WS21351
OPERATION DESCRIPTION
ignition and improve lamp life longevity.Accordingly,
the oscillation frequency is changed in the following
sequence:
Under-Voltage Lockout (UVLO)Function
The WS21351 has UVLO circuits for both high-side and
low-side circuits. When VDD reaches VDDTH+, UVLO is
released and the WS21351 operates normally. Under
UVLO condition,WS21351 consumes little current, typically
50uA. After UVLO is released,WS21351 operates normally
until VDD goes below VDDTH-, the UVLO has hysteresis
which typically is 2.8V . At UVLO condition, all latches that
determine the status of the IC are reset.
Preheating freq > Ignition freq > running freq.
WS21351 has a high-side gate driver circuit. The supply
for the high-side driver is applied between VB and VOUT.
To protect from malfunction of the driver at low supply
voltage between VB and VOUT,WS21351 provides an
additional UVLO circuit between this supply rails. If
VB-VOUT is under VHSTH+, the driver holds low state to
turn off the high-side switch, when VB-VOUT is higher than
VHSTH– after VB-VOUT exceeds VHSTH+, operation of
the driver continues.
WS21351
Oscillator Operation
The ballast circuit for a fluorescent lamp is based on the
LCC resonant tank and a half-bridge inverter circuit, as
shown in Fig4 . To accomplish Zero-Voltage Switching
(ZVS) of the half-bridge inverter circuit, the LCC must be
driven at a higher frequency than its resonant Frequency,
which is determined by L, CS, CP, and RL; where RL is the
equivalent lamp's impedance. The transfer function of LCC
resonant tank is heavily dependent on the lamp impedance,
RL, as illustrated in Figure 4. The oscillator inWS21351
generates effective driving frequencies to assist lamp
Preheating and ignition Mode
When VDD exceeds VDDTH+ threshold, theWS21351
enters preheating mode. An internal current source charges
the external capacitor on pin CPH, and the voltage on pin
CPH starts ramping up linearly. The frequency ramps down
towards the resonance frequency of the high-Q ballast
output stage causing the lamp voltage and load current to
Increase.
The voltage on pin CPH continues to increase and the
frequency keeps decreasing until the lamp ignites. If the
4/7
Steady, keep you advance
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