L6728D STMicroelectronics, L6728D Datasheet - Page 12

• Manufacturer Part Number: L6728D
• Description: IC CTLR PWM 1PH POWERGOOD 10DFN
• Manufacturer: STMicroelectronics

Specifications of L6728D

• Pwm Type: Voltage Mode
• Number Of Outputs: 1
• Frequency - Max: 330kHz
• Duty Cycle: 80%
• 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: 10-DFN
• Frequency-max: 330kHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Soft-start
7
7.1
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Soft-start
The L6728D implements a soft-start to smoothly charge the output filter, avoiding the high
in-rush currents to be required from the input power supply. The device gradually increases
the internal reference from 0 V to 0.8 V in 4.5 ms (typ.), in closed-loop regulation, linearly
charging the output capacitors to the final regulation voltage.
In the event of overcurrent triggering during soft-start, the overcurrent logic overrides the
soft-start sequence and shuts down the PWM logic and both the high side and low side
gates. This condition is latched. Cycle the VCC to recover.
The device begins the soft-start phase only when the VCC power supply is above the UVLO
threshold and the overcurrent threshold setting phase has been completed.
Low-side-less startup (LSLess)
In order to avoid any kind of negative undershoot and dangerous return from the load during
startup, the L6728D performs a special sequence in enabling the LS driver to switch: during
the soft-start phase, the LS driver is disabled (LS = OFF) until the HS starts to switch. This
prevents the dangerous negative spike on the output voltage which can happen if starting
over a pre-biased output.
If the output voltage is pre-biased to a voltage higher than the final one, the HS would never
start to switch. In this case, at the end of soft-start time, LS is enabled and discharges the
output to the final regulation value.
This particular feature of the device masks the LS turn-on only from the control loop point of
view: protection features bypass this turning on of the LS MOSFET if needed.
Figure 4.
LSLess startup (left) vs. non-LSLess startup (right)
Doc ID 16498 Rev 1
L6728D