LTM4641
APPLICATIONS INFORMATION—POWER SUPPLY FEATURES
•ꢀ Whenꢀ V
and V
are operated from separate
to non-zero minimum off-time, dropout voltage, and
maximum achievable switching frequency of operation.
INL
supplies…
INH
…whyshouldR
ordinarilyconnecttotheV power
INH
When an R
resistor external to the LTM4641 is con-
fSET
IN
fSET
source rather than V (Figure 49)?
nected between V
and f
to decrease the default
INL
SET
on-time setting, the total I current (units: amperes) is
given by:
ON
…when is it okay for R
(Figure 47)?
to connect to V
INH
fSET
V
V
V
INL
INL
INL
For application circuits of the form found in Figure 45,
Figure 46, Figure 47 and Figure 51: see Figure 3 for the
maximum recommended value of R
of nominal target output voltage, and resulting full-load
switching frequency corresponding to those R values.
I
=
+
=
(4)
ION
1.3MΩ RfSET 1.3MΩ||RfSET
as a function
fSET
whereV isin units of volts andR
is in unitsof ohms.
INL
fSET
R
fSET
is needed for output voltage settings less than or
equal to 3V , and for rail-tracking applications.
fSET
OUT
Figure 3 can also be interpreted to provide the lowest
recommended switching frequency for a given target
output voltage. Table 1 summarizes nominal values of
Theminimumon-timetheLTM4641supportsis43ns,typi-
cal, but guard banded conservatively to 75ns, maximum.
Therefore, for a conservative design, t should be larger
ON
R
endorsed for some popular output voltages; use
fSET
than 75ns, typical. From Equation 1, it follows that I
ION
of commonly available 5% tolerance resistors or better
with 100ppm/°C temperature coefficient or better is
recommended.
should be designed to be less than 93.3μA.
When an external R
resistor is applied between V
INL
fSET
and R
(and V and V are operating from the same
fSET
INL INH
rail—Figure 45 and Figure 46), the switching frequency of
100
50
700
600
500
400
300
200
100
0
operation of the power stage at full load, in Hz, is given by:
R
NOT
fSET
R
vs V
OUT
fSET
NEEDED FOR
> 3V
REGION
V
OUT
TO AVOID
VOUT
fSW
=
(5)
is the desired nominal
0.7V • 1.3MΩ||R
•10pF
(
)
10
5
fSET
where R
is in ohms, and V
fSET
OUT
output voltage, in units of volts.
1
0.5
In the general case, the switching frequency of the buck
converter power stage at full load is given, in Hz, by:
MAX RECOMMENDED R
fSET
SWITCHING FREQUENCY
2.5 3.5 4.5 5.5
NOMINAL OUTPUT VOLTAGE (V)
0.1
VOUT
INH •tON
VOUT • I
ION
VINH •0.7V •10pF
0
0.5
1
1.5
2
3
4
5
6
fSW
=
=
(6)
V
4641 F03
See Appendix C for a detailed discussion on the following
topics:
Figure 3. ꢁaꢂiꢃuꢃ Recoꢃꢃended RfSET (Noꢃinal Values) for
Non-Tracking Applications, and Resulting Full-Load Operating
Switching Frequency vs Noꢃinal Output Voltage
•ꢀ Whyꢀshouldꢀtheꢀswitchingꢀcontrollerꢀbeꢀoperatedꢀatꢀaꢀ
higher switching frequency (i.e., programmed for a
shorter on-time with R ) than that yielded by the
fSET
internal 1.3MΩ V -to-f resistor alone…
INL
SET
…for nominal output voltages of 3V and less?
…in rail-tracking applications?
4641f
19
Linear [ Linear ]
