MP3388SGR [MPS]
50V, 8-String, Step-Up, White LED Driver;
| 型号： | MP3388SGR |
| 厂家： | 
MONOLITHIC POWER SYSTEMS |
| 描述： | 50V, 8-String, Step-Up, White LED Driver |
| 文件： | 总15页 (文件大小：477K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
MP3388S
50V, 8-String, Step-Up, White LED Driver
The Future of Analog IC Technology
DESCRIPTION
FEATURES
The MP3388S is a step-up converter with 8
current-source channels designed to drive arrays
of white LED for LCD-panel backlighting
applications.
High Efficiency
Compact Size
4.3V-to-25V Input Voltage Range
50V Maximum Step-Up Voltage
Balanced Driver for 8 WLED Strings
2.5% Current-Matching Accuracy between
Strings
Selectable Switching Frequency of 625kHz
or 1.25MHz
PWM or DC Input-Burst PWM Dimming
Programmable Over-Voltage Protection
Under-Voltage Lockout
The
MP3388S
uses
current-mode,
fixed-frequency architecture, and users can select
a switching frequency of either 625kHz or
1.25MHz. It generates an output voltage up to
50V from a 4.3V-to-25V input supply. The
MP3388S independently regulates the current in
each LED string set by an external current setting
resistor to a user-programmed value.
Open- and Short-LED Protection
Thermal Shutdown
Small QFN24 (4mm × 4mm) Package
The MP3388S applies an internal current source
to each LED string terminal, and can achieve a
current balance of 2.5% regulation accuracy
between strings. Its low 600mV regulation voltage
on LED current sources reduces power loss and
improves efficiency.
APPLICATIONS
Notebook PCs
Tablet Computers
Small LCD TV
Handheld Terminal Displays
Automotive Electronics Equipments
The MP3388S features external PWM dimming or
DC-input PWM dimming, which allows for flexible
backlight luminance control. The dimming PWM
signal can be generated internally, and the
dimming frequency is programmed by an external
setting capacitor.
All MPS parts are lead-free and adhere to the RoHS directive. For MPS green
status, please visit MPS website under Quality Assurance. “MPS” and “The
Future of Analog IC Technology” are Registered Trademarks of Monolithic
Power Systems, Inc.
TYPICAL APPLICATION
M1
L1
VOUT
D1
VIN
C1
C2
4.5V-25V
R1
R2
C4
C3
VCC VIN COMPVFAULTSW1 SW2
EN
PGND1
PGND2
Enable
OSC
NC
FPWM
PWMO
GND
OVP
LED1
LED2
MP3388S
C6
C5
PWM
DIMMING
PWMI
LED3
LED8 LED7 LED6 LED5 ISET LED4
RSET
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
1
MP3388S—50V, 8-STRING, STEP-UP, WHITE LED DRIVER
ORDERING INFORMATION
Part Number
Package
Top Marking
QFN24 (4x4mm)
MP3388SGR*
M3388S
* For Tape & Reel, add suffix –Z (e.g. MP3388SGR–Z).
PACKAGE REFERENCE
TOP VIEW
24
23 22
21
20 19
1
2
3
4
5
6
18
17
16
15
14
13
EN
OSC
PGND1
PGND2
OVP
Exposed Pad
Connect to GND
FPWM
PWMO
GND
LED1
LED2
LED3
PWMI
7
8
9
10 11 12
ABSOLUTE MAXIMUM RATINGS (1)
Thermal Resistance (4)
θJA
θJC
VIN .................................................-0.3V to +30V
QFN24 (4 x 4mm) .................42 ....... 9....°C/W
VVFAULT........................................... VIN –6V to VIN
Notes:
VSW, VLED1 to VLED8............................-1V to +50V
All Other Pins..................................-0.3V to +6V
Continuous Power Dissipation... (TA = +25°C) (2)
QFN24(4mm×4mm)................................... 2.9W
Junction Temperature...............................150°C
Lead Temperature ....................................260°C
Storage Temperature............... -65°C to +150°C
Recommended Operating Conditions (3)
Supply Voltage VIN ...........................4.3V to 25V
Operating Junction Temp. (TJ). -40°C to +125°C
1) Exceeding these ratings may damage the device.
2) The maximum allowable power dissipation is a function of the
maximum
junction
temperature
TJ(MAX),
the
junction-to-ambient thermal resistance θJA, and the ambient
temperature TA. The maximum allowable continuous power
dissipation at any ambient temperature is calculated by
PD(MAX)=(TJ(MAX)-TA)/θJA. Exceeding the maximum
allowable power dissipation will cause excessive die
temperature, and the regulator will go into thermal shutdown.
Internal thermal shutdown circuitry protects the device from
permanent damage.
3) The device is not guaranteed to function outside of its
operation conditions.
4) Measured on JESD51-7, 4-layer PCB.
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
2
MP3388S—50V, 8-STRING, STEP-UP, WHITE LED DRIVER
ELECTRICAL CHARACTERISTICS
VIN =12V, VEN = 5V, TA = +25°C, unless otherwise noted.
Parameters
Symbol
Condition
Min
Typ
Max Units
Operating Input Voltage
VIN
IQ
4.3
25
V
Supply Current (Quiescent)
VIN=12V, VEN=5V, no load.
VEN=0V, VIN=12V
1.8
mA
Supply Current (Shutdown)
LDO Output Voltage
IST
2
μA
VEN=5V, 6V<VIN<25V,
0<IVCC<10mA
VCC
4.5
3.4
5
5.5
4.3
V
Input UVLO Threshold
Input UVLO Hysteresis
EN ON Threshold
VIN_UVLO
Rising Edge
3.9
V
mV
V
200
VEN_ON
VEN_OFF
REN PD
VEN Rising
VEN Falling
1.5
EN OFF Threshold
EN Pull-Down Resistor
EN Source Current
EN Sink Current
0.8
V
500
10
kΩ
μA
μA
IEN SOURCE
IEN SINK
VEN=5V
VEN=0V
15
1
STEP-UP CONVERTER
SW ON-Resistance
SW Leakage Current
RDS_ON
ISW_LK
IDS=20mA
VSW=45V
0.18
0.3
1
Ω
μA
MHz
kHz
V
VOSC=VCC or Floating
1.0
500
2.1
1.25
625
1.5
750
Switching Frequency
fSW
VOSC=0V
OSC High-Level Threshold
OSC Low-Level Threshold
VOSC_H
VOSC_L
fSW=1.25MHz
fSW=625kHz
0.8
96
V
PWM Mode,
when no pulse skipping happens
Minimum ON Time
100
93
ns
τON_MIN
Maximum Duty Cycle
SW Current Limit
DMAX
ISW_LIMIT
GCOMP
ICOMP
90
%
A
Duty=90%
2.0
COMP Transconductance
COMP Output Current
ΔICOMP=±10μA
100
60
μA/V
μA
PWM DIMMING
PWMI HIGH Threshold
PWMI LOW Threshold
PWMO Output Impedance
PWMI Leakage Current
PWMI Source Current
PWMI Sink Current
VPWMI_H
VPWMI_L
RPWMO
1.5
V
V
0.8
500
+1
1
300
-1
400
kΩ
μA
μA
μA
kHz
IPWMI_LK
IPWM SOURCE VPWMI=5V
IPWM SINK
fDPWM
VPWMI=0V
3
6
DPWM Frequency
CFPWM=2.2nF
1.2
1.6
2
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
3
MP3388S—50V, 8-STRING, STEP-UP, WHITE LED DRIVER
ELECTRICAL CHARACTERISTICS (continued)
VIN =12V, VEN = 5V, TA = +25°C, unless otherwise noted.
Parameters
Symbol Condition
Min
Typ
Max Units
LED CURRENT REGULATION
LEDx Average Current
Current Matching (5)
ILED
RISET=60.4kꢀ
19.4
20
20.6
2.5
mA
%
ILED=20mA
Maximum LED Current per
String
ILEDmax
30
mA
ISET Regulation Voltage
LEDx Regulation Voltage
1.18
500
1.22 1.26
V
VLEDX
ILED=20mA
600
700
mV
PROTECTION
OVP Over-Voltage Threshold
OVP UVLO threshold
VOVP_OV Rising Edge
1.17
48
1.23
70
1.3
102
5.9
V
mV
V
VOVP_UV Step-up Converter Fails
LEDx Over-Voltage Threshold
LEDx UVLO Threshold
Thermal Shutdown Threshold(6)
LEDx Over-Voltage Fault Timer
VFAULT Pull-Down Current
VLEDX_OV VIN>5.5V
VLEDX_UV
5.1
150
130
1.3
40
5.5
200
150
1.6
55
250
mV
℃
TST
Vosc=high
IFAULT
1.9
70
ms
μA
VFAULT Blocking-Off Voltage
(with Respect to VIN)
VFAULT
VIN =12V, VIN–VFAULT
6
V
Notes:
5) Matching is defined as the difference of the maximum to minimum current divided by 2 times average currents.
6) Guarantee by design.
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
4
MP3388S—50V, 8-STRING, STEP-UP, WHITE LED DRIVER
PIN FUNCTIONS
Pin #
Name
Description
1
EN
Enable Control Input. Do not let this pin floating.
Switching-Frequency Selection. When left floating or connected to VCC, the step-up
converter switching frequency is 1.25MHz. When connected to GND, the step-up converter
switching frequency is 625kHz.
2
3
4
OSC
Dimming PWM Frequency Setting. Connect a capacitor, CFPWM, between FPWM and GND to
FPWM
set the DPWM frequency using the equation: fDPWM=3.5μF/CFPWM
.
PWM Filter Output. For external PWM dimming, connect a capacitor between PWMO and
PWMO GND. For DC-input PWM dimming, directly apply a DC voltage between 0.2V and 1.2V. The
DC-input PWM dimming polarity is negative.
5
6
GND
Analog Ground.
PWM Signal Input. For external PWM dimming mode, apply a PWM signal. For DC-input
PWM dimming mode, leave floating.
PWMI
LED String 8 Current Input. This pin is the open-drain output of an internal dimming-control
switch. Connect the LED String 8 cathode to this pin.
7
LED8
LED7
LED6
LED5
ISET
LED String 7 Current Input. This pin is the open-drain output of an internal dimming-control
switch. Connect the LED String 7 cathode to this pin.
8
LED String 6 Current Input. This pin is the open-drain output of an internal dimming-control
switch. Connect the LED String 6 cathode to this pin.
9
LED String 5 Current Input. This pin is the open-drain output of an internal dimming-control
switch. Connect the LED String 8 cathode to this pin.
10
11
12
13
14
LED Current Setting. Tie a current setting resistor from this pin to ground to program the
current in each LED string. ILED = 1000 x 1.21V / RSET
LED String 4 Current Input. This pin is the open-drain output of an internal dimming-control
switch. Connect the LED String 4 cathode to this pin.
LED4
LED3
LED2
LED String 3 Current Input. This pin is the open-drain output of an internal dimming-control
switch. Connect the LED String 3 cathode to this pin.
LED String 2 Current Input. This pin is the open-drain output of an internal dimming-control
switch. Connect the LED String 2 cathode to this pin.
LED String 1 Current Input. This pin is the open-drain output of an internal dimming-control
switch. Connect the LED String 1 cathode to this pin.
15
LED1
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
5
MP3388S—50V, 8-STRING, STEP-UP, WHITE LED DRIVER
PIN FUNCTIONS (continued)
Pin #
Name
Description
Over-Voltage Protection Input. Connect to the tap of a resistor divider from the output to
AGND to program the OVP threshold. When this pin voltage reaches 1.23V, the MP3388S
triggers OVP mode.
16
OVP
17
18
PGND2
PGND1
Step-Up Converter Power Ground. PGND1 and PGND2 should be shorted externally.
Step-Up Converter Power Ground. PGND1 and PGND2 should be shorted externally.
Step-Up Converter Power Switch Output. SW2 is the drain of the internal MOSFET switch.
Connect the power inductor and output rectifier to SW2. SW2 can swing between GND and
50V. SW1 and SW2 should be shorted externally.
19
20
SW2
SW1
Step-Up Converter Power Switch Output. SW1 is the drain of the internal MOSFET switch.
Connect the power inductor and output rectifier to SW1. SW1 can swing between GND and
50V. SW1 and SW2 should be shorted externally.
Fault-Disconnection–Switch Driver Output. When the system starts up normally, this pin
turns on the external PMOS. When the MP3388S is disabled, the external PMOS turns off
to disconnect the input and output.
21
22
23
VFAULT
COMP
VIN
Step-Up Converter Compensation. This pin compensates the regulation control loop.
Connect a capacitor or a series RC network from COMP to GND.
Supply Input. VIN supplies the power to the MP3388S chip. Drive VIN with a 4.3V-to-25V
power source. Must be locally bypassed.
The Internal 5V Linear Regulator Output. VCC provides power for the internal MOSFET
gate driver and the internal control circuitry. Bypass VCC to GND with a ceramic capacitor.
If VIN is less than 5.5V, apply an external 5V supply directly on VCC.
24
VCC
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
6
MP3388S—50V, 8-STRING, STEP-UP, WHITE LED DRIVER
TYPICAL PERFORMANCE CHARACTERISTICS
VIN =15V, 10 LEDs in series, 8 strings in parallel, 20mA/string, unless otherwise noted.
Steady State
Efficiency vs. Input Voltage
Vin Startup
100
95
V
V
sw
20V/div
sw
20V/div
90
V
OUT
20V/div
V
85
LED8
500mV/div
V
IN
80
10V/div
V
OUT/AC
500mV/div
LED
100mA/div
75
70
I
I
LED
100mA/div
4
9
14
19
24
29
2ms/div
400ns/div
INPUT VOLTAGE (V)
Ven Startup
Open LED Protection
open all LED strings at working
PWM Dimming
fPWM=2kHz, DPWM=50%
V
V
sw
sw
V
sw
20V/div
20V/div
20V/div
V
OUT
20V/div
V
OUT
20V/div
V
V
PWMI
OUT
20V/div
V
5V/div
EN
5V/div
V
FAULT
5V/div
I
I
LED
LED
I
LED
100mA/div
100mA/div
100mA/div
400ms/div
2ms/div
200us/div
Short LED Protection
Short LED Protecton
short Vout to GND at working
short Vout to LEDx at working
V
V
sw
sw
20V/div
20V/div
V
OUT
20V/div
V
OUT
20V/div
V
LED8
20V/div
V
FAULT
10V/div
I
I
LED
LED
100mA/div
100mA/div
1ms/div
200us/div
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
7
MP3388S—50V, 8-STRING, STEP-UP, WHITE LED DRIVER
BLOCK DIAGRAM
VCC
VFAULT
SW
Regulator
VIN
GND
-
Control
Logic
+
PWM
Comparator
Current Sense
Amplifier
+
-
PGND
OVP
OV
Comparator
+
-
Oscillator
OSC
1.23V
LED OV
Comparator
-
5.5V
Short String
Protection
+
-
Max
Feedback
Control
Min
EA
+
COMP
EN
600mV
Enable
Control
LED1
+
1.21V
-
ISET
Current Control
DPWM
Oscillator
LED8
FPWM
PWMI
+
-
1.2V
0.2V
DPWM
Comparator
400
PWMO
Figure 1: Functional Block Diagram
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
8
MP3388S—50V, 8-STRING, STEP-UP, WHITE LED DRIVER
OPERATION
The MP3388S employs a constant-frequency,
peak-current–mode step-up converter and 8
channels of regulated current sources to regulate
up to 8 strings of white LEDs.
Internal 5V Regulator
The MP3388S includes an internal linear regulator
(VCC). When VIN exceeds 5.5V, this regulator
provides a 5V power supply for the internal
MOSFET switch gate driver and the internal
control circuitry. VCC drops to 0V when the chip
shuts down. In applications where VIN is less than
5.5V, tie VCC and VIN together. The MP3388S
features under-voltage lockout (UVLO). The chip
is disabled until VCC exceeds the UVLO
threshold. The UVLO hysteresis is approximately
200mV.
(b)
Figure 2: Recommended Start-Up Sequence
If using any other start-up sequence, as shown in
Figure 3(a), and the unused LEDx pins are not
connected to VOUT
,
then decrease the
input-voltage rise-time (τ). For a 6-string LED
application, select τ less than 20ms as shown in
Figure 3(b)
System Startup
When the MP3388S is enabled, the chip checks
the topology connection first. The VFAULT pin
slowly turns on the external fault disconnection
PMOS. And after a 400µs delay, the chip monitors
the OVP pin to check for the Schottky diode or if
the boost output is shorted to GND. If the OVP
voltage is below 70mV, the chip is disabled and
the external PMOS is turned off together. The
MP3388S also checks other safety limits,
including UVLO and OTP if the OVP test passes.
If the circuit passes all tests, then the MP3388S
uses an internal soft-start to boosting the step-up
converter.
(a)
VIN
τ
4.5V
For best results, use the following start-up
sequence: either VIN or the PWM dimming signal
first, and then the enable signal (see Figure 2).
The MP3388S does not require a power-off
sequence (see Figure 4)
0V
(b)
Figure 3: Input Voltage Rise-Time Indicator for
a 6-String LED Application
Note: Use any power sequence if the VIN
rise-time from 0V to 4.5V is less than 20ms.
(a)
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
9
MP3388S—50V, 8-STRING, STEP-UP, WHITE LED DRIVER
and the feedback voltage. The converter
automatically chooses the lowest active LEDx pin
voltage to provide the bus voltage to power all the
LED arrays.
VIN
VPWM
If the feedback voltage drops below the 600mV
reference, the output of the error amplifier
increases to increase the current flowing through
the power MOSFET, thus increasing the power
delivered to the output. This forms a closed loop to
regulate the output voltage.
VEN
(a)
At light-load operation—or VOUT≈VIN—the
converter enters pulse-skipping mode where the
MOSFET turns on for a minimum ON-time of
approximately 100ns before the converter
discharges the power to the output. The MOSFET
repeats this cycle until the output voltage requires
a boost.
VIN
VPWM
VEN
Dimming Control
The MP3388S provides two PWM dimming
methods: external PWM signal from the PWMI pin
or DC-input PWM dimming mode (see Figure 5).
(b)
VIN
VPWM
VEN
(c)
Figure 5: PWM Dimming
Figure 4: Recommended Power-Off Sequence
When applying a PWM signal to the PWMI pin, the
MP3388S generates a DC voltage on the PWMO
pin that is proportional to the duty cycle of the
PWMI signal. By comparing the PWMO pin signal
to the FPWM pin triangle waveform, the converter
gets a choppy low-frequency signal with a duty
cycle the same as the input’s. This choppy
low-frequency signal modulates the LED current.
Step-Up Converter
The converter operation frequency is selectable
(625kHz or 1.25MHz), which can optimize for
external component sizes and improve efficiency.
At the beginning of each cycle, the internal clock
turns on the power MOSFET. To prevent
sub-harmonic oscillations at D>50%, add a
stabilizing ramp to the output of the current-sense
amplifier; the output goes to the positive input of
the PWM comparator. When the PWM
comparator’s positive input voltage equals the
output voltage of the error amplifier (VCOMP) the
power MOSFET turns off.
Directly applying a DC analog signal to the PWMO
pin also modulates the LED current: the DC signal
is converted to a DPWM signal set at the
oscillation frequency. The polarity is negative. The
brightness of the LED array is proportional to the
duty cycle of the DPWM signal. The capacitor at
the FPWM pin sets the DPWM signal frequency.
The internal error amplifier amplifies the
difference between the 600mV reference voltage
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
10
MP3388S—50V, 8-STRING, WHITE LED DRIVERS
Open-String Protection
Short-String Protection
The open string protection is achieved through
over-voltage protection (OVP). If one or more
strings are open, the device pulls the respective
LEDx pins to ground and the output voltage
continues rising until it reaches the OVP
threshold. Then the device records and disable
the open LEDx strings with pin voltages less than
175mV. Once recording completes, the remaining
LED strings force the output voltage back into tight
regulation. The string with the highest voltage
drop determines the output regulation level.
The MP3388S monitors each LEDx pin voltage to
determine if there is a short string. If one or more
strings are shorted, the respective LEDx pins will
be pulled up to the boosted output and tolerate
high voltage stress. If the LEDx pin voltage
exceeds 5.5V, the device treats this condition as a
short string fault (LEDx over-voltage fault). If the
voltage remains above 5.5V more than than
1.6ms (VOSC=HIGH), the device records the string
and disables it. Once a string is recorded as a
short, its current regulation is forced to disconnect
from the output voltage loop regulation. The
recorded LED strings remain OFF until the part
restarts. If all strings in use are shorted, the
MP3388S shuts down the step-up converter.
The MP3388S will always try to light at least one
string. If all strings in use are open, the MP3388S
shuts down the step-up converter. The part
retains low-voltage LEDx string information until
the part shuts down.
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
11
MP3388S—50V, 8-STRING, STEP-UP, WHITE LED DRIVER
The minimum recommended PWM signal
amplitude is 2.1V (See Figure 6).
APPLICATION INFORMATION
Selecting the Switching Frequency
The user can select either a 625kHz or a 1.25MHz
step-up converter switching frequency. A bi-level
switching frequency selection input (the OSC pin)
sets the internal oscillator frequency. Tie the OSC
pin to GND corresponds for 625kHz, and to VCC
or float for 1.25MHz.
Setting the LED Current
The ISET pin sets the LED string currents to the
same level as per the equation below:
Figure 6: PWM Dimming with Internal Triangle
Waveform Generator
1.21V
ILED 1000
RSET
Direct PWM Dimming with Positive Logic
To use a direct external PWM dimming signal,
connect a 100kꢀ resistor from the FPWM pin to
GND and apply a 100Hz-to-30kHz (high SW
frequency) PWM dimming signal to the PWMI pin.
The minimum recommended amplitude of the
PWM signal is 1.5V (See Figure 7).
For RSET=60.4kꢀ, the LED current is 20mA. Do
not leave the ISET pin open.
Setting the Over-Voltage Protection Threshold
The open string protection is achieved through
OVP. In some cases, an LED string failure results
in a 0V feedback voltage. The MP3388S then
continues boosting the output voltage. OVP
triggers if the output voltage reaches the
programmed OVP threshold.
To ensure proper chip function, set the OVP
setting resistor divider with appropriate values.
Select an OVP threshold about 1.3x higher than
the output voltage for normal operation.
Figure 7: Direct PWM Dimming with Positive
Logic
R1 R2
VOVP 1.23
R2
Table 1 shows the PWM dimming duty range with
different PWM dimming frequencies.
Selecting the Dimming-Control Mode
The MP3388S provides four different dimming
methods:
PWM Dimming Mode with Internal Triangle
Waveform Generator
Apply a 100Hz-to-50kHz square waveform to the
PWMI pin. The internal 400kꢀ resistor and the
external capacitor on the PWMO pin filters the
dimming signal to a DC voltage (0.2V to 1.2V).
Then an internal PWM dimming signal whose
frequency is set by a capacitor on the FPWM pin
modulates the DC voltage, where:
3.5F
CFPWM
fDPWM
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
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© 2012 MPS. All Rights Reserved.
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MP3388S—50V, 8-STRING, WHITE LED DRIVERS
Table 1: PWM Frequency Range and Dimming
Duty Cycle
f
PWM(Hz)
DMIN
DMAX
100%
100%
100%
100%
100%
100%
100%
100%
100%
100%
100<f≤200
200<f≤500
500<f≤1k
1k<f≤2k
0.16%
0.40%
0.80%
1.60%
4.00%
8.00%
16.00%
18.00%
20.00%
24.00%
Figure 9: DC-Input PWM Dimming
Selecting the Inductor
A larger inductor results in less ripple current,
lowering both the peak inductor current and stress
on the internal N-channel MOSFET. However, a
larger value inductor is larger physical size with a
higher series resistance and a lower saturation
current.
2k<f≤5k
5k<f≤10k
10k<f≤20k
20k<f≤22k
22k<f≤25k
25k<f≤30k
Choose an inductor that does not saturate under
worst-case load conditions. Select the minimum
inductance value to ensure that the boost
converter works in continuous conduction mode,
for high efficiency and good EMI performance.
Direct PWM Dimming with Negative Logic
Calculate the minimum inductance value with:
VOUT D(1 D)2
Similar to direct PWM dimming with positive logic.
Apply a 100Hz-to-30 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
8).
L
2ILOAD fSW
V
IN
D 1
VOUT
Where VIN is the input voltage, VOUT is the output
voltage, fSW is the switching frequency, ILOAD is the
LED load current, and η is the efficiency.
Use either a 10μH (at a 1.25MHz switching
frequency) or a 22µH (at a 625kHz switching
frequency) inductor with a DC current rating of at
least 40% higher than the maximum input current
for most applications. Select an inductor with the
smallest-possible DC resistance for greatest
efficiency.
Figure 8: Direct PWM Dimming
DC-Input PWM Dimming with Negative Logic
Selecting the Input Capacitor
For negative-logic DC-input PWM dimming, apply
an analog signal (from 0.2V to 1.2V) to the PWMO
pin. If applying a PWMO DC voltage <0.2V, the
PWM duty cycle will be 100%. For PWMO DC
voltages >1.2V, the output will be 0% (See Figure
9). The capacitor on the FPWM pin sets the
internal triangle waveform frequency.
The input capacitor reduces both 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. For best results, use
ceramic capacitors with X5R or X7R dielectrics
because of their low ESR and small temperature
coefficients. For most applications, use a 4.7μF
capacitor.
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
13
MP3388S—50V, 8-STRING, WHITE LED DRIVERS
LAYOUT CONSIDERATIONS
Selecting the Output Capacitor
The output capacitor minimizes the output voltage
ripple and ensures a stable feedback loop. The
output capacitor impedance should be low at the
switching frequency. Use ceramic capacitors with
X7R dielectrics for their low ESR characteristics.
Layout and component placement on the PCB
requires careful attention. Proper high-frequency
switching-path layout can prevent noise and
electromagnetic interference problems. The loop
from SW (U1), to the output diode (D1), to the
output capacitor (C3), to PGND (U1) carries a
high-frequency pulse current: minimize the loop
length and enclosed area (See Figure 10).
The output voltage ripple is estimated as:
V
IN
1 -
I
LED
VOUT
VRIPPLE
C2 fSW
Where VRIPPLE is the output ripple voltage, VIN and
VOUT are the input and output voltages
respectively, ILED is the LED current, fSW is the
switching frequency, and C2 is the output
capacitor.
For most applications, use a 2.2μF ceramic
capacitor.
Figure 10: Sample Layout
The IC exposed pad is internally connected to
GND pin, and all logic signals are refer to the GND.
Externally connects PGND to GND, and avoid
placing PGND near logic signals.
MP3388S Rev.1.01
10/29/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
14
MP3388S—50V, 8-STRING, STEP-UP, WHITE LED DRIVER
PACKAGE INFORMATION
QFN24 (4mm × 4mm)
NOTICE: The information in this document is subject to change without notice. Users should warrant and guarantee that third
party Intellectual Property rights are not infringed upon when integrating MPS products into any application. MPS will not
assume any legal responsibility for any said applications.
MP3388S Rev. 1.01
10/29/2012
www.MonolithicPower.com
MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
15
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