IR3820MTRPBF International Rectifier, IR3820MTRPBF Datasheet - Page 9

• Manufacturer Part Number: IR3820MTRPBF
• Description: IC REG SYNC BUCK 12A 5X6 15QFN
• Manufacturer: International Rectifier
• Series: SupIRBuck™r
• Type: Step-Down (Buck)r

Specifications of IR3820MTRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 12 V
• Current - Output: 12A
• Frequency - Switching: 600kHz
• Voltage - Input: 2.5 ~ 21 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 15-PowerVQFN
• Power - Output: 3.7W
• Part Status: Active
• Package: PQFN / 5 x 6
• Circuit: Single Output
• Iout (a): 12
• Switch Freq (khz): 600
• Input Range (v): 2.5 - 21
• Output Range (v): 0.6 - 12
• Ocp Otp Uvlo Pre-bias Soft Start And: PGOOD
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: IR3820MTRPBFTR

For a given start-up time, the soft-start capacitor
can be estimated as:
Soft-Start
The IR3820 has programmable soft-start to
control the output voltage rise and limit the inrush
current during start-up.
To
sequence initiates when Vcc and Vc rise above
their threshold and generate the Power On
Ready (POR) signal. The soft-start function
operates by sourcing current to charge an
external capacitor to about 3V.
Initially, the soft-start function clamps the output
of error amplifier by injecting a current (40uA)
into the Fb pin and generates a voltage about
0.96V (40ux24K) across the negative input of
error amplifier (see figure 5).
The magnitude of the injected current is inversely
proportional to the voltage at the soft-start pin. As
the soft-start voltage ramps up, the injected
current decreases linearly and so does the
voltage at negative input of error amplifier.
When the soft-start capacitor is around 1V, the
voltage at the positive input of the error amplifier
is approximately 0.6V.
The output of error amplifier will start increasing
and generating the first PWM signal. As the soft-
start capacitor voltage continues to rise up, the
current flowing into the Fb pin will keep
decreasing.
The feedback voltage increases linearly as the
soft start voltage ramps up. When soft-start
voltage is around 2V, the output voltage reaches
the steady state and the injected current is
Figure
waveforms during soft-start.
The output voltage start-up time is the time
period
increases from 1V to 2V.
The start-up time will be dependent on the size of
the external soft-start capacitor and can be
estimated by:
11/04/08
ensure
C
6
when
SS
20
≅
shows
20
μA
correct
μ
∗
A
soft-start
T
C
*
start
ss
T
start
the
=
(
2
start-up,
ms
V
−
theoretical
)
1
V
capacitor
--(
) 1
the
operating
soft-start
voltage
zero.
Voltage at negative input
SS/SD
Comp
Voltage at Fb pin
Output of UVLO
Current flowing
0.6V
Fig. 6: Theoretical operation waveforms
Fig. 5: Soft-Start circuit for IR3820
Fb
of Error Amp
POR
into Fb pin
Soft-Start
20uA
Voltage
POR
≅ 0.96V
during soft-start
24K
24K
40uA
0V
0V
IR3820MPbF
3V
40uA
Error Amp
PD-60329
0uA
0.6V
0.6V
≅ 1V
≅ 2V
3V
9