Rainbow Electronics MAX782 - Manual (Page 24)

MAX782
Triple-Output Power-Supply
Controller for Notebook Computers
24 ______________________________________________________________________________________
Total Shutdown Circuits
When the +5V and +3.3V supplies are off, the complete
MAX782 circuit consumes only about 70µA, plus any
leakage through the off MOSFETs. Some NiCd batter-
ies can be damaged if they are fully discharged and
then left for long periods (months) under load. Even
100µA can do damage if left long enough.
The complete power-supply system can be shut down
by taking ON5 low, cutting the supply to the MAX782’s
V+ pin, while the bootstrapped +5V supply is turned
off. This removes the supply from the controller, and
turns off all the supplies. In this condition, the current
consumption drops to the level of the leakage currents
in the off transistors. Switching the V+ supply off is
easy because the V+ line draws very little power;
switching the entire power input from the battery would
be more difficult.
Figure 12 shows a logic interface for a momentary
switch that toggles the whole system on and off. The
logic circuit runs from the battery supply, so the input
voltage from the battery is limited to the normal operat-
ing range for the flip-flop gates, which is usually 18V for
4000-series CMOS circuits. The active-high OFF input
permits the supplies to be turned off under logic control
as well as when the switch is pushed. If this logic input
is not required, omit R1 and Q1. The supplies can only
be turned on using the hardware switch. For automatic
turn-off, connect the OFF input to a battery-voltage
sensing comparator or to a timer powered from VL.
Ensure that any signal connected to OFF does not
glitch high at power-up.
Generating Additional VPP Outputs
Using External Linear Regulators
Figure 13 shows a low-dropout linear regulator
designed to provide an additional VPP output from the
VDD line. It can be turned off with a logic-level signal;
its output can be switched to 5V or 12V; and it pro-
vides excellent rejection of the high-frequency noise
on VDD. If a monolithic linear regulator is used,
choose one having good PSRR performance at
300kHz.
ZETEX
ZVP2106G
SOT 223
V BATT
ON/
OFF
1M
1 µ F
R
1M
CD4011B
S
1M
1M
R1
1M
1N4148
1 µ F
OFF
Q1
2N4401
ON5
1N4148
PGND
ON
ON
(+5.5V TO +18V)
1N4148
ON5
VL
V+
1M
MAX782
1N4148
Figure 12. Hardware/Software Total Shutdown Circuit
Q2
2N7002
Q1
2N4403
C1
100nF
R4
1k
C3
10nF
R1
16.9k
OUT
C2
6.8 µ F
R2
6.34k
R3
26.7k
Q3
2N7002 12/5 OFF/ON
6
4
3
2
7
R5
100k
REF
C4
10nF
VDD
U1
OP27
OFF/ON 12/5 V OUT (V)
1 X 0
0 0 5
0 1 12
X = DON'T CARE
FROM MAX782
FROM MAX782
Figure 13. External Regulator For Additional VPP Outputs