Pin description, Operating principle – Rainbow Electronics MAX742 User Manual
Page 6
MAX742
Switch-Mode Regulator with
+5V to ±12V or ±15V Dual Output
6
_______________________________________________________________________________________
______________________________________________________________Pin Description
Inverting Compensation Capacitor
CC-
9
Inverting Section Feedback Input
FB-
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.
100/200
5
Selects V
OUT
. Ground for ±15V, or tie to V+ for ±12V.
12/15
6
Reference Voltage Output (+2.00V). Force to GND or V+ to disable chip.
VREF
7
Soft-Start Timing Capacitor (sources 5µA)
SS
8
Analog Supply Voltage Input (+5V)
AV+
4
Analog Ground
AGND
3
PIN
Step-Up Compensation Capacitor
CC+
2
Step-Up Feedback Input
FB+
1
FUNCTION
NAME
Charge-Pump Driver—clock output at 1/2 oscillator frequency.
PUMP
16
Push-Pull Output—drives external logic-level N-channel MOSFET.
EXT+
17
High-Current Ground
GND
18
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.
PDRV
15
________________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.