MP3313 Rev1.0

www.MonolithicPower.com

13

8/2/2017

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© 2017 MPS. All Rights Reserved.

OPERATION

The MP3313 is a step-up converter with

peak-current-mode

control

architecture

that

employs three channels of current sink to drive

three strings of white LED for an LCD panel.

The MP3313 supports 11-bit resolution analog

dimming via an internal register or external

pulse-width modulation (PWM) input signal.

Boost Converter Switching Frequency

The MP3313 has two selectable switching

When the register bit FS = 1, the switching

frequency is set to 1MHz. When the register bit

FS = 0, the switching frequency is set to 500kHz.

Additionally, the switching frequency can be

shifted down 12% by the register FS_SHFT.

Auto-Switching Frequency

To optimize the efficiency in different loads, the

MP3313 can select the switching frequency

automatically by comparing the auto-switching

frequency low threshold (register 0x16) and

auto-switching

frequency

high

threshold

(register 0x15) to 8MSBs of the brightness code

(register 0x19).

The auto-switching frequency function includes

three different working frequency points: 1MHz,

500kHz, and 250kHz. At the high threshold, the

device switches from 1MHz to 500kHz. At the

low threshold, the device switches from 250kHz

from 500kHz (see Table 1).

Table 1: Switching Frequency Auto-Function

8MSBs of Brightness Code

(register 0x19)

Switching

Frequency

<auto switching frequency low

threshold (register 0x16)

250kHz

>auto switching frequency low

threshold (register 0x16)

<auto switching frequency high

threshold (register 0x15)

500kHz

>auto switching frequency high

threshold (register 0x15)

1MHz

To

disable

auto-frequency,

both

the

auto-switching frequency high threshold and low

threshold must be set to zero. Once the

auto-function is disabled, the MP3313 works at

a fixed frequency set by the FS bit. The

auto-frequency function can be enabled by

setting a non-zero code for any one of

auto-switching frequency thresholds.

Note that the switching frequency -12% shift is

still active when the auto-switching frequency

function is enabled.

Minimum Inductor Selection

To optimize the boost converter control loop, the

minimum inductance is limited for the MP3313,

which is set by the minimum inductor L_MIN bit.

When the L_MIN bit is set to 0, the minimum

inductor is 4.7μH. When the L_MIN bit is set to 1,

the minimum inductor is 10μH.

System Start-Up

If input voltage is higher than the under-voltage

lockout (UVLO) threshold and EN is pulled high,

the MP3313 enters standby mode. In this mode,

with the host. Meanwhile, the MP3313 monitors

the topology connection and safety limits,

including two checks:

 The MP3313 checks whether the OUT pin

connection is correct or not. An OUT voltage

device switching and sets the FT_UVP bit to

1.

 The MP3313 checks if the device has

triggered

LED

open/short

protection,

over-current

limit

protection,

or

over-temperature protection (OTP). If all

protections pass, the MP3313 then starts

boosting the step-up converter with an

internal soft start.

To

prevent

a

large

inrush

current,

it

is

recommended that the power-on sequence be

from VIN power-on to PWM power-on to EN

power-on. If the external EN pin is always pulled

high or to VIN by a resistor, it is requested to

write EN to 1 after VIN powers on. When

dimming is done by the internal code only, the

PWM signal can be ignored.

Boost Converter Operation

The MP3313 uses peak-current mode to control

the output voltage. At the start of the internal

oscillator cycle, the low-side MOSFET (LS-FET)

(M1) is turned on. To prevent sub-harmonic

oscillation at duty cycles greater than 50%, a

stabilizing ramp is added to the output of the

current sense amplifier, and the result is fed into

the positive input of the PWM generation

comparator. When this voltage equals the