MAX742
Switch-Mode Regulator with
+5V to ±12V or ±15V Dual Output
6 _______________________________________________________________________________________
______________________________________________________________Pin Description
Inverting Compensation CapacitorCC-9
Inverting Section Feedback InputFB-10
Current-Sense Low (inverting section)CSL-11
Current-Sense High (inverting section)CSH-12
Supply Voltage Input (+5V)V+13
Selects oscillator frequency. Ground for 200kHz, or tie to V+ for 100kHz.100/2005
Selects V
OUT
. Ground for ±15V, or tie to V+ for ±12V.12/156
Reference Voltage Output (+2.00V). Force to GND or V+ to disable chip.VREF7
Soft-Start Timing Capacitor (sources 5µA)SS8
Analog Supply Voltage Input (+5V)AV+4
Analog GroundAGND3
PIN
Step-Up Compensation CapacitorCC+2
Step-Up Feedback InputFB+1
FUNCTIONNAME
Charge-Pump Driver—clock output at 1/2 oscillator frequency.PUMP16
Push-Pull Output—drives external logic-level N-channel MOSFET.EXT+17
High-Current GroundGND18
Current-Sense Low (step-up section)CSL+19
Current-Sense High (step-up section)CSH+20
Push-Pull Output—drives external P-channel MOSFET.EXT-14
Voltage Input—negative supply for P-channel MOSFET driver.PDRV15
________________Operating Principle
Each current-mode controller consists of a summing
amplifier that adds three signals: the current waveform
from the power switch FET, an output-voltage error sig-
nal, and a ramp signal for AC compensation generated
by the oscillator. The output of the summing amplifier
resets a flip-flop, which in turn activates the power FET
driver stage (Figure 1).
Both external transistor switches are synchronized to
the oscillator and turn on simultaneously when the flip-
flop is set. The switches turn off individually when their
source currents reach a trip threshold determined by
the output-voltage error signal. This creates a duty-
cycle modulated pulse train at the oscillator frequency,
where the on time is proportional to both the output-
voltage error signal and the peak inductor current. Low
peak currents or high output-voltage error signals result
in a high duty cycle (up to 90% maximum).
AC stability is enhanced by the internal ramp signal
applied to the error amplifier. This scheme eliminates
regenerative “staircasing” of the inductor current, which
is otherwise a problem when in continuous current
mode with greater than 50% duty cycle.
