MP3388

AN022
MP3388 White LED Driver

for LCD Backlighting
The Future of Analog IC Technology
INDEX
INTRODUCTION………………………………………………………..……………….……………….2
REFERENCE DESIGN………………………………………………………..……………….… ……. 2
CURRENT BALANCE………………………………………………………..………………….……… 5
PWM DIMMING………………………………………………………..………………………..….....….5
FAULT PROTECTION………………………………………………………..…………………… ..….8
START UP SEQUENCES……………………………………………..……………………………….10
FLEXIBLE APPLICATION……………………………………………..……………………………….10
LAYOUT CONSIDERATIONS……………………………………………..…………………….…….11
CONCLUSION……………………………………………..…………………………………………….12
AN022 Rev. 0.1 www.MonolithicPower.com 1

12/30/2013 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
AN022
MP3388 White LED Driver
for LCD Backlighting
The Future of Analog IC Technology
MP3388 INTRODUCTION
The MP3388 is a step-up converter with 8-channel current sources designed for driving typically 80
white LEDs (10 LEDs in series per string and 8 strings in parallel) for large size LCD panel backlighting
applications.
The MP3388 uses current mode, fixed frequency architecture. The switching frequency can be selected at
1.25MHz or 625 kHz. It generates an output voltage up to 50V from a 4.5V to 25V input supply. The
MP3388 regulates the current in each LED string to the user programmed value set by an external current
setting resistor.
The MP3388 applies an internal current source in each LED string terminal to balance the connects.
The current matching achieves 3% regulation accuracy between strings. Its low 600mV regulation
voltage on LED current sources reduces power loss and improves efficiency.
The MP3388 has multiple rich PWM dimming modes, including external PWM signal dimming, DC input
keep PWM dimming or PWM input burst dimming with capacitor set frequency.
The MP3388 has multiple features to protect the converter from fault conditions, including input UVLO,
current limiting, output over-voltage, output short to GND, short LED, open LED and thermal shut-down
protection. If the fault conditions occur in several strings, the MP3388 marks them off while the
remaining LED strings are still in normal operation.
REFERENCE DESIGN
A typical application circuit with MP3388 for large size LCD panel backlight application is shown in
Figure 1. The LED array consists of 80 LEDs with 10 LEDs in series and 8 strings in parallel. It is
suitable for notebook, LCD TV, handy terminals displays and automotive display system backlighting
where the input power comes from either the batteries or the adapter.
Figure 1—Typical Application for Driving WLED Array

AN022 – MP3388 WHITE LED DRIVER FOR LCD BACKLIGHTING
The detailed design specifications and BOM list are shown in Table 1 and Table 2.
Table 1. Specifications for Typical Application

Input Voltage 4.5V~21V
LED Forward Voltage 10x3.2V
LED Current 8x20mA
Open LED Protection Yes
Short Circuit Protection Yes
Table 2. BOM List for Typical Application

Qty Ref Value Description Package Manufacturer Manufacturer P/N
Ceramic Capacitor, 50V,
1 C1 4.7µF 1210 Murata GRM32ER71H475KA8
X7R
1 C2 2.2uF 1210 Murata GRM32ER72A225KA3
X7R
1 C3 4.7uF 0805 Murata GRM21BR61C475KA8
X5R
1 C4 68nF 0603 Murata GRM188R71H683KA0
X7R
1 C5 1.8nF 0603 Murata GRM188R71H182KA0
X7R
1 C6 68nF 0603 Murata GRM188R71H683KA9
X7R
1 D1 Diode Schottky, 60V, 1A SMA Diodes Inc DFLS160-F
1 L1 10µH Inductor,2.47A SMD Cooper DR73-100
1 M1 P- channel MOSFET SO8 AM4499P
1 R1 324kΩ Resistor, 1% 0603 Yageo RC0603FR-07324KL
1 R2 10kΩ Resistor, 1% 0603 Yageo RC0603FR-0710KL
1 R3 60.4kΩ Resistor, 1% 0603 Yageo RC0603FR-0760K4L
1 U1 LED Driver IC QFN24 MPS MP3388DR
Setting the LED Current

The current flowing into each LED strings is set through the current setting resistor on the ISET pin.
1.21V
I LED = 1000 ×
R SET
Where ILED=20mA, RSET =60.4kΩ
Selecting the Switching Frequency
MP3388 has two available switching frequencies, 1.25MHz and 625 kHz. The Switching Frequency
Selection (OSC) input sets the internal oscillator frequency as shown in Table 3.Connect OSC pin to
GND corresponds to the frequency 625 kHz. OSC pin to VCC or floating it corresponds to 1.25MHz. In
this application, the OSC pin is floated for 1.25MHz switching frequency. The high-frequency operation
optimizes the solution for the smallest component size.

Table 3. Frequency Selection
OSC PIN CONNECTION SWITCHING FREQUENCY(kHz)
Vcc /Floating 1250
GND 625
Selecting the Inductor

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:
VIN × (VOUT − VIN )

L=
VOUT × f SW × ΔI
VOUT × I LOAD(MAX)
I IN(MAX) =
VIN × η
ΔI = (30% ~ 40%) × I IN(MAX)
Where VIN is the minimum input voltage, fSW is the switching frequency, ILOAD(MAX) is the maximum load
current, ∆I is the peak-to-peak inductor ripple current and ηis the efficiency.
When VIN =4.5V, fSW =1.25MHz, ILOAD(MAX)=8x20mA=160mA, VOUT =10x3.5V+0.5=35.5V, the inductor is
calculated L=7.3uH. So a 10uH inductor with a DC current rating of at least 40% higher than the
maximum input peak current is employed. For highest efficiency, the inductor’s DC resistance should
be as small as possible.
Selecting the Schottky Diode
The output rectifier diode supplies current to the output capacitor when the internal MOSFET is off. A
Schottky diode is recommended due to lower diode forward voltage and short recovery time. The diode
should be rated for a reverse voltage greater than the output voltage used. The average current rating
must be greater than the maximum load current expected, and the peak current rating must be greater
than the peak inductor current.
Selecting the Input and Output Capacitor

The input capacitor reduces the surge current drawn from the input supply and the switching noise from
the device. Ceramic capacitors with X5R or X7R dielectrics are highly recommended because of their
low ESR and small temperature coefficients. For most applications, a 4.7μF~10μF capacitor is sufficient.
The output capacitor keeps the output voltage ripple small and ensures feedback loop stability. Ceramic
capacitors with X7R dielectrics are recommended for their low ESR characteristics. For most
applications, a 1μF~4.7μF ceramic capacitor is sufficient.
Base on these components with above parameters, the efficiency is up to about 90% at 21V input
voltage. Figure 2 shows the bench-test efficiency curve and steady state waveform for this application
design.

Efficiency vs Vin
100
95
Efficiency(%)
90
85
80
75
70
4 9 14 19 24 29
Vin(V)
(a) Efficiency Curve (b) Steady State Waveform (Vin=15V)
Figure 2. The Bench-Test Efficiency Curve and Steady State Waveform

CURRENT BALANCE
The MP3388 uses 8-channel current control to regulate their corresponding LED string currents. The
internal current source implements 3% current matching balance between strings. The proportion of the
current through the current setting resistor and the LED string is 1:1000. The current matching is defined as
(ILED_MAX – ILED_MIN)/ (2 ILED_AVERAGE)
PWM DIMMING
The MP3388 provides 4 different dimming methods
1, PWM dimming mode with internal triangle waveform generator
Apply a 100Hz to 50kHz square waveform to the PWMI pin. The internal 400kΩ and external capacitor
on PWMO pin filters the dimming signal to a DC voltage(0.2V~1.2V).Then the DC voltage is modulated
to an internal PWM dimming signal whose frequency is set via the capacitor on FSET pin according to
the equation:
fDPWM = 3.5uF / CFSET
DPWM Oscillator has an internal 7.5uA (±15% tolerance) constant current source to charge CFSET.
When the voltage amplitude exceeds 1.2V, another 7.5uA internal constant current source discharges
CFSET to 0.2V. These repetitive behaviors generate a triangle waveform with 1.2V amplitude.
The minimum recommended amplitude of the PWM signal is 1.5V (See Figure 3)
Figure 3. PWM Dimming with Internal Triangle Waveform Generator

Figure 4 shows the PWM Dimming waveforms and curves. The ILED increases linearly with PWM duty.
ILED Vs. PWM DUTY
160
140
120
100
ILED(mA)
80
60
500HZ
40 5kHZ
10kHZ
20 100kHZ
0
0 0.2 0.4 0.6 0.8 1
PWM DUTY(%)
(a) PWM Dimming Waveform (b) PWM Dimming Curve

Figure 4. PWM Dimming Waveform and Curve
2, Direct PWM dimming with positive logic

An external PWM dimming signal is directly employed to achieve PWM dimming control without a low
pass filter. In order to set the voltage on FSET pin between 0.2V and 1.2V with internal 7.5uA (±15%
tolerance) current source, connect a 100kΩ resistor from FSET pin to GND. Apply a 100Hz to 2 kHz
external square waveform to the PWMI pin for positive logic dimming. The minimum recommended
amplitude of the PWM signal is 1.5V (See Figure 5).
Figure 5. Direct PWM Dimming with Positive Logic

Figure 6 shows the PWM Dimming waveforms and curves. The ILED increases linearly with PWM duty.

ILED Vs. PWM DUTY
180
160
140
120
ILED(mA)
100
80
100Hz
60
200Hz
40 2kHz
20
0
0 0.2 0.4 0.6 0.8 1
PWM DUTY
(a) PWM Dimming Waveform (b) PWM Dimming Curve

Figure 6. PWM Dimming Waveform and Curve
3, Direct PWM dimming with negative logic
Similar to method 2, apply a 100Hz to 2 kHz external square waveform to the PWMO pin for negative
logic PWM dimming. The minimum recommended amplitude of the PWM signal is 1.5V (See Figure 7),
Figure 7. Direct PWM Dimming with Negative Logic

4, DC input PWM dimming
To apply DC input PWM dimming, apply an analog signal (range from 0.2V to 1.2V) at the PWMO pin
to modulate the LED current directly. If the PWMO is applied with a DC voltage<0.2V, the PWM duty
cycle will be at its maximum. If the PWMO pin is applied with a DC voltage>1.2V, the output will be at
its minimum duty cycle (See Figure 8). The capacitor on FSET pin set the frequency of internal triangle
waveform.
Figure 8. DC Input PWM Dimming

FAULT PROTECTION
The MP3388 has multiple features to protect the converter from fault conditions, including input Under
Voltage Lock Out (UVLO), programmable Over Voltage Protection (OVP), Over Current Protection,
short LED, open LED and Over Temperature Protection. Figure 9 describes the condition and operation
flow chart of MP3388.
Figure 9. Flow Chart of Operation

Setting the Over Voltage Protection
The open string protection is achieved through the over voltage protection. In some cases, an LED
string failure results in the feedback voltage always equal to zero. The part will run at maximum duty
cycle boosting the output voltage higher and higher. If the output voltage exceeds the programmed
OVP threshold, the protection will be triggered.
To make sure the chip functions properly, the OVP setting resistor divider must be set to a proper value.
The recommended OVP point is about 1.1 times higher than the output voltage for normal operation.
VOVP=1.23V*(R1+R2)/R2

Open Strings Protection

If one or more strings are open, the corresponding 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 210mV. Once the mark-off operation completes, the remaining
LED strings will force the output voltage back into tight regulation (Figure 10-a). The string with the
highest voltage drop is the ruling string during output regulation.
The MP3388 always tries to light at least one if all strings in use are open, the MP3388 shuts down the
step-up converter (Figure 10-b). The part will maintain mark-off information until the part shuts down.
Figure 10 shows the open load protection waveforms when all loads are open and one string of LED is
open at working.
(a) Open One String of LED at Working (b) Open All Load at Working
Figure 10. Open Load Protection
Short Strings Protection
The MP3388 monitors the LEDX pin voltage to judge if shorted string occurs. If one or more strings are
shorted, the corresponding LEDX pins will be pulled up to high voltage. If the LEDX pin voltage is
higher than 5.5V, the short string condition is detected. The string is marked off and disabled after
1.6ms delay time. Once a string is marked off, its current regulation is forced to disconnect from the
output voltage loop regulation. The remaining LED strings will operate normally without effect (Figure
11-a).
When the Schottky is not connected or the output is short to GND, MP3388 detects the OVP pin below
84mV. Then the converter is latched off and turns off the external P-channel MOSFET to disconnect
the system from input voltage source（Figure 11-b）. Figure 11 shows the waveforms when shorting
one string of LEDs and while shorting output to GND during operation.

(a) Short One String of LED during operation (b) Short Vout to GND during operation
Figure 11. Short Load Protection
START UP SEQUENCES
Figure 12 shows the start up sequence for normal operation and open/short LED conditions.
Short one string
Mark off short string
12V
Vin
5V
Ven
PWMI
12V
6V
Vfault
400us
<2V
Vcomp
0.8V
>5.5V
≥0.6V 5.5V
Floating
1.6ms
Vledx
160mA
140mA
Iled
1V
Vovp
(a) Start Up and One String Open LED Protection (b) Start Up and One String Short LED Protection
Figure 12. Short Load Protection
FLEXIBLE APPLICATION
MP3388 can be employed in situations where the LED strings are less than 8. For these applications,
connect the unused string pins to Vout (See Figure 13). The unused strings are marked off when high
voltage is detected for about 1.6ms.

Figure 13. Typical Application for Driving 6 Strings of WLED

LAYOUT CONSIDERATIONS
Careful attention must be paid to the PCB board layout and components placement. Proper layout of
the high frequency switching path is critical to prevent noise and electromagnetic interference problems.
The loops of the MP3388 SW to PGND pin (U1), output diode (D1), and output capacitor (C2) all have
high frequency pulsed current. It must be as short as possible (See Figure 14). The resistor
programmed LED current should be close to Iset pin and the loop of Iset to GND (Signal Ground)
should be minimized.
Figure 14. Layout Consideration

The IC exposed pad is internally connected to GND pin, and all logic signals are refer to the GND. The
PGND should be externally connected to GND and should be kept to keep away from the logic signals.

CONCLUSION
MP3388 totally integrates the power MOSFET, control circuit, current balance circuit, PWM dimming
circuit and complete circuit. The external component count is minimized, which reduces the cost and
reduces PCB layout size with the small QFN package. MP3388 has the following advantages:
z High Efficiency and Small Size
z 4.5V to 28V Wide Input Voltage Range Selectable Switching Frequency: 1.2MHz or 600kHz
z 3% Current Matching Accuracy Between Strings
z Flexible PWM Dimming
z Open LED Protection
z Short LED protection
MP3388 offers a high performance solution for WLED backlighting. It is suitable to notebook PC, .small
LCD TV, handy terminate display and automotive system WLED backlighting.


MP3388

Uploaded by

Copyright:

Available Formats

MP3388

Uploaded by

Document Information

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

MP3388

Uploaded by

Copyright:

Available Formats

AN022

MP3388 White LED Driver

REFERENCE DESIGN………………………………………………………..……………….… ……. 2

FAULT PROTECTION………………………………………………………..…………………… ..….8

AN022 Rev. 0.1 www.MonolithicPower.com 1

Figure 1—Typical Application for Driving WLED Array

AN022 Rev. 0.1 www.MonolithicPower.com 2

Table 1. Specifications for Typical Application

Table 2. BOM List for Typical Application

Setting the LED Current

AN022 Rev. 0.1 www.MonolithicPower.com 3

Table 3. Frequency Selection

OSC PIN CONNECTION SWITCHING FREQUENCY(kHz)

Vcc /Floating 1250

Selecting the Inductor

VIN × (VOUT − VIN )

Selecting the Input and Output Capacitor

AN022 Rev. 0.1 www.MonolithicPower.com 4

(a) Efficiency Curve (b) Steady State Waveform (Vin=15V)

Figure 2. The Bench-Test Efficiency Curve and Steady State Waveform

Figure 3. PWM Dimming with Internal Triangle Waveform Generator

AN022 Rev. 0.1 www.MonolithicPower.com 5

ILED Vs. PWM DUTY

(a) PWM Dimming Waveform (b) PWM Dimming Curve

2, Direct PWM dimming with positive logic

Figure 5. Direct PWM Dimming with Positive Logic

AN022 Rev. 0.1 www.MonolithicPower.com 6

ILED Vs. PWM DUTY

(a) PWM Dimming Waveform (b) PWM Dimming Curve

Figure 7. Direct PWM Dimming with Negative Logic

Figure 8. DC Input PWM Dimming

AN022 Rev. 0.1 www.MonolithicPower.com 7

Figure 9. Flow Chart of Operation

AN022 Rev. 0.1 www.MonolithicPower.com 8

Open Strings Protection

AN022 Rev. 0.1 www.MonolithicPower.com 9

AN022 Rev. 0.1 www.MonolithicPower.com 10

Figure 13. Typical Application for Driving 6 Strings of WLED

Figure 14. Layout Consideration

AN022 Rev. 0.1 www.MonolithicPower.com 11

AN022 Rev. 0.1 www.MonolithicPower.com 12

You might also like