Theory of Operation
Power Supply Functional Blocks
2
2-9
Table 2-4. Power Source Detection
POWER TYPE DETECTION
LOW BATTERY SOFT POWER-
OFF
SCALE
FACTOR
Battery Eliminator connected BATT_ELIM
≥ 1.0 Volts
na na 0.0951
NiCd Battery Pack installed BATT_TYPE
5 ≤ V ≤10.5 Volts
6.2V 5.7V 0.0909
Alkaline (AA) Batteries installed BATT_TYPE
V ≤ 4.5 or
V ≥ 11 Volts
6.2V 4.8V 0.0909
Power Supply Functional Blocks 2-11.
Refer to the Schematic Diagram in Chapter 7 during the following discussion. The power
supply consists of the circuit blocks listed below. Each block is identified with a letter
(A-J) keyed to Figure 2-2.
• Input power selector (A)
• Boost Preregulator (B)
• NiCd battery charger and timeout (C)
• DC-DC Converter (D)
• Backlight current sink (E)
• Power switch, wake-up and power down circuitry (F)
• LCD contrast control and temperature compensator (G)
• Power on reset (H)
• Linear post-regulators (I)
• +5 volt bias supply (J)
Input Power Selector (A) 2-12.
The input power selector automatically selects either battery eliminator power (line) or
battery. If a battery eliminator is connected to the GMM, +12 volts (±5%) is applied
through line filter FL1 to the anode of CR21. This 12 volts turns on Q13, which pulls
down the gate of Q14, turning it on. Current can then flow from the battery eliminator to
the boost circuit (B). Note that CR22 is back-biased.
If an eliminator is not connected, Q13 is off. Current is not allowed to flow through Q14
until Q12 is turned on. Q12 is turned on when enough voltage remains on the battery to
supply the bias regulator (J) and the power wake-up circuitry is enabled. When the GMM
is off (with no eliminator connected), Q14 is off and only the bias supply draws current.
The maximum off-state current draw is 100 µA.
CR21 and CR22 perform the power selection process.
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