iru3072 International Rectifier Corp., iru3072 Datasheet - Page 9

• Manufacturer Part Number: iru3072
• Description: 20-pin Synchronous Pwm Controller/ 3 Ldo Controller -
• Manufacturer: International Rectifier Corp.

When the power voltage such as Vcc go into steady
state and the output of voltage UVLO “POR” goes high,
a 20uA current source charges the external soft-start
capacitors. The soft-start voltage ramps up. In the mean
time, the current flowing into pin Fb1 starts to decrease
linearly and so does the voltage at the positive pin of
feedback UVLO comparator and the voltage at the nega-
tive input of Error amplifier. When the soft-start capacitor
voltage is around 1V, the current flowing into the Fb1 pin
is approximately 32mA. The voltage at the positive input
of the Error amplifier is approximately:
The Error Amplifier will start to operate and the output
voltage starts to increase. As the soft-start capacitor
voltage continues to go up, the current flowing into the
Fb1 pin will keep decreasing. Because the voltage at pin
of Error Amplier is regulated to reference voltage 0.8V,
the voltage at the Fb1 pin is:
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.
Figure 8 shows that the voltage at the positive pin of
feedback UVLO comparator is always higher than 0.4V,
therefore, feedback UVLO is not functional during soft-
start.
Rev. 1.0
3/25/04
of Error Amp and Feedback
Voltage at negative input
32mA325KV = 0.8V
V
FB1
Figure 8 - Theoretical operation waveforms
= 0.8V-25KV3(Injecting Current)
UVLO comparator
Voltage at Fb1 pin
Output of UVLO
Current flowing
into Fb1 pin
Soft-Start
Voltage
during soft-start.
POR
64uA
1.6V
0V
0V
0uA
0.8V
0.8V
3V
1V
2V
www.irf.com
From the above analysis, the output start up time is the
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:
For 5ms start up time, a 0.1mF soft-start capacitor is
required. In practice, the 20mA current will slightly de-
crease as the soft-start voltage goes up. Therefore, for a
0.1mF soft-start capacitor, start up time may be slightly
longer, e.g. 6ms.
The soft-start waveforms are shown in Figure 9. In this
figure, the start up time for the buck converter V
LDOs is different by selecting separate soft-start capaci-
tors.
Shutdown
The PWM output and LDO output can be turned on and
off individually by pulling up and down the correspond-
ing soft start capacitors.
20mA3t
C
For PWM: C
For LDOs: C
SS
Figure 9 - Soft-start of buck converter
20mA3t
START
SSLDO
SS
/C
START
(PWM) and LDO.
= 0.1mF, t
SS
= 33nF, t
/1V
2V-1V
START
---(1)
START
5ms
2ms
IRU3072
OUT1
and
9