LTM4641
APPLICATIONS INFORMATION—POWER SUPPLY FEATURES
where:
Table 1. Endorsed RfSET Resistor Value vs Output Voltage for
Non-Tracking Applications—and Resulting Full-Load Switching
Frequency (cf. Figure 45, Figure 46, Figure 47, and Figure 51
Circuits)
•ꢀ V
is nominal output voltage in volts.
OUT
•ꢀ t
is the minimum length of time M
can
OFF(MIN)
BOT
R
fSET
(ꢁΩ) (Nearest
be on, after M
turns off. For a conservative de-
TOP
V
(V)
EIA-Standard Values)
f
(kHz)
SW
OUT(NOꢁ)
sign, use a value of 300ns, taken from the Electrical
Characteristics Table.
0.6
0.787
175
0.7
0.8
0.9
1.0
1.2
1.5
1.8
2.0
2.5
0.825
200
215
235
255
285
315
325
330
335
0.887
•ꢀ t is the on-time of the power control MOSFET,
ON
0.931
M
, as programmed by the current flowing into
ON
TOP
1.00
the I pin of LTM4641’s internal control IC.
1.13
•ꢀ R is the series resistance of the module’s power
PS
1.43
stage, from V to V . For V ≥ 6V, this is less
INH
OUT
IN
2.00
than 50mΩ, even at extreme temperatures (T ≈
J
2.55
125°C). For V < 6V, the effective series resistance
IN
5.76
increases due to drop in INTV voltage and cor-
CC
Greater Than 3.0
∞ (Not Used)
∞ (Not Used)
∞ (Not Used)
∞ (Not Used)
See Figure 2
360
responding decreased gate-drive enhancement of
3.3
5.0
6.0
M
. Printed circuit board (PCB) and/or cable
TOP
550
resistancepresentinthecopperplanesand/orwires
that physically connect the output of the module to
660
the load adds to R ’s effective value.
PS
In rail-tracking applications, it is recommended to use the
value corresponding to the lowest voltage needed
•ꢀ I
is the load current on V
in amperes.
R
fSET
OUT
OUT
to be regulated during output voltage ramp down. For
example: to ramp V down to 0.5V requires R to be
For applications of the form shown in Figure 45, Fig-
OUT
fSET
ure 46 and Figure 47: the minimum allowable V
INH
OUT
not more than 750kΩ (nominal) per Figure 3.
voltage of operation to avoid dropout for 3V < V
≤ 6V is shown in Figure 4. The curves are a result of
It is often permissible to use lower R values than those
fSET
realizing that V
equals V
(neglecting
indicated in Figure 3 and Table 1 if, for example, lower
output ripple voltage and/or a lower output capacitance
is desired. However, be aware of three guiding principles:
IN(DROPOUT)
INH
MSP voltage drop) when dropout actually occurs, and
that Equations 1 and 2 yield an expression for t as
ON
a function of V . M
will be less fully enhanced
INH
TOP
I. Minimum On-Time. Ensure I
tions 1 and 4.
< 93.3µA. See Equa-
ION
during its on-time if DRV is less than its nominal
CC
value of 5.3V (for example, when V < 6V and when
INL
II. Minimum Off-Time and Dropout Operation. The mini-
mum off-time, t , is the shortest time required
DRV bias is provided by INTV ). DRV ’s effect on
CC
CC
CC
R
PS
at low line is illustrated in Figure 4.
OFF(MIN)
for the LTM4641 to perform the following tasks: turn
on its power synchronous MOSFET (M ), trip the
III. Maximum Attainable f . The maximum attainable
SW
BOT
switching frequency of operation (in units of Hz) for a
control loop’s current comparator, and turn off M
.
BOT
givenon-time(t ,inseconds)isgovernedsimplyby:
ON
The minimum input voltage on V , in volts, that one
INH
1
can regulate the output at and still avoid dropout is
fMAX
=
(8)
given by:
tON + tOFF(MIN)
t
OFF(MIN)
where a conservative value of 300ns can be used for
V
IN(DROPOUT) = VOUT • 1+
+RPS •IOUT (7)
tON
t
.
OFF(MIN)
4641f
20
Linear [ Linear ]
