MAX1744 데이터 시트보기 (PDF) - Maxim Integrated

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MAX1744

High-Voltage / Step-Down DC-DC Controller in MAX

Maxim Integrated 

High-Voltage, Step-Down DC-DC

Controllers in µMAX

EXT IN

REF

VH

SHDN

VL

VH

LINEAR

REGULATOR

VL

LINEAR

REGULATOR

Q

TRIG

MINIMUM

ON-TIME

ONE SHOT

Q

TRIG

MINIMUM

OFF-TIME

ONE SHOT

1.25

REFERENCE

ERROR

COMPARATOR

Q

SR

SHDN

OUT

(FB)

3/5

( ) MAX1745 ONLY

- - - MAX1744 ONLY

Figure 2. Simplified Functional Diagram

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

error comparator’s (VOUT / 1.25V) ✕ 8mV hysteresis,

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:

ESRCRITICAL = (VOUT / 1.25V) ✕ 8mV / IRIPPLE

CS

100mV

where IRIPPLE is the inductor ripple current, and can be

determined by:

IRIPPLE = (VIN - VOUT) ✕ tON(MIN) / L

where tON(MIN) is the minimum on-time (1μs) for mini-

mum on-time-control, or:

IRIPPLE = (VOUT) ✕ tOFF(MIN) / L

where tOFF(MIN) is the minimum off-time (2μs) for mini-

mum off-time-control.

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