_______________Detailed Description
100kHz/200kHz Oscillator
The MAX742 oscillator frequency is generated without
external components and can be set at 100kHz or
200kHz by pin strapping. Operating the device at
100kHz results in lower supply current and improved
efficiency, particularly with light loads. However, com-
ponent stresses increase and noise becomes more dif-
ficult to filter. For a given inductor value, the lower
operating frequency results in slightly higher peak cur-
rents in the inductor and switch transistor (see
Typical
Operating Characteristics
, Peak Inductor Current vs.
Load Current graph). When the lower frequency is used
in conjunction with an LC-type output filter (optional
components in Figure 2), larger component values are
required for equivalent filtering.
Charge-Pump Voltage Inverter
The charge-pump (PUMP) output is a rail-to-rail square
wave at half the oscillator frequency. The square wave
drives an external diode-capacitor circuit to generate a
negative DC voltage (Point A in Figure 2), which in turn
biases the inverting-output drive stage via PDRV. The
charge pump thus increases the gate-source voltage
applied to the external P-channel FET. The low on-
resistance resulting from increased gate drive ensures
high efficiency and guarantees start-up under heavy
loads. If a -5V to -8V supply is already available, it can
be tied directly to PDRV and all of the charge-pump
components removed. For input voltages greater than
8V, ground PDRV to prevent overvoltage. Observe
PDRV absolute maximum ratings.
MAX742
Switch-Mode Regulator with
+5V to ±12V or ±15V Dual Output
_______________________________________________________________________________________ 7
MAX742
PULSE
SQUARE
RAMP
OSC
12/15
SELECT
VREF
SOFT-START
AND THERMAL
SHUTDOWN
S
RQ
S
TO V+
RQ
∑
∑
AGND
VREF
12/15
CC-FB-
CC+FB+
CSH+
CSL+
V+
EXT+
GND
SS
PUMP
EXT-
PDRV
CSH-
CSL-
AV+100/200
Figure1. MAX742 Detailed Block Diagram
