Rainbow-electronics MAX1837 Bedienungsanleitung PDF herunterladen (Seite 8)

MAX1836/MAX1837

24V Internal Switch, 100% Duty Cycle,

Step-Down Converters

8 _______________________________________________________________________________________

stant-frequency pulse-width-modulation (PWM) con-

trollers that switch the MOSFET unnecessarily.

When the output voltage is too low, an error comparator

sets a flip-flop, which turns on the internal P-channel

MOSFET and begins a switching cycle (Figure 3). As

shown in Figure 4, the inductor current ramps up linear-

ly, charging the output capacitor and servicing the

load. The MOSFET turns off when the current limit is

reached, or when the maximum on-time is exceeded

while the output voltage is in regulation. Otherwise, the

MOSFET remains on, allowing a duty cycle up to 100%

to ensure the lowest possible dropout voltage. Once

the MOSFET turns off, the flip-flop resets, diode D1

turns on, and the current through the inductor ramps

back down, transferring the stored energy to the output

capacitor and load. The MOSFET remains off until the

0.5µs minimum off-time expires and the inductor cur-

rent ramps down to zero, and the output voltage drops

back below the set point.

4µs/div

CIRCUIT OF FIGURE 2, V

= 12V

A. V

, 5V/div

B. V

OUT

= 3.3V, 20mV/div, 200mA LOAD

C. INDUCTOR CURRENT, 500mA/div

10V

500mA

3.3V

Figure 4. Discontinuous-Conduction Operation

INPUT

4.5V OR 24V

OUTPUT

3.3V OR 5V

OUT

MAX1836

MAX1837

GND

SHDN

OUT

MAXIMUM

ON-TIME

DELAY

TRIG

MAXIMUM

OFF-TIME

DELAY

TRIG

SENSE

SET

1.25V

100mV

Figure 3. Functional Diagram

1 2 3 4 5 6 7 8 9 10 11 12 13

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