MP3309 2.7V-5.5V INPUT, 35V OUTPUT, f
s
-PROGRAMMABLE, WHITE LED DRIVER WITH I
2
C INTERFACE
MP3309 Rev. 1.01 www.MonolithicPower.com 8
5/14/2014 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2014 MPS. All Rights Reserved.
OPERATION
The MP3309 uses the peak current mode
control architecture to regulate the current
flowing through the white LEDs string. The
operation of the MP3309 can be understood by
referring to the function block diagram.
At the start of each oscillator cycle, the control
circuit turns on the low-side MOSFET (LS-FET,
M1). A stabilizing ramp added to the output of
the current-sense amplifier, which then feeds
into the positive input of the PWM comparator,
prevents sub-harmonic oscillations at duty
cycles greater than 50%. When the input to the
PWM comparator equals the output voltage of
the error amplifier. M1 turns off. Then the
inductor current flows through the synchronous
power MOSFET or the external Schottky diode,
which forces the inductor current to decrease.
The voltage at the output of the error amplifier
is the amplified difference between the 200mV
reference voltage and the feedback voltage. If
the feedback voltage starts to drop, the output
of the error amplifier increases, increasing the
current flowing through the M1, thus increasing
the power to the inductor and output power.
This method accurately regulates the LED
current.
System Startup
When enabled, the MP3309 checks safety
limits, including UVLO and over-temperature
protection (OTP), over-current protection after
passing the OVP test. If all the protection tests
pass, the chip then starts boosting the step-up
converter with an internal soft-start. If by
software enable,during the soft-start, the
reference voltage gradually ramps up with a
time step of 320s for a total of 32 steps. If by
hardware enable, during soft-start,the current
limit drops to 0.8A during startup to prevent a
large inrush current. This low-current-limit time
lasts for 5ms after the IC is enabled.
Synchronous or Non-synchronous Mode
Selection
In default mode (internal register bit SYNC=1),
the converter works in synchronous mode.
Synchronous mode can reduce diode power
consumption caused by a large voltage drop.
This synchronous mode increases the
converter efficiency for low LED string voltage.
This mode does not need an external Schottky
diode to reduce PCB space requirements and
BOM costs.
In some applications as below, select non-
synchronous mode by setting the internal
register bit SYNC=0. Be sure to connect an
external schottky diode between SW and VOUT.
1. When the LED string voltage is high, the
duty cycle is large, which means the
synchronous-power-MOSFET turn-on time
is very short;
2. When the switching frequency is set too
high, the synchronous power MOSFET
driver loss is large.
Dimming Control
The MP3309 provides two dimming methods:
PWM and analog dimming mode.
For PWM dimming, apply a PWM signal to the
EN/PWML pin. The LED current is then
segmented as per the PWM signal and the
average LED current equals:
dimset
DI × .Where
D
dim
is the PWM duty cycle, and I
set
is the LED
current amplitude. When PWM signal is high,
the converter operates in normal mode.
Conversely, when the PWM amplitude is low,
the converter stops switching. Make sure that
the PWM-signal low-level time is less than
20ms or the IC will shutdown. Use a 200Hz-to-
2kHz PWM-dimming frequency for most
dimming ratio requests.
For analog dimming, set the LED current
amplitude through I
2
C interface or an external
PWM signal. For MP3309C, control the LED
current-regulation voltage by setting the DIMS
bit(D0-D4) . For MP3309, implement analog
dimming by connecting the SCL and SDA pin
together as the PWMH pin and apply a PWM
signal. An internal RC fliter filters the PWM
signal. The LED current amplitude equals
I
set
×D
dim
, where D
dim
is the PWM duty cycle and
I
set
is the LED current amplitude. Use
a>20kHzPWM signal to improve filtering
performance.
