IRU3037 / IRU3037A
APPLICATION INFORMATION
Design Example:
The following example is a typical application for IRU3037,
the schematic is Figure 18 on page 14.
V
IN
= 5V
V
OUT
= 3.3V
I
OUT
= 4A
DV
OUT
= 100mV
f
S
= 200KHz
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
1.25V (0.8V for IRU3037A). The divider is ratioed to pro-
vide 1.25V at the Fb pin when the output is at its desired
value. The output voltage is defined by using the follow-
ing equation:
V
OUT
= V
REF
3
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:
t
START
= 753Css (ms)
---(2)
Where:
C
SS
is the soft-start capacitor (mF)
For a start-up time of 7.5ms, the soft-start capacitor will
be 0.1mF. Choose a ceramic capacitor at 0.1mF.
Shutdown
The converter can be shutdown by pulling the soft-start
pin below 0.5V. The control MOSFET turns off and the
synchronous MOSFET turns on during shutdown.
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 18. 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. In application, when a separate voltage
source is available the boost circuit can be avoided as
shown in Figure 1.
Input Capacitor Selection
The input filter capacitor should be based on how much
ripple the supply can tolerate on the DC input line. The
larger capacitor, the less ripple expected but consider
should be taken for the higher surge current during the
power-up. The IRU3037 provides the soft-start function
which controls and limits the current surge. The value of
the input capacitor can be calculated by the following
formula:
C
IN
=
I
IN
3 Dt
DV
---(3)
(
R
6
1+
R
5
)
---(1)
When an external resistor divider is connected to the
output as shown in Figure 3.
V
OUT
IRU3037
Fb
R
5
R
6
Figure 3 - Typical application of the IRU3037 for
programming the output voltage.
Equation (1) can be rewritten as:
R
6
= R
5
3
(
V
V
OUT
REF
-1
)
Choose R
5
= 1KV
This will result to R
6
= 1.65KV
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.
Where:
C
IN
is the input capacitance (mF)
I
IN
is the input current (A)
Dt
is the turn on time of the high-side switch (ms)
DV
is the allowable peak to peak voltage ripple (V)
Rev. 2.8
03/10/03
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