MAX768 Maxim, MAX768 Datasheet - Page 9

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

Figure 3. MAX768 Adjustable Configuration
For applications sensitive to the MAX768’s internal
switching frequency, connect an external TTL/CMOS
(within IN and GND) clock to SYNC. The clock must be
a 20kHz to 240kHz square wave between 40% and
60% duty cycle.
When under heavy loads, where POUT is sourcing into
NOUT (i.e., load current flows from POUT to NOUT,
rather than from supply to ground), do not allow NOUT
to pull above ground. In applications where large cur-
rents from POUT to NOUT are likely, use a Schottky
diode (1N5817) between GND and NOUT, with the
anode connected to GND (Figure 4).
Connect a IN5817-type Schottky diode from C2- to V-
to assure proper start-up.
Low-Noise, Dual-Output, Regulated Charge Pump
V
IN
C2
C1
IN
SYNC
C1+
C1-
C2+
C2-
NSHDN
PSHDN
Switching-Frequency Control
_______________________________________________________________________________________
MAX768
NOUT
POUT
SETP
SETN
GND
RDY
V+
V-
for GaAsFET, LCD, and VCO Supplies
Schottky Diodes
V
V
NOUT
POUT
R2
R1
R3
R4
C4
= (-1.25) (1 +
= (1.25) (1+
C3
R2
R1
R4
R3
)
C5
C6
)
Figure 4. A Schottky diode protects the MAX768 when a large
current flows from POUT to NOUT.
Good layout is important, primarily for good noise per-
formance:
1) Mount all components as close together as possi-
2) Keep traces short to minimize parasitic inductance
3) Use a ground plane.
Accurately measuring output noise and ripple can be
difficult. Brief differences in ground potential between
the circuit and the oscilloscope (which result from the
charge pump’s switching action) cause ground cur-
rents in the probe’s wires, inducing sharp voltage
spikes. For best results, measure directly across output
capacitor C3, C4, C5, or C6. Do not use the oscillo-
scope probe’s ground lead; instead, remove the
cover’s ground lead and touch the ground ring on the
probe directly to the ground terminal of C3, C4, C5, or
C6. Or, use a Tektronix chassis-mount test jack (part
no. 131-0258) to connect your scope probe directly.
This direct connection provides the most accurate
noise and ripple measurement.
ble.
and capacitance. This includes connections to
SETP and SETN.
Noise and Ripple Measurement
MAX768
Layout and Grounding
NOUT
GND
9