iru3055 International Rectifier Corp., iru3055 Datasheet - Page 10

• Manufacturer Part Number: iru3055
• Description: 5-bit Programmable 3-phase Synchronous Buck Controller Ic -
• Manufacturer: International Rectifier Corp.

IRU3055
When the inductor current is large enough, the voltage
across the low side switch is low enough so that the
voltage at OCSet node is below zero and the compara-
tor will flip and trigger a switch to discharge the soft-start
capacitor at a certain slope rate. The system enters into
a hiccup mode. The over current threshold can be set by
resistor R
then the current flowing into phase 1 will be one third of
the total output current. The maximum allowed output
current can be represented as:
Where R
In practice, the R
pendent. The overhead has to be considered. For prac-
tice, over current threshold has to be at least 50% higher
than the nominal current plus ripple. In the demo-board,
the maximum output current is set to be:
For each phase, the maximum current is one third (33A),
assuming good current sharing. The low side of MOSFET
is IRF3711S. The On resistor at 150 degrees is given
from the data sheet:
The over current setting resistor can be set as
10
Figure 9 - Operation waveforms at short circuit.
I
R
I
Consider ripple current, select I
R
R
Select R
MAX
MAX
SET
DS(ON)
SET
Ch1: Input current, 5A/div.
Ch2: Phase 1 inductor current, 10A/div.
Ch3: Soft-start capacitor voltage, 5V/div.
Ch4: Output voltage, 2V/div.
= 160mA 3 R
= (1+50%)3I
DS(ON)
= 33A 3 0.009/160mA = 1.86K
= I
SET
= 1.536mV = 9mV
MAX
. Suppose the current sharing is perfect,
SET
is the ON resistance of low side MOSFET.
3R
= 2.2K
DS(ON)
DS(ON)
(Hiccup mode)
OUT
SET
/3/160mA
of MOSFET is temperature de-
= 1.5360A = 90A
/ (R
DS(ON)
/3)
MAX
=100A
---(3)
www.irf.com
Over Voltage Protection
The Fb pin is connected to the output voltage. An over-
voltage condition is detected when the voltage at Fb pin
is 15% higher than the programmed voltage by DAC.
When the overvoltage occurs, the soft-start capacitor is
discharged. The high side MOSFETs are turned off and
the low side MOSFETs are turned on. As a result, the
low side MOSFET of synchronous rectifier conduct and
shunt the output voltage to ground and protect the load.
In the meantime, the PGood pin is held to low.
Soft-Start
The IRU3055 has a soft-start function to limit the current
surge at the start-up. An external capacitor which is
charged by a 10mA internal current source is used to
program the soft-start timing. The voltage of the external
capacitor linearly increases, which forces the output volt-
age to go up linearly until the voltage at soft-start reaches
the desired voltage. The following equation can be used
to calculate the start up time.
For a 7.5ms start-up time and 1.5V output, the required
capacitor will be 33nF.
Operation Frequency Selection
The operation switching frequency is determined by an
external resistor (Rt). The switching frequency is approxi-
mately inversely proportioned to resistance (see Fig.10).
The switching frequency can also be estimated by:
For example, if the 150KHz switching frequency is se-
lected, the required Rt is calculated as:
10mA3t
t
Where:
Css is the soft-start capacitor (mF).
V
sired output voltage.
Fs
Where Rt is in KV and Fs is in KHz.
Rt
START
SET
Figure 10 - The operation frequency vs. Rt.
500
400
300
200
100
7500/150 = 50KV
is the voltage from DAC and equal to the de-
= (V
7500/Rt
0
0
START
SET
Rt(K V )
10
+0.7V)3Css/10mA
/Css = V
Frequency versus Rt
20
---(5)
SET
30
+0.7V
40
frequency
50
60
---(4)
08/13/02
70
Rev. 1.4