LT1683 datasheet(20/24 Pages) LINER

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LT1683

1683f

Solid Tantalum: Small size and low impedance. Typically

the maximum voltage rating is 50V. With large surge

currents the capacitor may need to be derated or you need

a special type such as AVX TPS line.

OS-CON: Lower impedance than aluminum but only avail-

able for 35V or less. Form factor may be a problem.

Ceramic: Generally used for high frequency and high

voltage bypass. They may resonate with their ESL before

ESR becomes dominant. Recent multilayer ceramic (MLC)

capacitors provide larger capacitance with low ESR.

There are continuous improvements being made in ca-

pacitors so consult with manufacturers as to your specific

needs.

Input Capacitors

The input capacitor should have low ESR at high frequen-

cies since this will be an important factor concerning how

much conducted noise is created.

There are two separate requirements for input capacitors.

The first is for supply to the part’s VIN pin. The VIN pin will

provide current for the part itself and the gate charge

current.

The worst component from an AC point is the gate charge

current. The actual peak current depends on gate capaci-

tance and slew rate, being higher for larger values of each.

The total current can be estimated by gate charge and

frequency of operation. Because of the slewing with this

part gate charge is spread out over a longer time period

than with a normal FET driver. This reduces capacitance

requirements.

Typically the current will have spikes of under 100mA

located at the gate voltage transitions. This is charge/

discharge to and from the threshold voltage. Most slewing

occurs with the gate voltage near threshold.

Since the part’s VIN will typically be under 15V many

options are available for choice of capacitor. Values of

input capacitor for just VIN requirement will typically be in

the 50

µF range with an ESR of under 0.1Ω.

In addition to the part supply, decoupling of the supply to

the transformer needs to be considered. If this is the same

supply as the VIN pin then that capacitor will need to be

increased. However, often with this part the transformer

supply will be a higher voltage and as such a separate

capacitor.

The transformer decoupling capacitor will see the switch

current as ripple.

The above switch current computation can be used to

estimate the capacity for these capacitors.

ESR

CAP

SW MAX

MIN

∆

−

()

•

where

∆VCAP is the allowed sag on the input capacitor.

ESR is the equivalent series resistance for the cap. In

general allowed sag will be a few tenths of volts.

Output Filter Capacitor

The output capacitor is chosen both for capacity and ESR.

The capacity must supply the load current in the switch off

state. While slew control reduces higher frequency com-

ponents of the ripple current in the capacitor, the capacitor

ESR and the magnitude of the output ripple current

controls the fundamental component. ESR should also be

low to reduce capacitor dissipation.

The capacitance value can be computed by consideration

of desired load ripple, duty cycle and ESR.

ESR

OUT

LMAX

MIN

∆

−

()

•

APPLICATIO S I FOR ATIO

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