L6728A STMicroelectronics, L6728A Datasheet - Page 11

• Manufacturer Part Number: L6728A
• Description: Voltage Mode PWM Controllers Hi-Freq. singl-phase PWM controller w/Po
• Manufacturer: STMicroelectronics

Specifications of L6728A

• Number Of Outputs: Dual Output
• Topology: Buck
• Output Voltage: 0.8 V
• Output Current: 1500 mA
• Switching Frequency: 660 KHz
• Duty Cycle (max): 67 %
• Maximum Operating Temperature: + 125 C
• Minimum Operating Temperature: - 40 C
• Package / Case: VFDFPN-10 EP
• Synchronous Pin: No
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

L6728A
6
6.1
Soft-start
L6728A implements a soft-start to smoothly charge the output filter avoiding 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 msec (typ.), in closed loop regulation, linearly
charging the output capacitors to the final regulation voltage. A pre-charged output voltage
will affect the soft-start duration, resulting in a reduction of this period of time (< 4 msec).
During the soft-start process all the protections but the UVP are active: the UVP becomes
active as soon as the soft-start ends up.
The device begins soft-start phase only when
and over-current threshold setting phase has been completed.
Low-side-less start up (LSLess)
In order to avoid any kind of negative undershoot and dangerous return from the load during
start-up, L6728A performs a special sequence in enabling LS driver to switch: during the
soft-start phase, the LS driver results disabled (LS = OFF) until the HS starts to switch. This
avoid the dangerous negative spike on the output voltage that 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 discharge 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: protections by-pass this turning ON the LS MOSFET in case of need.
Figure 4.
LSLess startup (left) vs. non-LSLess startup (right)
V
CC
power supply is above UVLO threshold
Soft-start
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