PM6681A STMicroelectronics, PM6681A Datasheet - Page 25

• Manufacturer Part Number: PM6681A
• Description: Dual synchronous step down controller with adjustable LDO
• Manufacturer: STMicroelectronics

PM6681A
7.7
7.8
soft-start and soft-end
Each switching section is enabled separately by asserting high EN1/EN2 pins respectively.
In order to realize the soft-start, at the startup the overcurrent threshold is set 25 % of the
nominal value and the undervoltage protection (see related sections) is disabled. The
controller starts charging the output capacitor working in current limit. The overcurrent
threshold is increased from 25 % to 100 % of the nominal value with steps of 25 % every
700 µs (typ.). After 2.8 ms (typ.) the undervoltage protection is enabled. The s oft start time
is not programmable. A minimum capacitor C
any overshoot on the output:
Equation 10
Figure 37. Soft-start waveforms
When a switching section is turned off (EN1/EN2 pins low), the controller enters in soft-end
mode. The output capacitor is discharged through an internal 18 Ω P-MOSFET switch;
when the output voltage reaches 0.3 V, the low-side MOSFET turns on, keeping the output
to ground. The soft-end time also depends on load condition.
Gate drivers
The integrated high-current drivers allow to use different power MOSFETs. The high side
driver MOSFET uses a bootstrap circuit which is indirectly supplied by LDO5 output. The
BOOT and PHASE pins work respectively as supply and return rails for the HS driver.
The low side driver uses the internal LDO5 output for the supply rail and PGND pin as return
rail.
An important feature of the gate drivers is the adaptive anti-cross conduction protection,
which prevents high side and low side MOSFETs from being on at the same time. When the
Switching
Current limit threshold
EN1/EN2
C
INT
≥
I
Lvalley
4
6
INT
uA
+
is required to ensure a soft-start without
∆
2
I
L
×
C
out
Device description
Time
25/47