iru3039 International Rectifier Corp., iru3039 Datasheet - Page 6

• Manufacturer Part Number: iru3039
• Description: Ic Ctrlr Pwm Sync Switch 20-mlpq
• Manufacturer: International Rectifier Corp.

IRU3039(PbF)
The feedback voltage increases linearly as the injecting
current goes down. The injecting current drops to zero
when soft-start voltage is around 2V and the output volt-
age goes into steady state.
As shown in Figure 4, the positive pin of feedback UVLO
comparator is always higher than 0.4V, therefore, feed-
back UVLO is not functional during soft-start.
the output start-up time is the time period when soft-
start capacitor voltage increases from 1V to 2V. The start-
up time will be dependent on the size of the external
soft-start capacitor. The start-up time can be estimated
by:
For a given start up time, the soft-start capacitor can be
estimated as:
Internal Regulator
The regulator powers directly from Vcc and generates a
regulated voltage (6V @ 40mA). The output is protected
for short circuit. This voltage can be used for charge
pump circuitry as shown in Figure 1.
Supply Voltage Under-Voltage Lockout
The under-voltage lockout circuit assures that the
MOSFET driver outputs remain in the off state whenever
the supply voltage drops below set parameters. Lockout
occurs if Vc or Vcc fall below 3.4V and 4.4V respec-
tively. Normal operation resumes once Vc and Vcc rise
above the set values.
6
of Error Amp and Feedback
Voltage at negative input
20µA×T
C
Figure 4 - Theoretical operational waveforms
SS
UVLO comparator
20µA×T
Voltage at Fb pin
Output of UVLO
Current flowing
START
into Fb pin
Soft-Start
Voltage
/C
START
during soft-start.
POR
SS
= 2V-1V
/1V
64uA
1.6V
0V
0V
0uA
0.8V
0.8V
3V
1V
2V
www.irf.com
Shutdown
The converter can be shutdown by pulling the soft-start
pin below 0.4V. The control MOSFET turns off and the
synchronous MOSFET turns on during shutdown.
Over-Current Protection
Over-current protection is achieved with a cycle by cycle
scheme and it is performed by sensing current through
the R
an external resistor (R
pin and the drain of low side MOSFET (Q2) and sets the
current limit set point. The internal current source devel-
ops a voltage across R
turned on, the inductor current flows through the Q2 and
results a voltage which is given by:
When voltage V
comparator flips and disables the oscillator. The high
side MOSFET is turned off and the low side MOSFET is
turned on until the inductor current reduces to below
current set value. The critical inductor current can be
calculated by setting:
If the over-current condition is temporary and goes away
quickly, the IRU3039 will resume its normal operation.
If output is shorted or over-current condition persists,
the output voltage will keep going down until it is below
0.4V. Then the output under-voltage lock out comparator
goes high and turns off both MOSFETs. The operation
waveforms are shown in Figure 6.
Osc
V
V
I
Figure 5 - Diagram of the over current sensing.
SET
DS(ON)
OCSET
OCSET
IRU3039
= I
L(CRITICAL)
of low side MOSFET. As shown in Figure 5,
= I
= I
OCSET
OCSET
I
OCSET
OCSET
×R
×R
=
R
is below zero, the current sensing
SET
SET
SET
SET
SET
R
) is connected between OCSet
OCSet
-R
×I
DS(ON)
- R
. When the low side switch is
DS(ON)
OCSET
DS(ON)
R
SET
×i
×I
L
L
= 0
Q1
Q2
---(1)
---(2)
L1
V
OUT