iru3033 International Rectifier Corp., iru3033 Datasheet - Page 10

• Manufacturer Part Number: iru3033
• Description: 8-pin Pwm Switcher And Linear Controller Ic
• Manufacturer: International Rectifier Corp.

IRU3033
For example, if Rt=1K and R
ripple is:
The advantage of fixed output ripple is that when the
output voltage changes from 2V to 3.5V, the ripple volt-
age remains the same which is important in meeting the
Intel maximum tolerance specification.
Switcher Output Voltage Setting
The output voltage of the switcher 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=1KV , 1%:
Assuming, DVo=44mV:
The bottom resistor of the divider is then calculated us-
ing the following equations:
10
DVo = (1/422)311 = 26mV
R
Where:
Rt = Top resistor of the resistor divider.
R
and 4 of the IRU3033.
DVo = Selected output ripple (typically 1% to 2% of
output voltage).
R
Select R
R
Where:
R
V
X = (Vo +
X = (3.38 +
R
Select R
H
H
H
B
B
REF
B
= Hysteresis resistor connected between pins 3
= Rt / X
= Bottom resistor of the divider
= 1000 / 1.72 = 580V
= (113Rt) / DV
= (1131000) / 0.044 = 250KV
= 1.25V typical
H
B
= 576V, 1%
= 249KV, 1%
DVo
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0.044
2
2
)/V
)/1.25 - 1 = 1.72
REF
- 1
B
=422K, then the output
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Data and specifications subject to change without notice. 02/01
Frequency Calculation
The IRU3033 frequency of operation is calculated using
the following formula:
For our example:
The ESR=18mV for 2 of the Sanyo 1500mF, 6MV1500GX
caps. If L=3.5mH then, Fs is calculated as follows:
Fs = (Vo3(1 - D)3ESR) / (L3DVo)
D (Vo + Vf) / V
D = (3.38 + 0.5) / 5 = 0.78
Where:
Vo = Output voltage
D = Duty cycle
ESR = Output capacitor ESR
L = Output inductance
DVo = Output ripple voltage
Where:
Vf = Forward voltage drop of the Schottky diode.
Fs =
Fs = 87KHz
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(3.38 3 (1 - 0.78) 3 0.018)
(3.5 3 0.044)
IN
(V)
(mH)
TAC Fax: [phone]
(V)
(V)
= 0.087MHz
(MHz)
07/17/02
Rev. 1.7