TS19453 Datasheet, PDF (4/7 Pages) Taiwan Semiconductor Company, Ltd – AC/DC Programmable Current WLED Driver

Preliminary
TS19453
AC/DC Programmable Current WLED Driver
w/PWM Dimming Control
Application Information (Continue)
L is the inductance value, and C_L is the coil capacitance. Charging and discharging this capacitance every
switching cycle causes high-current spikes in the LED string. Therefore, connecting a small capacitor C_O(~100nF)
is recommended to bypass these spikes.
Using an ultra-fast rectifier diode for D1 is recommended to achieve high efficiency and reduce the risk of false
triggering of the current sense comparator. Using diodes with shorter reverse recovery time t_rr and lower junction
capacitance C_J achieves better performance. The reverse voltage rating V_R of the diode must be greater than the
maximum input voltage of LED lamps.
The total parasitic capacitance present at the Drain pin of the TS19453 can be calculated as:
C_P = C_DRAIN + C_PCB + CL + C_J
When the switching MOSFET turns on, the capacitance C_P is discharged into the Drain pin of the IC. The discharge
current is limited to about 50mA typically. However, it may become lower at increased junction temperature. The
duration of the leading edge current spike can be estimated as:
T_PSIKE = ((V_IN x C_P) / (I_SAT)) + t_rr
In order to avoid false trigger of the current sense comparator, C_P must be minimized in accordance with the
following expression.
C_P =< (Isat * ( T_BLANK(min) - t_rr )) / Vin(max)
When T_BLANK(min) is the minimum blanking time of 200nS, and Vin(max) is the maximum instantaneous input voltage.
Estimating Power Loss
Discharging the parasitic capacitance C_P in the the Drain output of TS19453 is responsible for the bulk of the
switching power loss. It can be estimated using as:
P_SWITCH = ((C_P * V_IN2 / 2 ) + (V_IN * I_SAT * t_rr)) * F_S
F_S is the switching frequency and is the saturated Drain current of TS19453, the switching loss is the greatest at
the maximum input voltage.
Disregarding the voltage drop at TS19453 and D1, the switching frequency is given as
F_S = (V_IN - V_OUT ) / (V_IN * T_OFF)
When the TS19453 is powered from full wave rectified AC input, the swithching power loss can be estimated as:
P_SWITCH = 1 / (2 * T_OFF) * ((V_AC * C_P + 2 * I_SAT * t_rr ) * (V_AC - V_OUT )))
When the TS19453 is powered from DC input voltage, conduction power loss can be estimated as:
P_COND = (D * I_OUT2 * R_ON) + I_DD * V_IN * (1 – D)
EMI Filter
As with all off-line converters, selecting an input filter is critical to obtaining good EMI. A switching side capacitor,
albeit of small value, is necessary in order to ensure low impedance to the high frequency switching currents of the
converter. As a rule of thumb, this capacitor should be approximately 0.1~0.2µF/W of LED output power.
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Version: Preliminary