MP3398E

– 4-STRING, 80V, STEP-UP, WLED CONTROLLER

MP3398E Rev. 1.02

www.MonolithicPower.com

14

7/31/2017

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2017 MPS. All Rights Reserved.

The on resistance of the MOSFET determines

the conduction loss, which is given by Equation

(9):

k

R

I

P

(on)

DS

2

RMS

cond







(9)

Where k is the temperature coefficient of the

MOSFET.

the charge between the threshold voltage and

the plateau voltage when a driver charges the

during

the

plateau

voltage.

These

two

parameters are needed to estimate the turn-on

and turn-off losses and can be calculated with

Equation (10):

SW

IN

DS

PLT

DR

G

GD

SW

IN

DS

TH

DR

G

GS1

SW

f

I

V

V

V

R

Q

f

I

V

V

V

R

Q

P

























(10)

and

is

the

gate

resistance.

is

recommended to be 10-20

Ω.

Please note that calculating the switching loss

is the most difficult part of loss estimation. The

formula above provides a simplified equation.

For more accurate estimates, the equation

becomes much more complex.

the gate drive loss and can be calculated with

Equation (11):

SW

DR

G

DR

f

V

Q

P







(11)

Selecting the Output Capacitor

The output capacitor keeps the output voltage

ripple small and ensures feedback loop stability.

The output capacitor impedance must be low at

the switching frequency. Ceramic capacitors

with X7R dielectrics are recommended for their

low ESR characteristics. A 4.7

μF ceramic

capacitor in parallel with a 22~47

μF electrolytic

capacitor is sufficient for most applications.

Setting the Over-Voltage Protection (OVP)

Open-string protection is achieved through the

detection of the voltage on OVP. In some cases,

an LED string failure results in the feedback

voltage always being zero. The MP3398E

continues boosting the output voltage higher

and higher. If the output voltage reaches the

programmed OVP threshold, the protection is

triggered.

To ensure that the chip functions properly, an

appropriate OVP voltage is required. The

recommended OVP point is about 1.1 to 1.2

times higher than the output voltage for normal

operation. The OVP voltage is set by an

external resistor on OVP and can be calculated

with Equation (12):

HIGH

OVP

LOW

R

V

2(V) (1

)

R



 

(12)

Expanding LED Channels

The MP3398E can expand the number of LED

channels by using two or three ICs in parallel.

To connect two ICs for a total of eight LED

strings, tie the VCC pins of the master IC and

the slave IC together to power the slave IC

internal logic circuitry. Tie the COMP pins of the

slave IC and the master IC together to regulate

the voltage of all eight LED strings. The slave

IC MOSFET driving signal is not used; the

boost converter can be driven by the master IC

only. Do not leave the ISENSE of the slave IC

floating; tie it to ground. Apply the EN and DIM

signals to both ICs.

PCB Layout Guidelines

Efficient

PCB

layout

is

critical

for

stable

operation and reducing EMI noise. For best

results, follow the guidelines below:

1. Ensure that the loop among the external

MOSFET, the output diode, and the output

ceramic capacitor is as small and short as

possible since it carries a high-frequency

pulse current.

2. Separate the power ground (PGND) and

signal

ground

(GND)

to

reduce

noise

affection.

3. Connect PGND and GND together. All logic

signals refer to the signal ground.

4. Place ceramic capacitors as close to VIN

and VCC as possible.