Datasheet

MAX1744/MAX1745
High-Voltage, Step-Down DC-DC
Controllers in µMAX
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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 (V
OUT
/ 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:
ESR
CRITICAL
= (V
OUT
/ 1.25V)
✕
8mV / I
RIPPLE
where I
RIPPLE
is the inductor ripple current, and can be
determined by:
I
RIPPLE
= (V
IN
- V
OUT
)
✕
t
ON(MIN)
/ L
where t
ON(MIN)
is the minimum on-time (1μs) for mini-
mum on-time-control, or:
I
RIPPLE
= (V
OUT
)
✕
t
OFF(MIN)
/ L
where t
OFF(MIN)
is the minimum off-time (2μs) for mini-
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
RS
VL
SHDN
VH
EXT
IN
MINIMUM
ON-TIME
ONE SHOT
Q TRIG
( ) MAX1745 ONLY
- - - MAX1744 ONLY
Figure 2. Simplified Functional Diagram