TPS92023 Datasheet, PDF (15/23 Pages) Texas Instruments – Resonant-Switching Driver Controller for LED Lighting

TPS92023
www.ti.com
SLUSBH3 – JUNE 2013
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.
4V
1.2 V
V_SS
Gate Driver
t_SS
t_SS(delay)
Figure 19. Soft-Start Sequence
UDG-10047
To prevent a long delay between the ON command and appearance 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 is calculated in Equation 15.
t_{SS(delay )}
=
1.2 V
175 mA
´
C_SS
(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 Equation 16.
f_SW
=
1_´
2
1
6ns ´1A
+ 150ns
I_RT
+
æ
ç^1.81mA
è
-
V_VSS
2.2kW
ö
÷
ø
(16)
After the SS pin voltage reaches 4 V, the soft-start period ends and the switching frequency equals that as
demanded by the RT pin current. The time used to charge the SS pin from 1.2 V to 4 V is defined as the soft-
start time and can be calculated in Equation 17.
2.8 V
t_SS = _{5 mA} ´ C_SS
(17)
To ensure reliable operation, the gate drivers restart with GD2 turning high. This prevents uncertainty during
system start up.
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