MP3389EY-LF Datasheet MP3389EY-LF-Z, MP3389EF-LF, MP3389EF-LF-Z | P10-P12

MP3389—12-STRING WHITE LED DRIVER WITH STEP-UP CONTROLLER

MP3389 Rev. 1.04 www.MonolithicPower.com 10

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directly applied to the DBRT pin to modulate the

LED current. And the DC signal is then converted

to a DPWM dimming signal at the setting

oscillation frequency.

The brightness of the LED array is proportional to

the duty cycle of the DPWM signal. The DPWM

signal frequency is set by the cap at the BOSC

pin.

Open String Protection

The open string protection is achieved through

the over voltage protection. If one or more strings

are open, the respective LEDX pins are pulled to

ground and the IC keeps charging the output

voltage until it reach OVP threshold. Then the

part will mark off the open strings whose LEDX

pin voltage is less than 180mV. Once the mark-

off operation completes, the remaining LED

strings will force the output voltage back into tight

regulation. The string with the highest voltage

drop is the ruling string during output regulation.

The MP3389 always tries to light at least one

string and if all strings in use are open, the

MP3389 shuts down the step-up converter. The

part will maintain mark-off information until the

part shuts down.

Short String Protection

The MP3389 monitors the LEDX pin voltage to

judge if the short string occurs. If one or more

strings are short, the respective LEDX pins will

be pulled up to the boost output and tolerate high

voltage stress. If the LEDX pin voltage is higher

than 5.5V, the short string condition is detected

on the respective string. When the short string

fault (LEDX over-voltage fault) continues for

greater than 512 switching clocks, the string is

marked off and disabled. Once a string is marked

off, its current regulation is forced to disconnect

from the output voltage loop regulation. The

marked-off LED strings will be shut off totally until

the part restarts. If all strings in use are short, the

MP3389 will shut down the step-up converter.

MP3389—12-STRING WHITE LED DRIVER WITH STEP-UP CONTROLLER

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APPLICATION INFORMATION

Selecting the Switching Frequency

The switching frequency of the step-up converter

is programmable from 100kHz to 500kHz. A

oscillator resistor on OSC pin sets the internal

oscillator frequency for the step-up converter

according to the equation:

= 67850 / R

OSC

（k）

For R

OSC

=191k, the switching frequency is set

to 355 kHz.

Setting the LED Current

The LED string currents are identical and set

through the current setting resistor on the ISET

pin.

LED

= 1000 x 1.22V / R

SET

For R

SET

=60.4k, the LED current is set to 20mA.

The ISET pin can not be open.

Selecting the Input Capacitor

The input capacitor reduces the surge current

drawn from the input supply and the switching

noise from the device. The input capacitor

impedance at the switching frequency should be

less than the input source impedance to prevent

high frequency switching current from passing

through the input. Ceramic capacitors with X5R

or X7R dielectrics are highly recommended

because of their low ESR and small temperature

coefficients. For most applications, a 4.7F

ceramic capacitor paralleled a 220uF electrolytic

capacitor is sufficient.

Selecting the Inductor and Current Sensing

Resistor

The inductor is required to force the higher output

voltage while being driven by the input voltage. A

larger value inductor results in less ripple current,

resulting in lower peak inductor current and

reducing stress on the internal N-Channel

MOSFET. However, the larger value inductor has

a larger physical size, higher series resistance,

and lower saturation current.

Choose an inductor that does not saturate under

the worst-case load conditions. A good rule for

determining the inductance is to allow the peak-

to-peak ripple current to be approximately 30% to

40% of the maximum input current. Calculate the

required inductance value by the equation:

IN OUT IN

OUT SW

V(V V)

VfI

×−

××

OUT LOAD(MAX)

IN(MAX)

V 

IN(MAX)

I(30%~40%)I

=×

Where V

is the minimum input voltage, f

is the

switching frequency, I

LOAD(MAX)

is the maximum

load current, I is the peak-to-peak inductor

ripple current and is the efficiency.

The switch current is usually used for the peak

current mode control. In order to avoid hitting the

current limit, the voltage across the sensing

resistor R

SENSE

should be less than 80% of the

worst case current limit voltage, V

SENSE.

SENSE

L(PEAK)

0.8 V

Where I

L(PEAK)

is the peak value of the inductor

current. V

SENSE

is shown in Figure 3.

SENSE

vs. Duty Cycle

DUTY CYCLE (%)

100

200

300

400

500

600

700

0 102030405060708090100

Figure 3—V

SENSE

vs Duty Cycle

Selecting the Power MOSFET

The MP3389 is capable of driving a wide variety

of N-Channel power MOSFETS. The critical

parameters of selection of a MOSFET are:

1. Maximum drain to source voltage, V

DS(MAX)

2. Maximum current, I

D(MAX)

3. On-resistance, R

DS(ON)

4. Gate source charge Q

and gate drain

charge Q

Total gate charge, Q

MP3389—12-STRING WHITE LED DRIVER WITH STEP-UP CONTROLLER

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Ideally, the off-state voltage across the MOSFET

is equal to the output voltage. Considering the

voltage spike when it turns off, V

DS(MAX)

should be

greater than 1.5 times of the output voltage.

The maximum current through the power

MOSFET happens when the input voltage is

minimum and the output power is maximum. The

maximum RMS current through the MOSFET is

given by

MAXIN(MAX))MAX(RMS

DII ×=

Where:

OUT

)MIN(INOUT

MAX

−

≈

The current rating of the MOSFET should be

greater than 1.5 times I

RMS,

The on resistance of the MOSFET determines

the conduction loss, which is given by:

kRIP

(on) DS

RMScond

××=

Where k is the temperature coefficient of the

MOSFET.

The switching loss is related to Q

and Q

GS1

which determine the commutation time. Q

GS1

the charge between the threshold voltage and

the plateau voltage when a driver charges the

gate, which can be read in the chart of V

vs. Q

of the MOSFET datasheet. Q

is the charge

during the plateau voltage. These two

parameters are needed to estimate the turn on

and turn off loss.

SWINDS

PLTDR

GGD

SWINDS

THDR

GGS1

fIV

×××

−

+×××

−

Where V

is the threshold voltage, V

PLT

is the

plateau voltage, R

is the gate resistance, V

the drain-source voltage. Please note that the

switching loss is the most difficult part in the loss

estimation. The formula above provides a simple

physical expression. If more accurate estimation

is required, the expressions will be much more

complex.

For extended knowledge of the power loss

estimation, readers should refer to the book

“Power MOSFET Theory and Applications”

written by Duncan A. Grant and John Gowar.

The total gate charge, Q

, is used to calculate

the gate drive loss. The expression is

SWDRGDR

fVQP ××

Where V

is the drive voltage.

Selecting the Output Capacitor

The output capacitor keeps the output voltage

ripple small and ensures feedback loop stability.

The output capacitor impedance should be low at

the switching frequency. Ceramic capacitors with

X7R dielectrics are recommended for their low

ESR characteristics. For most applications, a

4.7F ceramic capacitor paralleled 10uF

electrolytic capacitor will be sufficient.

Setting the Over Voltage Protection

The open string protection is achieved through

the over voltage protection (OVP). In some cases,

an LED string failure results in the feedback

voltage always zero. The part then keeps

boosting the output voltage higher and higher. If

the output voltage reaches the programmed OVP

threshold, the protection will be triggered.

To make sure the chip functions properly, the

OVP setting resistor divider must be set with a

proper value. The recommended OVP point is

about 1.2 times higher than the output voltage for

normal operation.

OVP

=1.23V*(R

)/R

Selecting Dimming Control Mode

The MP3389 provides 2 different dimming

methods

1. Direct PWM Dimming

An external PWM dimming signal is employed to

achieve PWM dimming control. Connect a 100k

resistor from BOSC pin to GND and apply the

100Hz to 2kHz PWM dimming signal to DBRT

pin. The minimum recommended amplitude of

the PWM signal is 1.2V. The low level should

less than 0.4V. (See Figure 4).

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