charge pump circuit.
- (V
t
START
V
is the desired start-up time (ms)
BUS
D1
+ V
(µF)
D2
)
C3
Vc
HDrv
C2
---(2)
D2
D1
C1
BUS
value, through
V
BUS
Q2
Q1
BUS
L
06/16/05
Rev.1.1
. The