TIL5942 Datasheet, PDF (3/19 Pages) Texas Instruments – CURRENT-MODE-PWM CONTROLLER WITH OPTOISOLATED VOLTAGE REFERENCE AND ERROR AMPLIFIER

TIL5942, TIL5942A
CURRENT-MODE-PWM CONTROLLER
WITH OPTOISOLATED VOLTAGE REFERENCE AND ERROR AMPLIFIER
SOES040 – OCTOBER 1997
theory of operation
The current through the LED is controlled by the precision reference amplifier according to the voltage error at
FB, with respect to the internal 2.5-V reference. The light emitted by the LED is coupled to the PWM controller
to change the modulation duty cycle and reduce the error voltage.
The PWM controller is in the off state until V_CC is greater than the turn-on threshold voltage level. Typically the
controller turns on at 16 V. It stays in the on state until the voltage drops below the turn-off threshold of 10 V
typical. Upon turning on, the controller generates an internal reference voltage V_ref of 5 V, which controls the
chip logic and can be used for low-power external applications. A voltage V_reg is also generated to supply current
to charge an external capacitor C_reg, which supplies instant current for the pulsing output.
The oscillator shown in Figure 1 consists of an external resistance-capacitance (RC) network, a voltage
comparator, logic, a 125-ns one-shot for the dead time, and a MOSFET used to discharge the external capacitor
to ground each cycle. At the end of the reset period, C_T charges toward the 5-V reference through timing resistor
R_T for a time given by one time constant R_TC_T. The oscillator period is given by:
T_OSC = R_TC_T + 0.125 µs.
A current I_IB(IS) supplied out of IS biases the current-sense resistor on the primary side of the power supply. If
the voltage V_(IS) at IS exceeds 1 V, the output pulse is reset. An internal current source I_(VC) supplies a nominal
current of –4 µA as a reference. This current charges a capacitor connected between VC and ground. The
photodiode is connected internally in parallel with the capacitor. The light-generated photodiode current tends
to discharge the capacitor until a steady-state balance is reached. When I_(VC) is greater than the photodiode
current, a net current is being sourced and the capacitor is charged. When I_(VC) is lower than the photodiode
current, a net current is being sinked and the capacitor is discharged. When I_(VC) is equal to the photodiode
current, the capacitor voltage stays constant. The voltage at VC is monitored by an internal comparator with
a threshold voltage given by V_(IS) + 2Vd, where Vd is a diode voltage drop of typically 0.7 V.
Whenever VC is below this threshold, the output pulse is reset. The output pulse is enabled if V_(IS) is less than
1 V and VC is greater than its threshold.
The 5-V reference on the PWM chip is brought out on REF and can be used as a reference and/or supply for
external circuits as long as the output current is limited to 20 mA and the power dissipation is not exceeded.
It is recommended that a 0.1-F ceramic capacitor be connected between REF and PWM_GND.
REF
RT/CT
PWM_GND
RT (see Note A)
CT
NOTE A: tosc = RTCT + 0.125 µs
Figure 1. Typical Oscillator Application
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