iru3034 International Rectifier Corp., iru3034 Datasheet - Page 9

• Manufacturer Part Number: iru3034
• Description: 8-pin Pwm Switcher Controller Ic With Current Limiting -
• Manufacturer: International Rectifier Corp.

The peak power dissipated in the CS resistor is:
However, the average power dissipated is much lower
than 2W due to the long off time caused by the hiccup
circuit of IRU3034. The average power is in fact the short
circuit period divided by the short circuit period plus the
off time or "Hiccup" period. For example, if the short
circuit lasts for Tsc=100 s before the IRU3034 enters
hiccup, the average power is calculated as:
For example:
Without "Hiccup" technique, the power dissipation of the
resistor is 2W.
Switcher Output Voltage Setting
The output voltage can be set using the following
equations:
Assuming, Vo=3.38V and the selected output ripple is
1.3%(44mV) of the output voltage, a set of equations
are derived that selects the resistor divider and the hys-
teresis resistor:
Assuming, Rt = 1K , 1%:
Assuming,
Ppk = I
P
Where:
Dsc = Tsc / T
T
M = 200ms/ F
Css = The soft-start capacitor
For Css=0.1 F and Tsc=500 s=0.5ms
T
P
R
Where:
Rt = Top resistor of the resistor divider
R
and 4 of the IRU3034
output voltage)
R
Select R
HCP
HCP
AVG
AVG
Vo = Selected output ripple (typically 1% to 2% of
H
H
H
= (11 Rt) / Vo
= (11 1000) / 0.044 = 250K
= Hysteresis resistor connected between pins 3
= Css M
= 0.1 200 = 20ms
= 0.5 Ppk Dsc
= 0.5 2 (0.5/20) = 25mW
CL
H
2
Vo=44mV:
= 249K , 1%
Rs = 20
HCP
2
0.005 = 2W
www.irf.com
The bottom resistor of the divider is then calculated us-
ing the following equations:
Select R
Frequency Calculation
The IRU3034 frequency of operation is calculated using
the following formula:
For our example:
The ESR=18m
6MV1500GX caps. If L=3.5 H then, Fs is calculated as
follows:
R
Where:
R
V
X = (Vo + ( Vo/2)) / V
X= (3.38+ (0.044/2)) / 1.25 - 1 = 1.72
R
Fs = (Vo (1-D) ESR) / (L
Where:
Vo = Output voltage (V)
D = Duty cycle
ESR = Output capacitor ESR (V)
L = Output inductance ( H)
D
Where:
Vf = Forward voltage drop of the Schottky diode.
D = (3.38 + 0.5) / 5 = 0.78
Fs =
Fs = 87KHz
REF
Vo = Output ripple voltage (V)
B
B
B
= 1000 / 1.72 = 580
= Rt / X
= Bottom resistor of the divider
(Vo + Vf) / V
B
= 1.25V Typical
= 576 , 1%
(3.38
(3.5
(1-0.78)
IN
for 2 of the Sanyo 1500 F,
0.044)
IRU3034 & (PbF)
REF
0.018)
- 1
Vo)
= 0.087
(MHz)
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