AS3696

austriamicrosystems

www.austriamicrosystems.com

Rev 1.10 / 1011-04-04

6 - 16

Digital output open drain

4

Detailed Block description

4.1

Current outputs

4.1.1 Precision current sink

All current sinks are built with an internal error amplifier A1 and an external power transistor. The

external transistor should be a NMOS type to keep the current accuracy.

The output current during PWM=1 can be calculated:

in normal mode ( 3D_Mode = 0 )

in 3D mode ( 3D_Mode = 1 )

4.1.2 Output voltage monitoring

In order to monitor the proper DCDC output voltage the voltage at pin “Dx” is measured during PWM=1.

If this voltage is too low a comparator turns on an transconductance amplifier which is able to control the

output voltage of the external power supply via pin FB1 or FB2.

4.1.3 Open LED detection

If a LED-string is broken the voltage at pin Dx gets lower than Vset_int/2. This status is detected and

accumulated by a comparator during PWM=1. If the accumulated status lasts longer than 100ms, a fault

is indicated and the corresponding power feedback function is turned off. After 500ms the fault is reset

and the detection starts again.

For proper detection the PWM high time has to be longer than 500us.

4.1.4 Short LED detection

Shorted LEDs in a LED-string will cause higher voltage at pin “Dx”. A higher voltage during PWM=1 is

detected by a comparator and will trigger a “short LED detection” fault. The duration of the fault is

7V

VDD

GND

Sx

Gx

FBx

Vset_int

Open

Monitor

<20V

Slew

Rate

control

Dx

Short

Monitor

Optional:

A1

200mV - 500mV

50V

Vset2

PWM_int

Rset

Vshort

200mV - 500mV

Vset1

3D_Mode

PWM_int

120k

30k

Vfb

OTA

HV-cascode

auto

turnoff

debounce

100ms

feedback

turnoff

debounce

100ms

VDD

Vset_int/2