APW7209 Datasheet, PDF (8/14 Pages) Anpec Electronics Coropration – 1MHz, High-Efficiency, Step-Up Converter for 2 to 10 White LEDs

APW7209
Function Description
Main Control Loop
The APW7209 is a constant frequency current-mode
switching regulator. During normal operation, the inter-
nal N-channel power MOSFET is turned on each cycle
when the oscillator sets an internal RS latch and turned
off when an internal comparator (ICMP) resets the latch.
The peak inductor current at which ICMP resets the RS
latch is controlled by the voltage on the COMP node, which
is the output of the error amplifier (EAMP). An external
current-sense resistor connected between cathode of the
lowest LED and ground allows the EAMP to receive a
current feedback voltage V_FB at FB pin. When the LEDs
voltage increases to cause the LEDs current to decrease,
it causes a slightly decrease in V_FB relative to the 0.3V
reference, which in turn causes the COMP voltage to in-
crease until the LEDs current reaches the set point.
VIN Under-Voltage Lockout (UVLO)
The Under-Voltage Lockout (UVLO) circuit compares the
input voltage at VIN with the UVLO threshold (2.2V rising,
typical) to ensure the input voltage is high enough for
reliable operation. The 100mV (typ) hysteresis prevents
supply transients from causing a restart. Once the input
voltage exceeds the UVLO rising threshold, startup begins.
When the input voltage falls below the UVLO falling
threshold, the controller turns off the converter.
Soft-Start
The APW7209 has a built-in soft-start to control the N-
channel MOSFET current rise during start-up. During soft-
start, an internal ramp voltage, connected to one of the
inverting inputs of the comoarator ICMP, raise up to re-
place the output voltage of error amplifier until the ramp
voltage reaches the V_COMP.
Current-Limit Protection
The APW7209 monitors the inductor current, flowing
through the N-channel MOSFET, and limits the current
peak at current-limit level to prevent loads and the
APW7209 from damages in overload conditions.
Over-Temperature Protection (OTP)
The over-temperature circuit limits the junction tempera-
ture of the APW7209. When the junction temperature ex-
ceeds 150^oC, a thermal sensor turns off the power
MOSFET, allowing the device to cool. The thermal sen-
sor allows the converter to start a soft-start process and
regulate the LEDs current again after the junction tem-
perature cools by 40^oC. The OTP is designed with a 40^oC
hysteresis to lower the average Junction Temperature
(T ) during continuous thermal overload conditions, in-
J
creasing the lifetime of the device.
Enable/Shutdown
Driving EN to ground places the APW7209 in shutdown
mode. When in shutdown, the internal power MOSFET
turns off, all internal circuitry shuts down and the quiescnet
supply current reduces to 1µA maximum.
This pin also could be used as a digital input allowing
brightness controlled by using a PWM signal with fre-
quency from 100Hz to 100kHz. The 0% duty cycle of PWM
signal corresponds to zero LEDs current and 100% cor-
responds to full one.
Open-LED Protection
In driving LED applications, the feedback voltage on FB
pin falls down if one of the LEDs, in series, is failed.
Meanwhile, the converter unceasingly boosts the output
voltage like a open-loop operation. Therefore, an over-
voltage protection (OVP), monitoring the output voltage
via OVP pin, is integrated into the chip to prevent the LX
and the output voltages from exceeding their maximum
voltage ratings. When the voltage on the OVP pin rises
above the OVP threshold (40V, typical), the converter
stops switching and prevents the output voltage from
rising. The converter can work again when the falling OVP
voltage falls below the OVP voltage threshold.
Copyright © ANPEC Electronics Corp.
8
Rev. A.3 - Jul., 2008
www.anpec.com.tw