Diodes AP3612 - Manual (Page 13)

Preliminary Datasheet
Boost Type LED Driver with 12-Channel Current Source AP3612
Aug. 2012 Rev. 1. 1 BCD Semiconductor Manufacturing Limited
13
Application Information
1. Enable
The AP3612 is enabled when the voltage to EN is
greater than approximately 2.4V, disabled when lower
than 0.5V.
2. Frequency Selection
An external resistor R
T
, placed between RT pin and
GND, can be used to set the operating frequency. The
operating frequency ranges from 100kHz to 1MHz.
The high frequency operation optimizes the regulator
for the smallest-sized component application, while
low frequency operation can help to reduce switch
loss. The approximate operating frequency can be
expressed as below:
] [
52
] [
Ω
=
K R
MHz f
RT
OSC
3. LED Current Setting
The maximum LED current per channel can be
adjusted up to 75mA via ISET pin. When ≥75mA
current is needed in application, two or more
channels can be paralleled to provide larger drive
current. Connect a resistor R
ISET
between ISET pin
and GND to set the reference current I
SET
. The LED
current can be expressed as below:
] [
400
] [
Ω
=
K R
mA I
ISET
LED
4. Dimming Control
Applying a PWM signal to DIM pin to adjust the
LED current, that means, the LED current of all
enabled channels can be adjusted at the same time
and the LED brightness can be adjusted from 1%×
I
CHX
_
MAX
to 100% × I
CHX
_
MAX
. During the “high
level” period of PWM signal, the LED is turned on
and 100% of the current flows through LED, while
during the “low level” period of the PWM signal, the
LED is turned off and almost no current flows
through the LED, thus changing the average current
through LED and finally adjusting LED brightness.
The external PWM signal frequency applied to PWM
pin is allowed to be 100Hz or higher.
5. Status Output
After IC is enabled, STATUS will output logic low if
any of the following conditions exists:
1) Any String is Open
2) LED Short Circuit Protection
3) Shut Down Under Abnormal Condition
4) Over Temperature Protection
5) Schottky Diode Short Protection
6) Over Voltage Protection
7) V
OUT
Short/Open Schottky Diode Protection
6. Over Voltage Protection
The AP3612 integrates an OVP circuit. The OVP pin
is connected to the center tap of voltage-divider (R
OV1
and R
OV2
) that placed between high voltage output
and GND. If the voltage on OVP pin exceeds 2.0V,
which may results from open loop or excessive
output voltage, all the functions of AP3612 will be
disabled with output voltage falling. The OVP
hysteresis is 250mV. The formula of OVP can be
expressed as below:
OV2
OV2 OV1
OVP R
2.0V ) R (R
V
× +
=
7. Over Current Protection
The AP3612 integrates an OCP circuit. The CS pin is
connected to the voltage-sensor (R
CS
) that placed
between the Drain of MOS and GND. If the voltage
on CS pin exceeds 0.54V, it is turned off immediately
and will not turn on until the next cycle begins.
8. LED Short-circuit Protection
The AP3612 integrates an LED Short-circuit
protection circuit. If the voltage at any of the
CH1-CH12 pins exceeds a threshold of
approximately 7.3V during normal operation, the
corresponding string is turned off and is latched off.
Toggle V
IN
and/or EN to reset the latch. LED short
detecting logic priority is lower than open LED and
OVP logic. The LED short detecting is triggered
when 0.1V<V
LED
_
MIN
under dimming on mode,
disabled when LED open occurs until output voltage
resumes to the regulated voltage.