Rainbow Electronics MAX768 - Manual (Page 8)

MAX768
Low-Noise, Dual-Output, Regulated Charge Pump
for GaAsFET, LCD, and VCO Supplies
8 _______________________________________________________________________________________
Shutdown
The MAX768 has two active-low, TTL logic-level shut-
down inputs:
PSHDN and NSHDN. When both inputs
are pulled low, the MAX768 shuts down and the supply
current is reduced to 10µA max over temperature.
Pulling
PSHDN low turns off the positive linear regula-
tor; the doubler charge pump remains active. Pulling
the
NSHDN input low while PSHDN remains high turns
off the inverter charge pump, the negative linear regula-
tor, and the output-ready indicator (Table 1).
Capacitors
The overall dropout voltage is a function of the charge
pump’s output resistance and the voltage drop across
the linear regulator. The charge-pump output resistance
is a function of the switching frequency and the capaci-
tor’s ESR value. Therefore, minimizing the charge-pump
capacitors’ ESR minimizes dropout voltage.
See Table 2 for capacitor selection. All capacitors
should be either surface-mount ceramic chip or tanta-
lum. External capacitor values may be adjusted to opti-
mize size, performance, and cost.
PSHDN
Positive output active 1 1
POUT
STATUS
NSHDN
Negative output active
NOUT
STATUS
0.8
SUPPLY
CURRENT
(mA)
Negative output inactive 0.7 Positive output active 0 1
Negative output active 0.3
Shutdown (low-power mode) 0.0001 Shutdown (low-power mode) 0 0
Positive output inactive 1 0
100 IN
20 to 240 External Clock
SYNC
INPUT
25 GND
FREQUENCY
(kHz)
2.2µF 10µF
C = 220µF/f (kHz)
10µF
4.7µF
CAPACITORS
C1, C2, C3, C4 C5, C6 C7
Figure 2. MAX768 Standard Application Circuit
Table 1. Shutdown-Control Logic
Table 2. Charge-Pump Capacitor Selection (Figure 2)
MAX768
C1-
GND
C4
C3
C2 C1
C7
C5
C6
C2-
V-
NOUT
SETN
NSHDN
PSHDN
SEE TABLE 2 FOR CAPACITOR VALUES
C2+
V IN
TO
V
IN
-5V
+5V
IN
C1+
V+
POUT
SETP
RDY
SYNC R 84 8 C1 C3
2
f C1
R 84 8 C1 4 C2
(C4 )
2
f C1
1
f C2
POUT ESR ESR
OSC
NOUT ESR ESR
ESR
OSC OSC
= +
( )
+
( )
+
= +
( )
+
( )
+
+ +