LT1432 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1432' PDF : 28 Pages View PDF
LT1432
APPLICATI S I FOR ATIO
Internal Restart Sequence
At very light load currents (>10mA), coupled with low
input voltages (<8.5V), it is possible for the basic architec-
ture used by the LT1432 to assume a stable output state
of less than 5V. To avoid this possibility, the LT1432 has
an internal timer which applies a temporary 20mA load to
the output if the output is below its regulated value for
more than 1.8ms. This action is normally transparent to
the user.
Auxiliary Outputs – “Free” Extra Voltages
Semi-regulated secondary outputs may be added to buck
converters by adding additional windings to the main
inductor. These outputs will have a typical regulation of 5
to 10%, but have one very important limitation. The total
output power of the auxiliary windings is limited by the
output power of the main output. If this limit is exceeded,
the auxiliary winding voltages will begin to collapse,
although the main 5V output is unaffected by collapse of
the secondary. The auxiliary power available is also a
function of input voltage. At higher input voltages signifi-
cantly more power is available.
Figure 19 shows the ratio of maximum auxiliary power to
main output power, versus input voltage. The auxiliary
output was loaded until its output voltage dropped 10%.
For applications which push the limit of theoretically
available current, care should be used in winding the
inductor. The effects of leakage inductance and series
resistance are magnified at low input voltage where aux-
iliary winding currents are many times DC load current.
Also, be aware that output voltage ripple on the 5V main
output can increase significantly when the auxiliary output
is heavily loaded. The inductor is acting partially like a
transformer, so the AC current delivered to the 5V output
capacitor increases in amplitude and shifts from a tri-wave
to a trapezoid with much faster edges.
A typical example would be a +5V buck converter with a
minimum load of 500mA. Output power is 5V × 0.5A =
2.5W. Maximum power from the auxiliary windings would
be 1.25W for input voltages of 9V and above. If we assume
a low dropout linear regulator on the auxiliary output, with
a regulated output voltage of minus 5V, the auxiliary
winding output would have to be about minus 7V. Maxi-
mum output current from the 7V output would be 1.25W/
7V = 178mA. Note that the power restriction is the total for
all auxiliary outputs.
The formula to calculate turns ratio for the auxiliary
windings versus main winding is simple:
[ ( ) ] NAUX
=
NMAIN
VAUX + VDO =
5V + VD
2V
+ VDA
NMAIN = Number of turns on main inductor winding
NAUX = Number of turns on auxiliary winding
VDA = Auxiliary diode forward voltage
VD = Main 5V catch diode forward voltage
VDO = Allowance for regulation of auxiliary winding and
dropout voltage of low-dropout linear regulator used on
auxiliary winding. Set equal to zero if no regulator is used.
2.0
1.5
1.0
0.5
0
0
5
10
15
20
INPUT VOLTAGE (V)
LT1432 F19
Figure 19. Auxiliary Power vs 5V Power
It is not necessary to use a linear regulator on the auxiliary
winding if 5 to 10% regulation is adequate. Line regulation
will be fairly good, but variations in auxiliary voltage will
occur with load changes on either the auxiliary winding or
the 5V output. For relatively constant loads, regulation will
be significantly better.
22
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