High-Voltage, Step-Down DC-DC

Controllers in µMAX

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9

lower frequencies, reducing the MOSFET gate drive

and switching losses. In discontinuous mode, under

most circumstances, the on-time will be the fixed mini-

mum on-time of 1μs. If the inductor value is small, or

the current-sense resistor large, the current limit will be

tripped before the minimum on-time, terminating the

on-time and thus setting the fixed on-time.

If the inductance is too large, or the output capacitance

high and equivalent series resistance (ESR) low, then

the MOSFET remains on longer than the minimum on-

time, until the output capacitor charges beyond the

causing the part to operate in hysteretic mode.

Operating in hysteretic mode results in lower frequency

operation. The transition to hysteretic mode occurs at

the critical output capacitor ESR:

determined by:

mum on-time-control, or:

mum off-time-control.

REF

MINIMUM

OFF-TIME

ONE SHOT

Q

TRIG

VL

LINEAR

REGULATOR

VH

LINEAR

REGULATOR

1.25

REFERENCE

(FB)

ERROR

COMPARATOR

OUT

3/5

SHDN

CS

100mV

+

-

Q

R

S

VL

SHDN

VH

EXT

IN

MINIMUM

ON-TIME

ONE SHOT

Q

TRIG

( ) MAX1745 ONLY

- - - MAX1744 ONLY

Figure 2. Simplified Functional Diagram