Mindray Passport 5-Lead, Service Manual

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Block Diagrams Theory of Operation
2 - 52 0070-00-0420 Passport 5-Lead, 5L, LT, XG Service Manual
FIGURE 2-20
Comparatory 20 Second Delay Circuit
Oscillator / Charge Pump Circuit
An oscillator is formed from the R11 - C4 combination around U2 (CD4093 Schmitt trigger
nand gate). The second input to the Nand gate from the comparator circuit provides an ON/
OFF switch to the oscillator. This oscillator, at a frequency of 200 KHz, is tied to another
Schmitt trigger to form complementary outputs. The two output signals are coupled to the
system power (FET) switch “ gate” through a pair of .01Uf capacitors (C5 and C6) and a
bridge rectifier (CR3-CR6) that converts ac voltage to dc. This circuit forms an approximate
voltage doubler. Therefore at a cutoff supply voltage of about 9.5V, this circuit will provide
9.5 volts at the gate of this FET. This FET needs a Gate to Source voltage of 5V at a Drain
current of 4A (the system only requires 1A), providing a safety factor of almost two.
CR7 clamps the input to U2 from going higher than the supply rail. This component offers
protection to the IC U2. R9 bleeds charge off of capacitors C5 and C6 to shut off the FET
when the oscillator is disabled. R14 limits current to the FET gate when the FET is initially
turned “ON”.
VIN
R5
R1
R3
Q2
2N7000
Q1
20 Sec Delay
2N7000
CR1
3
2
7
6
4
8
U1
R6
LT1004-1.2
R8
C1
CR2
C7
8
9
10
11
12
13
CD4093BE
R12
U2
COMPARATOR CIRCUIT
LM4250