MAX768 Maxim, MAX768 Datasheet - Page 8

• Manufacturer Part Number: MAX768
• Description: Low-Noise / Dual-Output / Regulated Charge Pump for GaAsFET / LCD / and VCO Supplies
• Manufacturer: Maxim

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).
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.
Low-Noise, Dual-Output, Regulated Charge Pump
for GaAsFET, LCD, and VCO Supplies
Table 2. Charge-Pump Capacitor Selection (Figure 2)
8
Table 1. Shutdown-Control Logic
External Clock
R
R
_______________________________________________________________________________________
POUT
NOUT
PSHDN
INPUT
SYNC
GND
IN
1
1
0
0
84
84
(C4
ESR
8 C1
8 C1
)
FREQUENCY
ESR
ESR
20 to 240
NSHDN
f
(kHz)
OSC
100
25
1
0
1
0
2
C1
4 C2
C3
ESR
ESR
f
OSC
Positive output active
Positive output active
Positive output inactive
Shutdown (low-power mode)
1
C2
f
Capacitors
OSC
C = 220µF/f (kHz)
Shutdown
C1, C2, C3, C4
2
C1
STATUS
2.2µF
10µF
POUT
Figure 2. MAX768 Standard Application Circuit
V
IN
C4
SEE TABLE 2 FOR CAPACITOR VALUES
C2
Negative output active
Negative output inactive
Negative output active
Shutdown (low-power mode)
CAPACITORS
C5, C6
C1-
C2-
V-
SETN
NSHDN
PSHDN
GND
NOUT
10µF
STATUS
NOUT
MAX768
SYNC
POUT
SETP
RDY
C2+
C1+
V+
IN
C3
TO
V
IN
4.7µF
CURRENT
SUPPLY
C7
C1
0.0001
(mA)
0.8
0.7
0.3
C5
C6
C7
+5V
-5V