STLD20D

5/8

FUNCTIONAL DESCRIPTION

1. BOOST CONTROLLER:

The STLD20D is a PWM mode control boost con-

verter operating at 500kHz in discontinuous mode.

An automatic compensation of the oscillation ramp

allows rejection of the battery voltage transient. The

LED constant regulation (referred

by sensing the LED current through the resistance

the feedback loop of the controller (pin FB).

2. PEAK INDUCTOR CURRENT LIMITATION

AND SOFT START FUNCTION:

An integrated current sensor senses the peak drain current of the switch SW in order to keep the inductor

STLD20D-C8), the RS flip flop turns off the switch SW. During start up, this peak drain current limitation

acts inherently like a soft start function .

3. PEAK INDUCTOR CURRENT ADJUSTABILITY

(STLD20D-C8 ONLY)

The peak current of the boost inductor should always

be below the saturation current. In order to provide

flexibility in the selection of the inductor, the maxi-

mum peak inductor current can be adjusted by con-

pected on the battery voltage bus, then the

maximum peak inductor current should be reduced.

4. SHUTDOWN:

The SHDN pin is a low logic input signal and allows turning off the controller without cutting the input volt-

age from the boost regulator circuit. An integrated LED disconnect switch LDS disconnects the LEDs

branch in shutdown mode .This arrangement allows eliminating the DC current path that normally exists

with traditional boost regulator in shutdown mode.

5. BRIGHTNESS CONTROL:

The brightness of the Led is adjusted by pulsing the shutdown pin with a low frequency PWM signal.

By using such a PWM signal the controller is alternatively ON and OFF and the LED current changes from

full current to zero. The duty cycle allows to regulate the average LED current .

This scheme ensures that when the LEDs are ON they are driven at the full current without risk of color change.

6. OTP:

An integrated temperature sensor senses the temperature of the junction of the controller. when this tem-

perature exceeds 110°C min fixed internally the controller is automatically turned OFF . When the tem-

perature is reduced the operation of the device automatically recovers.

Figure 6: LED current versus input voltage

V(V)

IN

I

(mA)

LED

19.00

19.20

19.40

19.60

19.80

20.00

20.20

20.40

20.60

20.80

21.00

2.5

3

3.5

4

4.5

5

I(A)

Lmax

R

= f(I

)

SET

Lmax

R(k )

SET

Ω

0

20

40

60

80

100

120

0.3

0.4

0.5

0.6

0.7

V =3.2V

IN

V =4.2V

IN