AS3695C datasheet(9/40 Pages) AMSCO

9 / 40 page

AS3695C

austriamicrosystems

www.austriamicrosystems.com

Rev 1.11 / 2010-12-08

8 - 39

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 can either be a NMOS or a NPN bipolar transistor. The driving current capability of

the output amplifier is 2mA. For low EMI radiation the slew rate of the amplifier output voltage can be

adjusted between 1us and 9us

4.1.2 Power supply feedback

The voltage on the pins “Dx” is monitored to adjust the DCDC output voltage. If this voltage is lower than

“FBvoltage” a comparator turns on a NMOS transistor which is able to control the output voltage of the

external power supply via pin FB1 or pin FB2. The feedback comparator can be assigned to either FB1

or FB2. The power supply feedback can be turned off for all channels by the flag “FBen”. If an output is

turned off with the corresponding bit in the CUR_ON_1/2 -registers, the feedback function of this output

is also automatically turned off.

4.1.3 Open LED detection

If a LED-string is broken the voltage at the current setting resistor goes below “OPENvoltage”. This

status is detected by a comparator and if this status lasts longer than 100ms a fault is indicated and

optionally the output and the corresponding power feedback function is turned off. This feature can also

be used for external resistor short detection during production. For proper detection the PWM high time

has to be longer than 50us.

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

accumulated and if the time exceeds 100ms a fault is indicated and optionally the output is turned off. If

the high-time of the waveform is shorter than 100ms it will take more periods to trigger this fault. For

proper detection the PWM high time has to be longer than 50us.

4.1.5 Fault detection

If an Open-LED or Short-LED condition is detected the fault-signal is debounced with an internal 100ms

clock. This means that the fault will be indicated in the OPENLED or SHORTLED registers between

100ms and 200ms. In order to clear the bit in these registers a “1” has to be written.

4.1.6 DAC

The reference voltage for the output stage is generated by an internal 10-bit DAC. The DAC reference

can be selected between 500mV and 800mV depending on register settings. The DAC is trimmed

during production with DACref = 800mV to guarantee an output current accuracy of ±0.5% on every

current output.

D Q

PWMx

Delay

20us/40us

500kHz

FAULT

D Q

100ms

S Q

CLEAR FAULT BIT BY WRITING

OPENLED

SHORTLED

registers

To STATUS register

From other

fault bits

1 to REGISTER

AS3695C Datasheet(PDF) 9 Page - ams AG

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