aoz1960 Alpha & Omega Semiconductor, aoz1960 Datasheet - Page 12

• Manufacturer Part Number: aoz1960
• Description: Multi-string Boost White Led Driver
• Manufacturer: Alpha & Omega Semiconductor

Detailed Description
AOZ1960 is a 6-string boost LED driver with integrated
boost switch and input protection switch. The input
voltage range is from 7V to 24V and output voltage can
go as high as 55V and drive up to 12 LEDs in series for
each string. LED current of each string is individually
regulated by an internal current regulator to achieve ±3%
current matching. AOZ1960 supports two brightness
control methods: analog dimming and digital dimming. In
analog dimming mode, the LED current can be directly
adjusted through ABRT pin to achieve 10% to 100%
brightness range. In digital dimming mode, a PWM signal
on DBRT controls the brightness by turning the current
regulators on and off. The brightness is proportional to
the duty cycle of the PWM signal. The maximum LED
current is set with an external resistor in ISET pin.
AOZ1960 features multiple protection functions. The
cycle by cycle over-current protection (OCP) limits the
inductor peak current to less than 2.5A. The input
protection switch turns off when a short circuit is detected
at the output. The cycle by cycle over-voltage protection
(OVP) prevents the output voltage from running away
when LED strings are open. OVP threshold can be
adjusted using a voltage divider. The maximum allowed
output voltage is 55V. Thermal shutdown function
provides another layer of protection when junction
temperature reaches 145ºC. Internal soft-start effectively
limits the inrush current during startup. In addition,
AOZ1960 provides an open-drain fault indicator when a
fault such as open LED, shorted LED, short circuit, or
over temperature occurs.
Normal Operation
AOZ1960 boost LED driver uses constant frequency
PWM control architecture. Different from regular boost
regulators which regulate output voltage, AOZ1960
regulates LED current.
There are six internal current regulators. The minimum of
the six current sense signals is fed to a transconductance
error amplifier and compared with an internal reference.
The PWM comparator compares the error amplifier
output with the inductor current ramp signal to set a
flip-flop, which turns off the boost switch. The boost
switch turns on when the oscillator clock comes,
connecting the inductor between the input supply and
ground. The inductor current linearly ramps up storing
energy in the magnetic field. The boost switch is turned
off by PWM comparator. When the boost switch is off, the
inductor current freewheels through the output Schottky
diode, connecting the inductor between the input supply
and output. The inductor current linearly ramps down and
Rev. 1.0 August 2010
www.aosmd.com
energy stored in the inductor is transferred to the output.
Therefore, if minimum of the six current sense signals is
below internal reference, AOZ1960 will keep boosting the
output voltage until it reaches regulation. This control
method ensures the output voltage is just high enough to
drive all six LED strings without wasting any extra power.
Input Protection Switch (LIN)
A major problem of traditional boost regulators is that
they cannot protect against output short-circuit fault.
When the output is shorted to ground, short-circuit
current will flow directly from input supply, through the
inductor and Schottky diode, to ground. Typically a fuse
is required for output short-circuit protection. AOZ1960
solves this issue by integrating a high-voltage (30V) low
on-resistance (50mΩ) P-channel input protection switch.
The input switch is connected between the input supply
(IN) and boost inductor (LIN). In normal operation, the
switch is fully turned on connecting the input supply to
the inductor. The input switch is turned off whenever
AOZ1960 detects the output voltage is below the input.
To ensure startup, a startup resistor (1kΩ) should be
connected between IN (source of input switch) and LIN
(drain of input switch) to make sure output is close to
input before AOZ1960 is enabled.
Linear Regulator (BIAS)
AOZ1960 has an internal 5V linear regulator that powers
all the internal circuitry including power MOSFET driver.
BIAS is the output of the internal regulator. Connect a
1μF or larger ceramic capacitor between BIAS and
ground. The regulator is used for internal circuitry only.
Do not connect any external load to BIAS pin.
The regulator is enabled when EN is logic high. If BIAS
voltage is above UVLO threshold (4.3V), AOZ1960 is
enabled: the input switch is turned on and the boost
regulator starts switching. If BIAS voltage is below UVLO
threshold, AOZ1960 remains off.
Current Regulators (LED1 to LED6)
There are six internal current regulators. Each consists of
a driver, a MOSFET, and a current sense resistor. The
driver controls the MOSFET gate voltage based on the
current sense feedback to achieve current regulation.
When current sense signal is below a preset regulation
point, the driver will reduce the MOSFET drain-to-source
voltage by increasing gate voltage, allowing more voltage
across the LED string. When current sense signal is
above the regulation point, the driver will increase the
MOSFET drain-to-source voltage by lowering gate
AOZ1960
Page 12 of 20