Datasheet

Vss
Gate driver
delayss
t
_
ss
t
1.2V
4V
1 2
175
SS _ delay SS
. V
t C
Am
=
1 1
6 1
2
150
1 81
2 2
=
´
+
æ ö
+ -
ç ÷
W
è ø
S
SS
RT
f
ns A
ns
V
I . mA
. k
2 8
5
SS SS
. V
t C
Am
=
UCC25600
www.ti.com SLUS846B –SEPTEMBER 2008– REVISED JULY 2011
Soft Start
During start up and fault recovery conditions, soft start is always implemented to prevent excessive resonant tank
current and ensure Zero-Voltage Switching (ZVS). During soft start, the switching frequency is increased. The
soft-start time can be programmed by placing a capacitor from SS pin to ground.
The soft-start pin also serves as an ON/OFF control pin of the device. By actively pulling the SS pin below 1 V,
the device is disabled. When the pull down is removed, SS pin voltage is increased because of internal charging
current. Once SS pin becomes above 1.2 V, the device starts to generated gate-driver signal and enters
soft-start mode. The time sequence of soft start is shown in Figure 19.
Figure 19. Soft-Start Sequence
To prevent a long delay between the ON command and apperance of a gate driver signal, the SS pin current is
set as two different levels. When SS pin voltage is below 1.2 V, its output current is 175 μA. This high current
could charge the soft-start pin capacitor to 1.2 V in a short period of time, and reduces the time delay. This time
delay can be calculated using following equation:
(15)
The switching frequency during soft start is determined by both the current flowing out of the RT pin and the
voltage on SS pin. The switching frequency can be calculated based on the following equation:
(16)
After SS pin voltage reaches 4 V, soft-start period is finished and switching frequency becomes the same as
demanded by the RT pin current. The time used to charge SS pin from 1.2 V to 4 V is defined as soft-start time
and can be calculated as:
(17)
To ensure reliable operation, the gate drivers restart with GD2 turning high. This prevents uncertainty during
system start up.
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