ir3637s International Rectifier Corp., ir3637s Datasheet - Page 5

• Manufacturer Part Number: ir3637s
• Description: 1% Accurate Synchronous Pwm Controller
• Manufacturer: International Rectifier Corp.

THEORY OF OPERATION
Soft-Start
The IR3637 has a programmable soft-start to control the
output voltage rise and limit the current surge at the start-
up. To ensure correct start-up, the soft-start sequence
initiates when the Vc and Vcc rise above their threshold
(3.3V and 4.2V respectively) and generates the Power
On Reset (POR) signal. Soft-start function operates by
sourcing an internal current to charge an external ca-
pacitor to about 3V. Initially, the soft-start function clamps
the E/A’s output of the PWM converter and disables the
short circuit protection. During the power up, the output
starts at zero and voltage at Fb is below 0.4V. The feed-
back UVLO is disabled during this time by injecting a
current (64µA) into the Fb. This generates a voltage
about 1.6V (64µA×25K) across the negative input of E/
A and positive input of the feedback UVLO comparator
(see Figure 3).
The magnitude of this current is inversely proportional to
the voltage at soft-start pin.
The 20µA current source starts to charge up the exter-
nal capacitor. In the mean time, the soft-start voltage
ramps up, the current flowing into Fb pin starts to de-
crease linearly and so does the voltage at the positive
pin of feedback UVLO comparator and the voltage nega-
tive input of E/A.
When the soft-start capacitor is around 1V, the current
flowing into the Fb pin is approximately 32µA. The volt-
age at the positive input of the E/A is approximately:
The E/A will start to operate and the output voltage starts
to increase. As the soft-start capacitor voltage contin-
ues to go up, the current flowing into the Fb pin will keep
decreasing. Because the voltage at pin of E/A is regu-
lated to reference voltage 0.8V, the voltage at the Fb 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.
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.
Rev. 1.1
06/16/05
V
FB
= 0.8-25K×(Injected Current)
32µA×25K = 0.8V
www.irf.com
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:
SS/SD
Comp
of Error Amp and Feedback
0.8V
Fb
Voltage at negative input
25µA×T
C
Figure 4 - Theoretical operational waveforms
POR
SS
64uA × 25K=1.6V
25uA
Figure 3 - Soft-start circuit for IR3637.
≅ 25µA×T
UVLO comparator
When SS=0
Voltage at Fb pin
Output of UVLO
Current flowing
0.4V
START
25K
25K
into Fb pin
Soft-Start
Voltage
/C
during soft-start.
START
POR
SS
= 2V-1V
3V
64uA
Max
/1V
Error Amp
64uA
≅ 1.6V
0V
0V
Feeback
UVLO Comp
IR3637SPBF
POR
0uA
0.8V
0.8V
≅ 1V
≅ 2V
3V
HDrv
LDrv
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