LT1683 Ver la hoja de datos (PDF) - Linear Technology

Número de pieza

componentes Descripción

Lista de partido

LT1683

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

Linear Technology 

LT1683

APPLICATIONS INFORMATION

for capacitance. However, noise depends more on the ESR

of the capacitors. In addition lower ESR can also improve

efficiency.

Input capacitors must also withstand surges that occur

during the switching of some types of loads. Some solid tan-

talum capacitors can fail under these surge conditions.

Design Note 95 offers more information but the following

is a brief summary of capacitor types and attributes.

Aluminum Electrolytic: Low cost and higher voltage. They

can be used in this application but in general you will

need higher capacitance to achieve low ESR. Additional

nonelectrolytic capacitors may be required to achieve

better performance.

Specialty Polymer Aluminum: Panasonic has come out

with their series CD capacitors. While they are only avail-

able for voltages below 16V, they have very low ESR and

good surge capability.

Solid Tantalum: Small size and low impedance. Typically

the maximum voltage rating is 50V. With large surge cur-

rents 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/dis-

charge 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 op-

tions are available for choice of capacitor. Values of input

capacitor for just VIN requirement will typically be in the

50µF 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.

This switch current computation can be used to estimate

the capacity for these capacitors:

CIN

=

1

∆ VCAP

∆ ISW (MAX )

− ESR

•

DCMIN

f

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

components of the ripple current in the capacitor, the

capacitor ESR and the magnitude of the output ripple

1683fd

20

Share Link: