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LT3797

3797f

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APPLICATIONS INFORMATION

High Side PMOS Disconnect Switch Selection

A PMOS with a minimum VGS(TH) of –1V to –2V is rec-

ommended for the high side disconnect switch in most

LT3797 applications to improve the PWM dimming ratio

and protect the LED array from excessive heating during

fault conditions. The PMOS BVDSS rating must be higher

than the open-LED regulation voltage set by the FBH pin.

The maximum continuous drain current ID(MAX) rating

should be higher than the maximum LED current.

Input Capacitor Selection

The input capacitor CIN supplies the AC ripple current to

the power inductor of the converter and must be placed

and sized according to the transient current requirements.

Theswitchingfrequency,outputcurrentandtolerableinput

voltage ripple are key inputs to estimating the required

capacitor value. The X5R or X7R type ceramic capacitors

are usually good choices since they have small variation

withtemperatureandDCbias.Typically,theboostorSEPIC

converter requires a lower value input capacitor than the

buck mode or buck-boost mode converter, due to the fact

that its inductor is in series with the input, and the input

current waveform is continuous. The input capacitor value

can be estimated based on the inductor ripple

∆IL (refer to

Inductor Selection section), the switching frequency, and

the acceptable input voltage ripple

∆VIN on CIN. CIN value

of the boost and SEPIC converter can be calculated by:

CIN = 0.125•

∆IL

∆VIN • f

CIN value of the buck mode and buck-boost mode LED

driver can be calculated by:

CIN =ILED •

VLED • VIN(MIN) – VLED

(

)

VIN(MIN)2 • ∆VIN • f

Output Capacitor Selection

The output filter capacitors should be sized to attenuate

the LED current ripple. Use of X5R or X7R type ceramic

capacitors is recommended. To achieve the same LED

ripple current, the required filter capacitor is smaller in the

buck mode applications than that in the boost, buck-boost

mode and SEPIC applications. This is due to the fact that,

in the buck converter, the inductor is in series with the

output and the ripple current flowing through the output

capacitor is continuous. Lower operating frequencies will

require proportionately higher capacitor values.

The DC Coupling Capacitor Selection for

SEPIC LED Driver

The DC voltage rating of the DC coupling capacitor, CDC,

connected between the primary and secondary inductors

should be larger than the maximum input voltage:

VCDC > VIN(MAX)

CDC has nearly a rectangular current waveform in CCM.

During the switch off-time, the current through CDCisIVIN,

while approximately –ILED flows during the on-time. The

CDC voltage ripple causes distortions on the primary and

secondary inductor current waveforms. The CDC should

be sized to limit its voltage ripple. The power loss on the

CDC ESR reduces the LED driver efficiency. Therefore, the

sufficient low ESR ceramic capacitors should be selected.

TheX5RorX7RceramiccapacitorisrecommendedforCDC.

Integrated INTVCC Power Supply

The LT3797 includes an internal switch mode DC/DC con-

verter to generate a regulated 7.5V INTVCC power supply

to power the NMOS gate drivers of the three channels

(IDRIVE). This INTVCC power supply can also be used to

drive external circuits (IEXT). This INTVCC power supply

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