SQ9912/SQ9912A
Universal High Brightness LED Driver
FEATURES
●
>90% Efficiency
●
Universal rectified 85 to 265V
AC
input range
●
Constant-current LED driver
●
Applications from a few mA to more than 1A
Output
●
LED string from one to hundreds of diodes
●
PWM Low-Frequency Dimming via Enable pin
●
Input voltage surge ratings up to 500V
● Power-on sequence control and Soft Start
● Spread Spectrum to reduce EMI filter Cost
● Open Loop Protection (OLP)
●
Internal Thermal Protection (OTP)
●
7.5V MOSFET drive – SQ9912
10V MOSFET drive – SQ9912A
PRODUCT DESCRIPTION
The SQ9912 is a PWM high-efficiency LED driver
control IC. The SQ9912 adds Open-Loop-
Protection (OLP) feature to SQ9911. The SQ9912
drives LED string with constant current and OLP
pin can detect the open load condition and
clamps the output voltage at certain level to
avoid damage to the output circuit. OLP pin has
internal pull-up resistor. The SQ9912 is pin-to-
pin compatible with SQ9911 if OLP pin is left
open. SQ9912 has Soft Start to reduce input
surge current during cold start. SQ9912 also has
pseudo-random oscillator hopping function
(Spread Sprectrum) to reduce EMI emmission so
that input EMI filter cost can be reduced. Typical
oscillator hopping range is approximately 8%
around base frquency set by
R
OSC.
SQ9912
allows efficient operation of High Brightness
(HB) LEDs from voltage sources ranging from
85V
AC
up to 265V
AC
.
SQ9912A allows wide range of external MOSFET
which has lower R
DSON
at higher V
GS
.
TYPICAL APPLICATIONS
●
DC/DC or AC/DC LED Driver application
●
RGB Backlighting LED Driver
●
Back Lighting of Flat Panel Displays
●
General purpose constant current source
●
Signage and Decorative LED Lighting
APPLICATION CIRCUIT
SQ9912/SQ9912A
APPLICATION INFORMATION
AC/DC Off-Line Applications
The SQ9912 is a low-cost off-line buck or boost
converter control IC specifically designed for driving
multi-LED stings or arrays. It can be operated from
either universal AC line or any DC voltage between
14-450V. Optionally, a passive power factor
correction circuit can be used in order to pass the AC
harmonic limits set by EN 61000-3-2 Class C for
lighting equipment having input power less than 25W.
The SQ9912 can drive up to hundreds of High-
Brightness (HB) LEDs or multiple strings of HB LEDs.
The LED arrays can be configured as a series or
series/parallel connection. The SQ9912 regulates
constant current that ensures controlled brightness
and spectrum of the LEDs, and extends their lifetime.
The SQ9912 features an enable pin (PWM_D) that
allows PWM control of brightness.
The SQ9912 can also control brightness of LEDs by
programming continuous output current of the LED
driver (so-called linear dimming) when a control
voltage is applied to the LD pin.
The SQ9912 is offered in a standard 8-pin SOIC
package.
The SQ9912 includes an internal high-voltage linear
regulator that powers all internal circuits and can also
serve as a bias supply for low voltage external
circuitry.
circuit, consisting of 3 diodes and 2 capacitors, can
be added as shown in the typical application circuit
diagram of Figure 2
.
Supply Current
A current of 1mA is needed to start the SQ9912.
As shown in the block diagram on page 4, this
current is internally generated in the SQ9912
without using bulky startup resistors typically
required in the offline applications. Moreover, in
many applications the SQ9912 can be
continuously powered using its internal linear
regulator that provides a regulated voltage of 7.5V
for all internal circuits.
Setting Light Output
When the buck converter topology of Figure 2 is
selected, the peak CS voltage is a good
representation of the average current in the LED.
However, there is a certain error associated with
this current sensing method that needs to be
accounted for. This error is introduced by the
difference between the peak and the average
current in the inductor. For example if the peak-to-
peak ripple current in the inductor is 150mA, to get
a 500mA LED current, the sense resistor should
be 250mV/(500mA+ 0.5*150mA) = 0.43Ω
Dimming
Dimming can be accomplished in two ways,
separately or combined, depending on the
application. Light output of the LED can be
controlled either by linear change of its current, or
by switching the current on and off while
maintaining it constant. The second dimming
method (so-called PWM dimming) controls the
LED brightness by varying the duty ratio of the
output current.
The linear dimming can be implemented by
applying a control voltage from 0 to 250mV to the
LD pin. This control voltage overrides the
internally set 250mV threshold level of the CS pin
and programs the output current accordingly. For
example, a potentiometer connected between V
DD
and ground can program the control voltage at the
CS pin. Applying a control voltage higher than
250mV will not change the output current setting.
When higher current is desired, select a smaller
sense resistor.
The PWM dimming scheme can be implemented
by applying an external PWM signal to the
PWM_D pin. The PWM signal can be generated
by a microcontroller or a pulse generator with a
duty cycle proportional to the amount of desired
light output. This signal enables and disables the
converter modulating the LED current in the PWM
LED Driver Operation
The SQ9912 can control all basic types of converters,
isolated or non-isolated, operating in continuous or
discontinuous conduction mode. When the gate
signal enhances the external power MOSFET, the
LED driver stores the input energy in an inductor or in
the primary inductance of a transformer and,
depending on the converter type, may partially deliver
the energy directly to LEDs The energy stored in the
magnetic component is further delivered to the output
during the off-cycle of the power MOSFET producing
current through the string of LEDs (Flyback mode of
operation).
When the voltage at the V
DD
pin exceeds the UVLO
threshold the gate drive is enabled. The output
current is controlled by means of limiting peak current
in the external power MOSFET. A current sense
resistor is connected in series with the source
terminal of the MOSFET. The voltage from the sense
resistor is applied to the CS pin of the SQ9912. When
the voltage at CS pin exceeds a peak current sense
voltage threshold, the gate drive signal terminates,
and the power MOSFET turns off. The threshold is
internally set to 250mV, or it can be programmed
externally by applying voltage to the LD pin.
Additionally, a simple passive power factor correction
stm32/stm8
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