User Guide

Technical Documentation
Cigarette Lighter Charger LCH–2
Page – 5
Original 50/96
Oscillator
The converter operates at a fixed frequency of 300 kHz, produced by os-
cillator 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 saw–tooth volt-
age 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–cir-
cuiting 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 reference 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 resis-
tor.
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
comparator 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.