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LT1683

Slew Rate Controlled Ultralow Noise Push-Pull DC/DC Controller

Linear Technology 

LT1683

APPLICATIONS INFORMATION

There are a number of ways to choose the inductance

value for L. We suggest as a starting point that L be selected

such that the converter is continuous at IOUT(MAX)/4. If your

minimum IOUT is higher than this or your components can

handle higher peak currents then use a higher number.

D1

1:N

L

VOUT

VIN

LPRI

C

ROUT

D2

RSENSE

1683 F07

Figure 7. Push-Pull Topology

Continuous operation occurs when the current in the

inductor never goes to zero. Discontinuous operation

occurs when the inductor current drops to zero before the

start of the next cycle and can occur with small inductors

and light loads. There is nothing inherently bad about

discontinuous operation, however, converter control and

operation are somewhat different. The inductor is smaller

for discontinuous operation but the peak currents in the

switch, the transformer, the diodes, inductor and capacitor

will be higher which may produce greater losses.

For continuous operation the inductor ripple current must

be less than twice the output current. The worst case for

this is at maximum input (lowest duty cycle, DCMIN) but

in the following we will evaluate at nominal input since

the IOUT/4 is somewhat arbitrary.

Note when both inputs are off, the inductor current splits

between both secondary outputs and the diode common

goes to 0V.

Looking at the inductor current during off time, output

ripple current is:

∆ IOUT = 2 • IOUT(MIN)

IOUT(MIN) = IOUT(MAX) / 4

( ) L = VOUT(MIN) + VF • (1− 2 • DC)

∆ IOUT • f

18

The inductance of the transformer primary should be such

that L, when reflected into the primary, dominates the

input current. In other words, we want the magnetizing

current of the transformer small with respect to the current

going through the transformer to L. In general, then, the

inductance of the primary should be at least five times that

of L reflected to the input. This ensures that most of the

power will be passed through the transformer to the load.

It also increases the power capability of the converter and

reduces the peak currents that the switch will see.

LPRI

=

5•L

N2

If the magnetizing current is small, say below 100mA, then

a smaller L can be used with a higher percentage of the

switch current generated by the magnetizing current.

With the value of L set, the ripple in the inductor is:

∆IL

=

( VOUT

+

VF ) • (1−

L•f

2

•

DC)

However, the peak inductor current is evaluated at maxi-

mum load and maximum input voltage (minimum DC).

IL(MAX )

=

IOUT(MAX

)

+

∆IL(MAX

2

)

The magnetizing ripple current can be shown to be:

∆IMAG

=

VOUT + VF

N •LPRI • f

and the peak current in the switch is:

ISW(PEAK) = N • IL(MAX) + ∆IMAG

This current should be less than the current limit.

Worst-case switch ripple is:

∆ISW(PEAK) = N • ∆IL(MAX) + ∆IMAG

In the push-pull converter the maximum switch voltage

will be 2 • VIN. Because voltage is slew-controlled, the

leakage spikes are small. So, the MOSFET should have a

maximum rated switch voltage at least 20% higher than

2 • VIN.

1683fd

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