8
Rev. 1.0
09/17/03
IRU3073
www.irf.com
VIN - VOUT = L3 ; Dt = D3 ; D =
1
fS
VOUT
VIN
Di
Dt
L = (VIN - VOUT)3 ---(11)
VOUT
VIN3Di3fS
Where:
VIN = Maximum Input Voltage
VOUT = Output Voltage
i = Inductor Ripple Current
fS = Switching Frequency
t = Turn On Time
D = Duty Cycle
APPLICATION INFORMATION
Design Example:
The following example is a typical application for IRU3073,
the schematic is Figure 17 on page 16.
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 (VP) is connected to reference voltage (VREF).
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.
Figure 8 - Typical application of the IRU3039 for
programming the output voltage.
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:
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:
VOUT = VP 3 1 + ---(7)
R6
R5
VP = VREF = 0.8V
( )
Fb
IRU3073
VOUT
R5
R6
VREF
VP
Css E 203tSTART (mF) ---(8)
Where tSTART is the desirable start-up time (s)
For VIN=5V, IOUT=8A and D=0.5, the IRMS=4A
IRMS = IOUT D3(1-D)     ---(9)
Where:
D is the Duty Cycle, D=VOUT/VIN.
IRMS is the RMS value of the input capacitor current.
IOUT is the output current for each channel.
R6 = R5 3 - 1
VOUT
VP
( )
Choose R5 = 1K. This will result to R6 = 2.15K
Switcher
VIN = 5V
VOUT = 2.5V
IOUT = 8A
DVOUT = 50mV
fS = 200KHz
Linear Regulator
VIN = 2.5V
VOUT = 1.6V
IOUT = 2A
Supply Voltage
VCC=VCL=VCH=12V