MAX742 Maxim, MAX742 Datasheet - Page 6

• Manufacturer Part Number: MAX742
• Description: Switch-Mode Regulator with +5V to 12V or 15V Dual Output
• Manufacturer: Maxim

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
Switch-Mode Regulator with
+5V to ±12V or ±15V Dual Output
6
______________________________________________________________Pin Description
________________Operating Principle
_______________________________________________________________________________________
PIN
10
11
12
13
14
15
16
17
18
19
20
1
2
3
4
5
6
7
8
9
100/200
NAME
AGND
PUMP
CSH+
PDRV
12/15
VREF
EXT+
CSL+
CSH-
CSL-
GND
CC+
EXT-
AV+
FB+
CC-
FB-
V+
SS
Step-Up Feedback Input
Step-Up Compensation Capacitor
Analog Ground
Analog Supply Voltage Input (+5V)
Selects oscillator frequency. Ground for 200kHz, or tie to V+ for 100kHz.
Selects V
Reference Voltage Output (+2.00V). Force to GND or V+ to disable chip.
Soft-Start Timing Capacitor (sources 5µA)
Inverting Compensation Capacitor
Inverting Section Feedback Input
Current-Sense Low (inverting section)
Current-Sense High (inverting section)
Supply Voltage Input (+5V)
Push-Pull Output—drives external P-channel MOSFET.
Voltage Input—negative supply for P-channel MOSFET driver.
Charge-Pump Driver—clock output at 1/2 oscillator frequency.
Push-Pull Output—drives external logic-level N-channel MOSFET.
High-Current Ground
Current-Sense Low (step-up section)
Current-Sense High (step-up section)
OUT
. Ground for ±15V, or tie to V+ for ±12V.
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.
FUNCTION