Dc-Dc Converter (D); Backlight Current Sink (E); Power Switch Circuitry (F); Lcd Contrast Control (G) - Fluke 867B Service Manual

867B,863
Service Manual

DC-DC Converter (D)

The DC-DC converter is a conventional push-pull transformer couple type. The center
tap of T1 is fed from the +15 from the boost preregulator. Q16 drives one side or the
other of the primary to ground at the rate of approximately 100 kHz, determined by the
oscillator (U34, R10, and C22) and a divide-by-2 flip-flop (U33). This provides a 30-volt
peak-peak signal on the primary of T1.
Diodes CR14 and CR10 and associated components use the 50% square wave from U33
to produce non-overlapping signals for the gate drives of Q16. Capacitors C24 and C57
ac couple the drive signal to prevent a stopped clock from causing burn out of Q16.
The outputs of the secondary windings are rectified and filtered to provide the raw DC
voltages needed by the linear post-regulators (I). The output of CR12 is filtered by C21
and provides the raw compliance voltage required by the backlight current sink (E).

Backlight Current Sink (E)

The backlight current sink is enabled by BACKLIGHT_ON* going low, which turns off
Q5, allowing its collector to go open. Voltage is then applied to the base of Q21. R16
turns this voltage, minus one base-emitter drop, into a current. This current flows in the
collector circuit of Q21, consisting of the raw voltage from C21 and the LEDs in the
LCD backlight. The tolerance on this current is approximately ±33% and is nominally 90
mA.

Power Switch Circuitry (F)

S1 (the rotary selector switch) is closed when it is in the OFF position. To turn on the
circuitry, U34 and its associated components are used to generate a POK-1 signal, which
turns on the GMM. R138, CR25, R139, and C10, in conjunction with U34, provide fast
turn-on (R138 and CR25) and slow turn-off (R139 and C10). This slow turn-off time
allows you to rotate through the OFF position on the selector without turning the GMM
off. If a POWER_DOWN command is sent to Q30, U33 is cleared, overriding the switch
command and shutting down the power supply. The WAKEUP* line resets U33,
allowing the GMM to restart. Note that this circuitry is powered from the bias supply.

LCD Contrast Control (G)

The LCD contrast control is enabled from the microprocessor by a high level (+3.3) on
the VEE_ON line. The level of VEE is determined by a pulse-width modulated signal on
the CONTRAST line. The voltage at VEE varies from approximately -24 volts to -14
volts as the pulse width is varied from 100% to 0%. This voltage allows control of the
LCD contrast.
The actual voltage applied to VEE is the average PWM voltage, minus two diode drops
(Q17 and Q20), and times the gain of U2.
Since the voltage required to achieve optimum contrast is a strong function of
temperature, the gain of U2 is selected so that the two-diode drop temperature coefficient
of about -4.6 mV/C compensates for the change in contrast. This gain is about 16.7,
corresponding to a change of about -77 mV/°C at the VEE output.
U2 is provided with power supplies of +5.2 regulated and approximately -24
unregulated.

Power-On Reset Circuit (H)

The power-on reset circuit receives a +2.5 reference voltage powered by the unregulated
output of the converter. The trip threshold for comparator U22 is set by R69 and R81 for
2-12
2-15.
2-16.
2-17.
2-18.
2-19.