MP3309L Rev. 1.01

www.MonolithicPower.com

8

7/18/2017

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OPERATION

The MP3309L uses peak-current-mode control

to regulate the current through the WLED string.

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, 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 MP3309L checks safety

limits, including UVLO and over-temperature

protection (OTP), over-current protection. If all

the protection tests pass, the chip then starts

boosting the step-up converter with an internal

soft-start. During the soft-start, the current limit

drops to 0.85A during startup to prevent a large

inrush current. This low-current-limit time lasts

for 5ms after the IC is enabled.

Dimming Control

The MP3309L 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:

dim

set

D

I



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 100Hz-to-

2kHz

PWM-dimming

frequency

for

most

dimming ratio requests.

For analog dimming, connect the PWM

dimming signal on PWMH pin, an internal RC

filter filters the PWM signal. The LED current

amplitude. Use a >20kHz PWM signal to

improve filtering performance.

Open-String Protection

The MP3309L monitors the VOUT pin for open-

string protection. If the LED string is open, the

feedback voltage is lower than the reference

voltage. The COMP then rises and charges the

output capacitor until the VOUT voltage

OVP threshold, the IC resumes function when

Input Under-Voltage Protection

When VIN exceeds 2.5V, the converter starts to

charge the internal reference and provides

power to the internal control circuitry. There is

UVLO hysteresis, approximately 200mV, as

VIN falls. The IC shuts down when the input

voltage drops below 2.3V.

Thermal-Shutdown Protection

Thermal shutdown prevents the IC from

operating at exceedingly high temperatures.

When the die temperature exceeds the upper

normal operation when the die temperature

drops below lower threshold. Typically, the

hysteresis is 25°C.