［AP2152］

MS1348-E-00

11

2012/05

2-9) LED short protection and Cout setting

The AP2152 has a built in LED short protection circuit. If an LED is broken as a short state and the CH

pin voltage exceeds 10.8V, the corresponding constant current circuit will be ceased.

If there is an unused CH pin, the LED anode voltage increases up to the OVP setting voltage by the

LED open detection circuit. In this case, the CH pin temporarily increases and becomes the same state

as the LED short protection, in order to distinguish this case from LED short protection operation, the

opened CH pin is excluded using the IC internal counter. For this operation, the Cout value is

constrained, so Cout should be determined using the following formula.

LED short protection count time＞CH pin voltage rising time (10.8V or higher)

①LED short protection count time

1920

×

Fsw

1

=

②

　

95

.

0

Iout

)

V

8

.

10

-

)

min

(

LEDVf

-

)

max

vp(

Vo

(

Cout

time

rising

voltage

pin

CH

×

×

=

2-10) PGOOD function

The PGOOD pin is an Nch open drain output. The internal FET on the PGOOD pin stays off when the

supply voltage is 0V (If the PGOOD pin is connected to a pull-up resistor, the PGOOD pin becomes high

level) If the supply voltage reaches to approximately 2V, the internal circuits are biased and the

internal PGOOD FET turns on. If the supply voltage is more than UVLO and the PWM pin is high, the

standby circuit will be released, the IC recognizes as normal operation and the internal PGOOD FET

turns off. If a protection circuit is activated after normal operation, the internal PGOOD turns on.

(Relationship between protection function and the PGOOD pin state is later described in “Protection

Function”). By applying a PWM low level for longer than 50ms, the IC will be reset and the internal

PGOOD FET turns on in a standby state.

When the LED anode is shorted to GND, the internal PGOOD FET immediately is turned on by the

OVP pin short circuit protection and the PGOOD signal can be used to cut the path from VIN to the LED

anode. (Refer to Figure 7)

The AP2152 employs a timer latch method for protection functions. If the FET over current protection,

thermal protection, CH short protection, LED open protection, or LED short protection is activated, the

IC is latched off and turns off the LEDs.

For example, when the PGOOD pin is monitored by a microcomputer and an abnormal state (PGOOD

pin with pull-up resistor is low level) is detected, the microcomputer controls a restart sequence after

the IC is reset, so a highly stable system configuration is possible.