LT1432CS8 datasheet(22/28 Pages) LINER

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LT1432

APPLICATI

I FOR ATIO

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:

AUX

MAIN

AUX

()+

[]

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.

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

INPUT VOLTAGE (V)

1.0

1.5

LT1432 F19

0.5

2.0

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.

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