IS31LT3360-SDLS3-TR datasheet(11/15 Pages) ISSI

Part #	IS31LT3360-SDLS3-TR

Description	40V/1.2A LED DRIVER WITH INTERNAL SWITCH

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Manufacturer	ISSI [Integrated Silicon Solution, Inc]

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11 / 15 page

IS31LT3360

Integrated Silicon Solution, Inc. – www.issi.com

11

Rev. C, 12/22/2013

series with the supply source impedance and lowers

overall efficiency. This capacitor has to supply the

relatively high peak current to the coil and smooth the

current ripple on the input supply.

If the source is DC supply, the capacitor is decided

by ripple of the source, the value is given by Equation

(3):

MAX

ON

F

MIN

U

t

I

C







(3)

IF is the value of output current，

MAX

U



is the ripple

of power supply. tON is the “ON” time of MOSFET.

The value is higher than the minimum value. A

100µF capacitor is recommended.

If the source is an AC supply, typical output voltages

ripple from a nominal 12V AC transformer can be

±10%.If the input capacitor value is lower than 220μF,

the AC input waveform is distorted, sometimes the

lowest value will be lower than the forward voltage of

LED strings. This lower the average current of the

LEDs. So it is recommended to set the value of the

capacitor bigger than 220µF.

INDUCTOR SELECTION

Recommended inductor values for the IS31LT3360

are in the range 47μH to 220μH.

Higher values of inductance are recommended at

higher supply voltages and low output current in

order to minimize errors due to switching delays,

which result in increased ripple and lower efficiency.

Higher values of inductance also result in a smaller

change in output current over the supply voltage

range. The inductor should be mounted as close to

the chip as possible with low resistance connections

to the LX and VIN pins.

The chosen coil should have a saturation current

higher than the peak output current and a continuous

current rating above the required mean output

current. It is recommended to use inductor with

saturation current bigger than 1.2A for 700mA output

current and inductor with saturation current bigger

than 500mA for 350mA output current.

The inductor value should be chosen to maintain

operating duty cycle and switch 'on/off' times within

the specified limits over the supply voltage and load

current range.

The following equations can be used as a guide.

LX Switch 'ON' time:

)

(

LX

L

S

AVG

LED

IN

ON

R

I

V

I

L

t











(4)

Note: tON_MIN > 200ns.

LX Switch 'OFF' time:

)

(

S

L

AVG

D

LED

OFF

R

I

V

I

L

t









(5)

Note: tOFF_MIN > 200ns.

Where:

L is the coil inductance (H)

RL is the coil resistance (Ω)

IAVG is the required LED current (A)

∆I is the coil peak-peak ripple current (A) {Internally

set to 0.3 × IAVG}

VIN is the supply voltage (V)

VLED is the total LED forward voltage (V)

RLX is the switch resistance (Ω)

VD is the diode forward voltage at the required load

current (V)

Example:

For VIN=12V, L=47μH, RL=0.26Ω, VLED=3.4V, IAVG

=333mA, VD =0.36V, RS = 0.3Ω, RLX=0.27Ω:

s

t

ON



564

.

0

)

27

.

0

26

.

0

3

.

0

(

333

.

0

4

.

3

12

333

.

0

3

.

0

47













s

t

OFF



19

.

1

)

3

.

0

26

.

0

(

333

.

0

36

.

0

4

.

3

333

.

0

3

.

0

47













This gives an operating frequency of 570kHz and a

duty cycle of 32%.

Optimum performance will be achieved by setting the

duty cycle close to 50% at the nominal supply voltage.

This helps to equalize the undershoot and overshoot

and improves temperature stability of the output

current.

DIODE SELECTION

For maximum efficiency and performance, the

rectifier (D1) should be a fast low capacitance

Schottky diode with low reverse leakage at the

maximum operating voltage and temperature.

If alternative diodes are used, it is important to select

parts with a peak current rating above the peak coil

current and a continuous current rating higher than

the maximum output load current. It is very important

to consider the reverse leakage of the diode when

operating at high temperature. Excess leakage will

increase the power dissipation in the device.

The higher forward voltage and overshoot due to

reverse recovery time in silicon diodes will increase

the peak voltage on the LX output. If a silicon diode is

used, care should be taken to ensure that the total

voltage appearing on the LX pin including supply

ripple, does not exceed the specified maximum

value.

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