L6738ATR STMicroelectronics, L6738ATR Datasheet - Page 14

• Manufacturer Part Number: L6738ATR
• Description: IC PWM CTLR 1PH STPDN 16-VFQFPN
• Manufacturer: STMicroelectronics

Specifications of L6738ATR

• Frequency - Max: 220kHz
• Pwm Type: Voltage Mode
• Number Of Outputs: 1
• Duty Cycle: 100%
• Voltage - Supply: 5 V ~ 12 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 32-VFQFN Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 497-11012-2

Soft-start
5
5.1
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Soft-start
The L6738A implements a soft-start to smoothly charge the output filter avoiding high in-
rush currents to be required to the input power supply. During this phase, the device
increases the internal reference from zero up to 0.8 V in closed loop regulation. The soft-
start is implemented only when VCC and VCCDR are above their own UVLO threshold and
the EN pin is set free.
When SS takes place, the IC initially waits for 1024 clock cycles and then starts ramping-up
the reference in 1024 clock cycles in closed-loop regulation. At the end of the digital soft-
start, the PWRGOOD signal is set free with 3x clock cycles delay.
Protections are active during this phase as follows:
●
●
●
Soft-start time depends on the programmed frequency, initial delay and reference ramp-up
lasts for 1024 clock cycles. SS time and initial delay can be determined as follows:
LS-less startup
In order to avoid any kind of negative undershoot on the load side during startup, the
L6738A performs a special sequence in enabling the drivers for both sections: during the
soft-start phase, the LS MOSFET is kept OFF until the first PWM pulse. This particular
sequence avoids the dangerous negative spike on the output voltage that can happen if
starting over a pre-biased output.
Low-side MOSFET turn-on is masked only from the control loop point of view: protections
are still allowed to turn on the low-side MOSFET in the case of overvoltage, if needed.
undervoltage is enabled when the reference voltage reaches 80 % of the final value
overvoltage is always enabled
FB disconnection is enabled
Doc ID 18134 Rev 1
T
SS
[
ms
]
=
------------------------------
Fsw kHz ]
1024
[
]
L6738A