MP3378E
4-Channel WLED Controller
with High-Efficiency Buck Converter
DESCRIPTION
FEATURES
The MP3378E is a one-chip solution specially
designed for monitor applications. The
MP3378E includes a step-up WLED controller
with four current channels for backlighting, and
a high-efficiency buck converter for internal bus
voltage or standby power.
WLED Controller:
The 4-string WLED controller drives an external
MOSFET to boost up the output voltage from
the input supply. It regulates the current in each
LED string to the programmed value set by an
external current setting resistor. It supports both
analog and PWM dimming independently to
meet special dimming mode requests. Full
protection features for the WLED controller
include OCP, OTP, UVP, OVP, LED short/open
protection, and inductor/diode short protection.
The high-efficiency buck converter operates in
the current mode operation with a built-in
MOSFET and a built-in synchronous rectifier. It
offers a very compact solution to achieve
excellent load and line regulation. Full
protection features for the buck converter
include OCP and thermal shutdown.
The MP3378E is available in a TSSOP-28 EP
package.
4-String,
Max
350mA/String,
WLED
Controller
Up to 24V Input Voltage Range
2.5% Current Matching Accuracy
Programmable Switching Frequency
PWM and Analog Dimming Mode
LED Open and Short Protection
Programmable Over-Voltage Protection
Recoverable Thermal Shutdown Protection
Over-Current Protection
Over-Temperature Protection
Inductor/Diode Short Protection
Buck Converter:
144mΩ/80mΩ Low RDS(ON) Internal Power
MOSFETs
Low Quiescent Current
Fixed 235kHz Switching Frequency
Frequency Sync from 250kHz to 2MHz
External Clock
Internal Soft Start
OCP and Hiccup Mode
Over-Temperature Protection
Output Adjustable from 0.8V
APPLICATIONS
Desktop LCD Flat Panel Displays
Flat Panel Video Displays
2D/3D LCD TVs and Monitors
All MPS parts are lead-free, halogen-free, and adhere to the RoHS directive. For
MPS green status, please visit the MPS website under Quality Assurance.
“MPS” and “The Future of Analog IC Technology” are registered trademarks of
Monolithic Power Systems, Inc.
MP3378E Rev. 1.02
www.MonolithicPower.com
5/26/2017
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1
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
TYPICAL APPLICATION
L1
D1
VIN
C2
C1
M1
24
C3
25
GATE
VIN1
ISENSE
VCC1
R10
23
R9
22
R2
2
4
28
R4
3
15
VIN
C4
22µF
21
EN1
OVP
OSC
LED1
ADIM
LED2
PWM
ISET
MP3378
E
VIN2
LED3
LED4
BST
SW
SYNC
8
7
6
5
20 R5
47Ω
16
C7
0.1µF
SYNC
14
C6
0.1µF
12
VCC2
R6
EN2
L2 10µH
R7
40.2k
5V
C5
66µF
FB
51k
13
String 4
9
String 3
27
C4
GND1
COMP
String 1
R3
1
String 2
R1
26
GND2
AGND
19
R8
7.5k
17, 18
MP3378E Rev. 1.02
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2
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
ORDERING INFORMATION
Part Number
MP3378EGF
Package
TSSOP-28 EP
Top Marking
See Below
* For Tape & Reel, add suffix –Z (e.g. MP3378EGF–Z)
TOP MARKING
MP3378E: Product code of MP3378EGF
MPS: MPS prefix
YY: Year code
WW: Week code
LLLLLLLLL: Lot number
PACKAGE REFERENCE
TOP VIEW
GND1
1
28
PWM
OSC
2
27
EN1
ISET
3
26
COMP
ADIM
4
25
VCC1
LED4
5
24
VIN1
LED3
6
23
GATE
LED2
7
22
ISENSE
LED1
8
21
SYNC
OVP
9
20
BST
NC
10
19
AGND
NC
11
18
GND2
Exposed Pad
Connect to GND
FB
12
17
GND2
EN2
13
16
SW
VCC2
14
15
VIN2
TSSOP-28 EP
MP3378E Rev. 1.02
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MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
ABSOLUTE MAXIMUM RATINGS (1)
Thermal Resistance
WLED Controller
VIN1 .............................................. -0.3V to +28V
VLED1 to VLED4 .................................. -1V to +55V
VGATE, VCC1, VISENSE ..................... -0.3V to +6.5V
All other pins .............................. –0.3V to VCC1
Buck Converter
VIN2, VSW ....................................... –0.3V to 28V
VBST ...................................................... VSW+6V
All other pins ................................... –0.3V to 6V
(2)
Continuous power dissipation (TA = 25°C)
TSSOP-28 EP ...........................................3.9W
Junction Temperature .............................. 150°C
Lead Temperature ................................... 260°C
TSSOP-28 EP……………......….32........6....°C/W
Recommended Operating Conditions
(4)
θJA
θJC
NOTES:
1) Exceeding these ratings may damage the device. The voltage
is measured with a 20MHz bandwidth limited oscilloscope.
2) The maximum allowable power dissipation is a function of the
maximum junction temperature TJ (MAX), the junction-toambient 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 produces an excessive die temperature, causing
the regulator to 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
operating conditions.
4) Measured on JESD51-7, 4-layer PCB.
(3)
Supply voltage (VIN1, VIN2) .................. 5V to 24V
Operating junction temp. (TJ). .. -40°C to +125°C
MP3378E Rev. 1.02
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4
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
ELECTRICAL CHARACTERISTICS (5)
VIN1 = VIN2 = 12V, VEN1 = VEN2 = 5V, TA = 25°C, unless otherwise noted.
Parameters
Symbol
Condition
Min
Typ
Max
Units
1.2
1.35
1.5
mA
1
μA
WLED Controller Section
Supply current (quiescent)
IQ1
Supply current (shutdown)
IST1
LDO output voltage
VCC1
VCC1 UVLO threshold
VCC1_UVLO
VIN1 = 12V, VEN1 = 5V,
no load without switching,
buck disabled
VEN1 = 0V, VIN = 12V,
buck disabled
VEN1 = 5V, 7V < VIN1 < 28V,
0 < IVCC1 < 10mA
Rising edge
5.4
6
6.6
V
3.6
4
4.4
V
VCC1 UVLO hysteresis
200
EN1 high voltage
VEN1_HIGH
VEN1 rising
EN1 low voltage
VEN1_LOW
VEN1 falling
Step-Up Converter
Gate driver impedance
(sourcing)
Gate driver impedance
(sinking)
mV
1.8
VCC1 = 6V,VGATE = 6V
VCC1 = 6V, IGATE = 10mA
V
0.6
V
4.1
7
Ω
3
5
Ω
ROSC = 115kΩ
470
530
590
kHz
ROSC = 374kΩ
150
180
210
kHz
1.20
1.23
1.26
V
Switching frequency
fSW1
OSC voltage
VOSC
Maximum duty cycle
Cycle-by-cycle ISENSE
current limit
COMP source current limit
DMAX1
ICOMP SOLI
1V < COMP < 1.9V
70
μA
COMP sink current limit
ICOMP SILI
1V < COMP < 1.9V
17
μA
ΔICOMP = ±10μA
440
μA/V
COMP transconductance
93
Max duty cycle
GCOMP
145
180
%
230
mV
Current Dimming
PWM input low threshold
VPWM_LO
VPWM falling
PWM input high threshold
Analog dimming input low
threshold
Analog dimming input high
threshold
LED Current Regulation
VPWM_HI
VPWM rising
ISET voltage
VISET
LEDX average current
Current matching
ILED
(5)
VCC max current limit
LED FET resistance
LEDX regulation voltage
RISET = 30.5kΩ
0.75
1.25
V
0.38
0.41
0.44
V
1.44
1.49
1.54
V
1.20
1.225
1.25
V
31.4
33
34.2
mA
2.5
%
100
mA
ILED = 32mA
ICC1_Limit
R_LED
VLEDX
V
50
75
ILED = 10mA
1.7
Ω
ILED = 330mA
800
mV
ILED = 60mA
260
mV
MP3378E Rev. 1.02
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5
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
ELECTRICAL CHARACTERISTICS (continued)
VIN1 = VIN2 = 12V, VEN1 = VEN2 = 5V, TA = 25°C, unless otherwise noted.
Parameters
Symbol
Condition
Min
Typ
Max
Units
Protection
OVP (over-voltage protection)
threshold
OVP (over-voltage protection)
threshold hysteresis
VOVP_OV
Rising edge
1.20
1.23
1.26
V
VOVP_HYS
OVP hysteresis
OVP UVLO threshold
VOVP_UV
Step-up converter fails
LEDX UVLO threshold
LEDX over-voltage threshold
80
mV
33
75
115
mV
VLEDX_UV
120
190
260
mV
VLEDX_OV
5.8
6.3
6.8
V
720
mV
LED short fault cycles
T_LED_OV
Latch-off current limit
VLMT
Thermal protection threshold
TST
4096
600
Thermal protection hysteresis
660
150
°C
25
°C
Buck Converter Section
Supply current (quiescent)
IQ2
Supply current (shutdown)
IST2
VIN2 under-voltage lockout
threshold
VIN2 under-voltage lockout
threshold hysteresis
VCC2 regulator
VIN2_UVLO
VFB = 1V,
WLED controller disabled
VEN2 = 0V,
WLED controller disabled
Rising edge
VCC2
VCC2 load regulation
ICC2 = 5mA
EN2 high threshold
VEN2_HI
EN2 rising threshold
EN2 low threshold
VEN2_LO
EN2 falling threshold
150
200
250
μA
10
μA
3.7
3.9
4.1
V
550
650
750
mV
4.65
4.9
5.15
V
0
1
3
%
1.8
V
0.6
V
HS switch-on resistance
HSRDS-ON
VBST-SW = 5V
144
mΩ
LS switch-on resistance
LSRDS-ON
VCC2 = 5V
80
mΩ
Current limit
ILIMIT
Duty cycle = 40%
4.8
6
7.2
A
Oscillator frequency
fSW2
VFB = 750mV
190
235
280
kHz
Foldback frequency
fFB
VFB = 200mV
Maximum duty cycle
DMAX2
VFB = 750mV
Minimum on time
(5)
90
TON_MIN
Sync frequency range
fSYNC
Feedback voltage
VFB
TA = 25ºC
Feedback current
IFB
VFB = 820mV
Soft-start period
TSS
10% to 90%
0.5
fSW2
95
%
80
ns
0.25
779
0.8
2
MHz
791
803
mV
10
50
nA
1.5
2.2
ms
MP3378E Rev. 1.02
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5/26/2017
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6
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
ELECTRICAL CHARACTERISTICS (continued)
VIN1 = VIN2 = 12V, VEN1 = VEN2 = 5V, TA = 25°C, unless otherwise noted.
Parameters
Symbol
SYNC high threshold
VSYNC_HI
SYNC low threshold
VSYNC_LO
Condition
Min
Typ
Max
1.8
Units
V
0.6
V
Thermal shutdown
150
°C
Thermal hysteresis
20
°C
NOTE:
5) Matching is defined as the difference between the maximum to minimum current divided by 2 times the average currents.
MP3378E Rev. 1.02
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5/26/2017
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7
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
TYPICAL PERFORMANCE CHARACTERISTICS
WLED Controller Section:
VIN = 16V, 10 LEDs in series, 4 strings parallel, 120mA/string, TA = 25°C, unless otherwise noted.
MP3378E Rev. 1.02
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5/26/2017
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MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
TYPICAL PERFORMANCE CHARACTERISTICS
Buck Converter Section:
VIN = 16V, VOUT = 5V, L2 = 10μH, TA = 25°C, unless otherwise noted.
MP3378E Rev. 1.02
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MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
PIN FUNCTIONS
Pin #
Name
Description
1
GND1
2
OSC
3
ISET
4
ADIM
5
LED4
6
LED3
7
LED2
8
LED1
9
OVP
10, 11
NC
12
FB
13
EN2
14
VCC2
15
VIN2
16
17, 18
SW
GND2
Ground for WLED controller.
Switching frequency set. Connect a resistor between OSC and GND to set the step-up
converter switching frequency. The voltage at OSC is regulated to 1.23V. The clock
frequency is proportional to the current sourced from OSC.
LED current set. Tie a current-setting resistor from ISET to ground to program the
current in each LED string. The ISET voltage is regulated to 1.225V. The LED current is
proportional to the current through the ISET resistor.
Input for analog brightness control. The LED current amplitude is determined by
ADIM, and the input signal can be either a PWM signal or a DC voltage signal. An
internal RC filter (10MΩ resistor and 100pF capacitor) is integrated into ADIM. If a PWM
signal is applied to ADIM, a >20kHz frequency is recommended for better PWM signal
filtering performance to ensure that the amplitude voltage is higher than 1.5V, and the
low-level voltage is less than 0.4V. For a DC signal input, apply a DC input signal
ranging from 0.41V to 1.49V to set the LED current linearly from minimum to full scale.
Internally pull to GND if ADIM is floated.
LED string 4 current input. LED4 is the open-drain output of an internal dimming
control switch. Connect the LED string 4 cathode to LED4.
LED string 3 current input. LED3 is the open-drain output of an internal dimming
control switch. Connect the LED string 3 cathode to LED3.
LED string 2 current input. LED2 is the open-drain output of an internal dimming
control switch. Connect the LED string 2 cathode to LED2.
LED string 1 current input. LED1 is the open-drain output of an internal dimming
control switch. Connect the LED string 1 cathode to LED1.
Over-voltage protection input. Connect a resistor divider from the output to OVP to
program the OVP threshold.
Not connected.
Buck converter feedback. An external resistor divider from the output to AGND tapped
to FB sets the output voltage. To prevent current-limit runaway during a short-circuit fault
condition, the frequency foldback comparator lowers the oscillator frequency when the
FB voltage is below 400mV.
Buck converter enable. Pull EN2 high to enable the buck converter.
Bias supply for buck converter. Decouple with a 0.1μF-0.22μF capacitor. The
capacitance should be no more than 0.22μF.
Supply voltage input for buck converter. A ceramic capacitor is needed to decouple
the input rail. Use a wide PCB trace to make the connection.
Switch output for buck converter. Use a wide PCB trace to make the connection.
Ground for buck converter.
19
AGND
Analog ground for buck converter.
MP3378E Rev. 1.02
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10
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
PIN FUNCTIONS (continued)
Pin #
Name
20
BST
21
SYNC
22
ISENSE
23
GATE
24
VIN1
25
VCC1
26
COMP
27
EN1
28
PWM
Description
Bootstrap for buck converter. A capacitor and a 47Ω resistor connected between SW
and BST are required to form a floating supply across the high-side switch driver.
Synchronization for buck converter. Apply a clock signal with a frequency higher than
250KHz. The frequency of the buck converter can be synchronized by an external clock.
The internal clock’s rising edge is synchronized to the external clock’s falling edge.
Current sense input for WLED controller. During normal operation, ISENSE senses
the voltage across the external inductor current-sensing resistor (RSENSE) for peakcurrent-mode control and also limits the inductor current during every switching cycle.
Power switch gate output for WLED controller. GATE drives the external power
N-channel MOSFET device.
Supply input for WLED controller.
Internal 6V linear regulator output for WLED controller. VCC1 provides a power
supply for the external MOSFET switch gate driver and the internal control circuitry.
Bypass VCC1 to GND with a ceramic capacitor.
Error amplifier output of WLED controller. Connect a capacitor and a resistor in
series to make the boost converter loop stable.
Enable input for WLED controller.
Input signal for PWM brightness control. The PWM pin should be used for dimming
and can not be used for turning on/off the WLED controller. Internally pull to GND if
PWM is floated.
MP3378E Rev. 1.02
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11
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
BLOCK DIAGRAM
VCC1
VIN1
Regulator
GND1
-
Control
Logic
+
PWM
Comparator
GATE
Current Sense
Amplifier
+
200ns
Blank Time
ISENSE
-
OV
Comparator
OVP
+
Oscillator
OSC
-
100 ns
Blanking
+
-
ILIMIT
PWM
STOP
+
-
COMP
-
UP_ CLAMP
Short-String
Protection
1.23V
6.3V
+
Max
-
Min
EA
Feedback
Control
+
Ref
EN1
WLED
Enable
LED1-4
1
Current Control
+
1.225V
ADIM
-
PWM
ISET
VIN2
VCC2
RSEN
VCC
Regulator
BST
LDO
HS
Driver
OSC
SYNC
1pF
EN2
Rference
50pF
400k
Current Limit
Comparator
On-Time
Control Logic
SW
VCC2
LS
Driver
FB
Error Amplifier
GND2
AGND
Figure 1: Functional Block Diagram
MP3378E Rev. 1.02
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12
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
OPERATION
WLED CONTROLLER SECTION:
The WLED controller employs a programmable,
constant frequency, peak current mode, step-up
converter with four channels that regulate
current sources to drive an array of up to four
strings of white LEDs.
Internal 6V Regulator
When VIN1 is greater than 6.5V, VCC1 outputs
a 6V power supply to the external MOSFET
switch gate driver and the internal control
circuitry. The VCC1 voltage drops to 0V when
the WLED controller shuts down.
System Start-Up
When enabled, the WLED controller checks the
topology connection first. The WLED controller
monitors the over-voltage protection (OVP) to
see if the schottky diode is connected, or if the
boost output is shorted to GND. An OVP
voltage of less than 75mV will disable the
WLED controller. Once all of the protection
tests pass, the WLED controller then boosts the
step-up converter with an internal soft start.
It is recommended that the enable signal occurs
after the establishment of the input voltage and
the PWM dimming signal during the start-up
sequence to avoid a large inrush current. The
PWM pin should be used for dimming function
and can not be used for turning on/off the
WLED controller.
If the PWM is used for turning on/off WLED
controller in some applications, a resistor RD is
recommended to parallel with COMP pin as
shown in figure 2. The recommended value for
RD is 1M Ω -10M Ω , considering the PWM
frequency.
COMP
RCOMP
RD
MP3378E
CCOMP
Figure 2 COMP Connection for turning on/off
WLED Controller with PWM signal
Step-Up Converter
The
converter
operating
frequency
is
programmable through an external resistor on
OSC. The operating frequency is recommended
between 300kHz to 500kHz. This helps
optimize efficiency and the size of the external
components.
At the beginning of each switching cycle, the
internal clock turns on the external MOSFET (in
normal operation, the minimum turn-on time is
200ns). A stabilizing ramp added to the output
of the current sense amplifier prevents sub
harmonic oscillations for duty cycles greater
than 50 percent. This result is fed into the PWM
comparator. When this resulted voltage reaches
the output voltage of the error amplifier (VCOMP),
the external MOSFET turns off.
The output voltage of the internal error amplifier
is an amplified signal of the difference between
the reference voltage and the feedback voltage.
Automatically, the converter chooses the lowest
active LEDX voltage to provide a high enough
bus voltage to power all the LED arrays.
If the feedback voltage drops below the
reference, the output of the error amplifier
increases. This results in more current flowing
through the MOSFET, increasing the power
delivered to the output. This forms a closed
loop that regulates the output voltage.
Under light-load operation, especially in the
case of VOUT1≈VIN1, the converter runs in pulseskipping mode, where the MOSFET turns on for
a minimum on time of approximately 200ns
before the converter discharges the power to
the output for the remaining period. The
external MOSFET remains off until the output
voltage needs to be boosted again.
Dimming Control
Two dimming methods, PWM and analog
dimming mode, are allowed.
For PWM dimming, apply a PWM signal to
PWM. The LED current is chopped by this
PWM signal, and the average LED current is
equal to ISET*DDIM, where DDIM is the duty cycle
of the PWM dimming signal, and ISET is the LED
current amplitude.
MP3378E Rev. 1.02
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MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
For analog dimming, either a PWM signal or a
DC signal can be applied to ADIM.
When a PWM signal is applied to ADIM, this
signal is filtered by the internal RC filter. The
LED current amplitude is equal to ISET*DDIM,
where DDIM is the duty cycle of the PWM
dimming signal, and ISET is the LED current
amplitude. A 20kHz or higher PWM signal is
recommended for better filtering performance.
When a DC signal is applied to ADIM, the
voltage ranging from 0.41V to 1.49V sets the
LED current linearly from minimum to full scale
directly.
Open-String Protection
Open-string protection is achieved through OVP
and LEDX (1 to 4). If one or more strings are
open, the respective LEDXs are pulled to
ground, and the WLED controller keeps
charging the output voltage until it reaches the
over-voltage protection (OVP) threshold. If the
OVP point has been triggered for >4µs, the
WLED controller stops switching and marks off
the strings which have an LEDX voltage lower
than 190mV. Once marked off, the remaining
LED strings force the output voltage back into
tight regulation. The string with the largest
voltage drop determines the output regulation. If
all strings are open, the WLED controller shuts
down until the WLED controller resets.
Short-String Protection
The WLED controller monitors the LEDX
voltages to determine if a short-string fault has
occurred. If one or more strings are shorted, the
respective LEDXs tolerate high-voltage stress.
If an LEDX voltage is higher than 6.3V, this
condition triggers the detection of a short string.
When a short-string fault (LEDX over-voltage
fault) remains for 4,096 switching cycles, the
fault string is marked off and disabled. Once a
string is marked off, it disconnects from the
output voltage loop. The marked LED strings
shut off completely until the boost part restarts.
In order to prevent mistriggering the short LED
protection when opening the LED string or
sharp ADIM, the short LED protection function
is disabled when the VLEDXs of all used LED
channels are higher than 1.5V.
Inductor/Diode Short Protection
To prevent WLED controller and external
MOSFET
damage
when
the
external
inductor/diode is shorted, the protection mode
operates in two methods.
When the inductor/diode is shorted, the output
cannot maintain enough energy to load the LED,
and the output voltage drops. Thus, the COMP
(error amplifier output) voltage rises until it can
be clamped high. If it lasts longer than 512
switching cycles, the WLED controller turns off
and latches.
However, in some cases, the COMP voltage
cannot be clamped to high when the
inductor/diode is shorted. The WLED controller
provides a protection mode by detecting the
current flowing through the power MOSFET. In
this mode, when the current senses voltage
across the sense resistor (connected between
MOSFET and GND) and hits the VLMT limit
value (lasting for 4 switching cycles), the WLED
controller turns off and latches.
Thermal Shutdown Protection
To prevent the WLED controller from operating
at exceedingly high temperatures, a thermal
shutdown is implemented in this chip and
monitors the silicon die temperature. When the
die temperature exceeds the upper threshold
(TST), the WLED controller shuts down. It
returns to normal operation when the die
temperature drops below the lower threshold.
Typically, the hysteresis value is 25°C.
MP3378E Rev. 1.02
www.MonolithicPower.com
5/26/2017
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© 2017 MPS. All Rights Reserved.
14
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
BUCK CONVERTER SECTION:
The buck converter is a step-down, switchmode converter with built-in internal power
MOSFETs and offers a very compact solution.
It operates in a fixed-frequency, peak-currentcontrol mode to regulate the output voltage. A
PWM cycle is initiated by an internal clock. The
integrated high-side power MOSFET is turned
on and remains on until its current reaches the
value set by the COMP_BUCK voltage (this
COMP_BUCK is one of the buck’s internal
control voltage, not the COMP pin). When the
power switch is off, it remains off until the next
clock cycle starts. If the current in the power
MOSFET does not reach the COMP_BUCK set
current value in 95% of one PWM period, the
power MOSFET is forced to turn off.
Internal Regulator
Most of the internal circuitries are powered by a
5V internal regulator. This regulator takes the
VIN2 input and operates in the full VIN2 range.
When VIN2 is greater than 5.0V, the output of
the regulator is in full regulation. When VIN2 is
lower than 5.0V, the output decreases. A 0.1µF
ceramic capacitor is required for decoupling
purposes.
Error Amplifier
The error amplifier compares the FB voltage
with the internal 0.8V reference (REF) and
outputs a COMP_BUCK voltage, which is used
to control the power MOSFET current. The
optimized internal compensation network
minimizes the external component count and
simplifies the control loop design.
SYNC Control
The buck converter can be synchronized to an
external clock ranging from 250kHz to 2MHz
through SYNC. The internal clock’s rising edge
is synchronized to the external clock’s falling
edge. The synchronized logic high voltage
should be higher than 1.8V, and the
synchronized logic low voltage should be lower
than 0.6V. The frequency of the external clock
should be higher than the internal clock.
Otherwise the internal clock may turn on the
high-side MOSFET.
Under-Voltage Lockout (UVLO)
Under-voltage lockout (UVLO) is implemented
to protect the buck converter from operating at
insufficient supply voltages by monitoring the
output voltage of the internal regulator (VCC2).
The UVLO rising threshold is about 3.9V while
its falling threshold is consistently 3.25V.
Internal Soft Start (SS)
The soft start is implemented to prevent the
converter output voltage from overshooting
during start-up. When the chip starts up, the
internal circuitry generates a soft-start voltage
(SS) and ramps up from 0V. The soft-start
period lasts until the voltage of the soft-start
capacitor exceeds the reference voltage of 0.8V.
At this point, the reference voltage takes over.
The soft-start time is set internally to around
1.5ms.
Over-Current Protection and Hiccup Mode
The cycle-by-cycle over-current limit is
implemented when the inductor current peak
value exceeds the set current limit threshold.
The output voltage drops until FB is below the
under-voltage (UV) threshold, typically 50%
below the reference. Once UV is triggered, the
buck converter enters hiccup mode to restart
the part periodically. This protection mode is
especially useful when the output is deadshorted to ground. The average short-circuit
current is greatly reduced to alleviate thermal
issues and protect the regulator. The buck
converter exits hiccup mode once the overcurrent condition is removed.
Thermal Shutdown
Thermal shutdown is implemented to prevent
the chip from operating at exceedingly high
temperatures. When the silicon die temperature
is higher than 150°C, the buck converter shuts
down. When the temperature is lower than its
lower threshold, typically 130°C, the buck
converter is enabled again.
MP3378E Rev. 1.02
www.MonolithicPower.com
5/26/2017
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© 2017 MPS. All Rights Reserved.
15
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
Floating Driver and Bootstrap Charging
The floating power MOSFET driver is powered
by an external bootstrap capacitor. This floating
driver has its own UVLO protection. The UVLO
rising threshold is 2.2V with a hysteresis of
150mV. The bootstrap capacitor voltage is
regulated internally by VIN2 through DB, R6,
C7, L2, and C5 (see Figure 3). If VIN2-VSW is
more than 5V, then U1 will regulate MB to
maintain a 5V BST voltage across C7.
DB
VIN2
MB
BST
5V
Start-Up and Shutdown
If VIN2 is higher than its appropriate thresholds,
the buck converter starts up. The reference
block starts first, generating stable reference
voltages and currents, and then the internal
regulator is enabled. The regulator provides a
stable supply for the remaining circuitries.
Three events can shut down the buck converter:
EN2 low, VIN2 UVLO, and thermal shutdown.
During the shutdown procedure, the signaling
path is first blocked to avoid any fault triggering.
The COMP voltage and the internal supply rail
are then pulled down. The floating driver is not
subject to this shutdown command.
R6
U1
C7
L2
SW
VOUT2
C5
Figure 3: Internal Bootstrap Charging Circuit
MP3378E Rev. 1.02
www.MonolithicPower.com
5/26/2017
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© 2017 MPS. All Rights Reserved.
16
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
APPLICATION INFORMATION
WLED CONTROLLER SECTION:
Selecting the Switching Frequency
The switching frequency of the step-up converter
is recommended to be between 300kHz to
500kHz for most applications. An oscillator
resistor on OSC sets the internal oscillator
frequency for the step-up converter, according to
Equation (1):
minimum inductor value to ensure that the boost
converter works in continuous conduction mode
with high efficiency and good EMI performance.
Calculate the required inductance values using
Equation (3) and Equation (4):
η VOUT1 D (1 D)2
L1
2 fSW1 ILOAD1
D 1
67320
FSW1 KHz
Rosc KΩ
(1)
When ROSC=224kΩ, the switching frequency is
set to 300kHz.
Setting the LED Current
Each LED string current can be set through the
current setting resistor on ISET using Equation
(2):
ILED(mA)
795 1.23
RSET KΩ
RSENSE
When RSET=8.06kΩ, the LED current is set to
120mA. Do NOT leave ISET open.
Selecting the Inductor and Current Sensing
Resistor
A larger value inductor results in less ripple
current, lower peak inductor currents, and
reduced stress on the N-channel MOSFET.
However, the larger value inductor has a larger
physical size, a higher series resistance, and a
lower saturation current.
(4)
Where VIN1 and VOUT1 are the input and output
voltages, fSW1 is the switching frequency, ILOAD1 is
the total LED load current, and η is the efficiency.
Usually, the switching current is used for peakcurrent-mode control. To avoid hitting the current
limit, the voltage across the sensing resistor
(RSENSE) must be less than 80% of the worst-case
current-limit voltage (VSENSE). See Equation (5)
and Equation (6):
(2)
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
the high-frequency switching current from
passing through to the input. Ceramic capacitors
with X5R or X7R dielectrics are recommended
because of their low ESR and small temperature
coefficients. For most applications, use a 4.7μF
ceramic capacitor in parallel with a 220µF
electrolytic capacitor.
VIN1
VOUT1
(3)
IL1(PEAK)
0.8 VSENSE
IL1(PEAK)
(5)
VOUT1 ILOAD1 VIN1 (VOUT1 VIN1 )
ηVIN1
2 L1 FSW1 VOUT1 (6)
Where IL1(PEAK) is the peak value of the inductor
current. VSENSE is shown in Figure 4.
Figure 4: VSENSE vs. Duty Cycle
Choose an inductor that does not saturate under
the worst-case load conditions. Select the
MP3378E Rev. 1.02
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5/26/2017
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© 2017 MPS. All Rights Reserved.
17
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
Selecting the Power MOSFET
The critical parameters for selecting a MOSFET
are listed below:
1. Maximum drain-to-source voltage, VDS(MAX).
2. Maximum current, ID(MAX).
3. On-resistance, RDS(ON).
4. Gate source charge (QGS) and gate drain
charge (QGD).
5. Total gate charge (QG).
Ideally, the off-state voltage across the MOSFET
is equal to the output voltage. Considering the
voltage spike when it turns off, VDS(MAX) should be
greater than 1.5 times the output voltage.
The maximum current through the power
MOSFET occurs at the minimum input voltage
and the maximum output power. The maximum
RMS current through the MOSFET is given by
Equation (7) and Equation (8):
IRMS(MAX) IIN1(MAX) DMAX , where:
DMAX
(7)
VOUT1 VIN1(MIN)
VOUT1
(8)
The current rating of the MOSFET should be
greater than 1.5xIRMS.
The on resistance of the MOSFET determines
the conduction loss, which is given by Equation
(9):
Pcond IRMS
2
R DS (on) k
(9)
Where k is the temperature coefficient of the
MOSFET.
The switching loss is related to QGD and QGS1,
which determine the commutation time. QGS1 is
the charge between the threshold voltage and
the plateau voltage when a driver charges the
gate, which can be read in the VGS vs QG chart of
the MOSFET datasheet. QGD is the charge 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):
PSW
Q GS1 R G
VDS I IN1 f SW1
VDR VTH
Q GD R G
VDS I IN1 f SW1
VDR VPLT
(10)
Where VTH is the threshold voltage, VPLT is the
plateau voltage, RG is the gate resistance, and
VDS is the drain source voltage. Please note that
calculating the switching loss is the most difficult
part in the loss estimation. The formula above is
a simplified equation. For a more accurate
estimation, the equation becomes much more
complex.
The total gate charge (QG) is used to calculate
the gate drive loss. See Equation (11):
PDR QG VDR fSW1
(11)
Where VDR 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 must be low at
the switching frequency. Ceramic capacitors with
X7R dielectrics are recommended for their low
ESR characteristics. For most applications, a
4.7μF ceramic capacitor in parallel with a 22μF
electrolytic capacitor is sufficient.
Setting the Over-Voltage Protection
The open-string protection is achieved through
the detection of the voltage on OVP. In some
cases, an LED string failure results in a feedback
voltage of zero. The part then boosts the output
voltage higher and higher. If the output voltage
reaches the programmed OVP threshold, the
protection will be triggered.
To ensure the chip functions properly, select the
resistor values for the OVP resistor divider to
provide an appropriate set voltage. The
recommended OVP point is about 1.1 to 1.2
times higher than the output voltage for normal
operation. See Equation (12):
R
VOVP 1.23 (1 HIGH )
RLOW
(12)
Selecting the Dimming Control Mode
Two different dimming methods are provided:
1. Direct PWM Dimming
An external PWM dimming signal is employed to
achieve PWM dimming control. Apply a PWM
dimming signal (ranging from 100Hz to 20kHz) to
PWM. The minimum recommended amplitude of
the PWM signal is 1.5V, and the low level should
be less than 0.4V (see Table 1).
MP3378E Rev. 1.02
www.MonolithicPower.com
5/26/2017
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© 2017 MPS. All Rights Reserved.
18
MP3378E – 4-CHANNEL WLED CONTROLLER WITH BUCK CONVERTER
Table 1: The Range of PWM Dimming Duty
BUCK CONVERTER SECTION:
fPWM (Hz)
100
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