User Guide

NHB–3
Technical Documentation
PAMS
Cigarette Lighter Charger LCH–2
Original 26/97 Page 6
Oscillator
The converter operates at a fixed frequency of 300 kHz, produced by oscillator
N110A. A suitable operating voltage for the comparator positive input is set by
resistor divider R112/R114. Positive feedback from the comparator output is
supplied by R111 in parallel with C104. When the comparator output is high
C110 is charged through R110 and R113 until the voltage at the negative input
exceeds the positive input. At that instant the comparator output turns low and
C110 is discharged via R113.
Pulse–Width Control
Pulse width is controlled by comparator N110B, where the sawtooth voltage
from the oscillator is compared to a d.c. control voltage. Max. control voltage is
set by resistor divider R121/R120 to make sure that pulse width cannot reach
100 % under any conditions as this would mean short–circuiting VB to GND via
V112.
When the control voltage drops, so do the pulse width and the converter output
current. The control voltage can be pulled down by output current comparator
N110C, output voltage comparator N110D, or by the ”error” output pin of a ref-
erence regulator N100.
Current Limiting
Output current is measured with series resistor R116 connected between L112
and GND. When the converter operates with the output loaded it generates a
negative voltage drop (as compared to GND) over this resistor.
The voltage drop is added to the positive reference value set by R122 and
R123 and used as the current comparator positive input. The current compara-
tor compares the level of this input to zero (ground) level. When the current
reaches the limit, voltage at the positive input falls to zero and the comparator
output goes down, limiting the duty cycle. The value of the current limit is set by
resistor divider R122, R123. A negative feedback for stabilize the comparator is
produced with R124.