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MAX20078
General Description
The MAX20078 is a high-voltage, synchronous n-channel
MOSFET controller for high-current buck LED drivers. The
device uses a proprietary average current-mode-control
scheme to regulate the inductor current. This control
method does not need any control-loop compensation
while maintaining nearly constant switching frequency.
Inductor current sense is achieved by sensing the
current in the bottom synchronous n-channel MOSFET. It
does not require any current sense at high voltages. The
device operates over a wide 4.5V to 65V input range. The
device is designed for high-frequency operation and can
operate at switching frequencies as high as 1MHz. The
high- and low-side gate drivers have peak source and sink
current capability of 2A. The driver block also includes
a logic circuit that provides an adaptive nonoverlap time
to prevent shoot-through currents during transition. The
device includes both analog and PWM dimming. The
device includes a 5V VCC regulator capable of delivering
10mA to external circuitry. The device also includes a
current monitor that provides an analog voltage
proportional to the inductor current. The device has
a fault flag that indicates open and shorts across the
output. Protection features include inductor current-limit
protection, overvoltage protection, and thermal shutdown.
The MAX20078 is available in a space-saving (3mm x
3mm), 16-pin TQFN or a 16-pin TSSOP package and is
specified to operate over the -40°C to +125°C automotive
temperature range.
Ordering Information appears at end of data sheet.
Synchronous Buck,
High-Brightness LED Controller
Benefits and Features
● Automotive Ready: AEC-Q100 Qualified
● Wide Input Voltage Range: 4.5V to 65V
● Easy to Design
• No Compensation Components
• Programmable Switching Frequency
● Wide Dimming Ratio Allows High Contrast Ratio
• Analog Dimming
• PWM Dimming
● Suitable for Matrix Lighting
• Maintains Current Regulation While Shorting/
Opening Individual LEDs in the String
• Ultrafast-Response Control Loop Prevents
Overshoots and Undershoots
● Fault Detection and Protection
• Overvoltage Protection
• Open and Short Detection
• Thermal Shutdown
• Inductor Current Monitor
● Low-Power Shutdown Mode
Applications
●
●
●
●
Automotive Front Lights
Automotive Matrix Lights
Head-Up Displays
Constant-Current Regulators
Simplified Schematic
INPUT
CIN
CVCC
R1
D1
BST
VCC
IN
DH
Q1
CBST
MAX20078
TON
PWM
LED CURRENT CONTROL
FAULT FLAG
CURRENT MONITOR
COUT
DL
DIM
REFI
Q2
R2
LED1
R3
LEDn
CSP
RCS
FLT
CSN
IOUTV
PGND
19-8717; Rev 5; 4/19
L1
LX
C1
AGND
OUT
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Absolute Maximum Ratings
VCC Short-Circuit Duration.........................................Continuous
Continuous Power Dissipation (TA = +70°C) (Note 1)
16-Pin TQFN-EP
(derate 24.4 mW/°C above +70°C).........................1951.2mW
16-Pin TSSOP-EP
(derate 25.6 mW/°C above +70°C).........................2051.3mW
Operating Temperature Range.......................... -40°C to +125°C
Junction Temperature.......................................................+150°C
Storage Temperature Range............................. -65°C to +150°C
Lead Temperature (soldering, 10s).................................. +300°C
Soldering Temperature (reflow)........................................+260°C
IN, DIM, TON to AGND..........................................-0.3V to +70V
LX to AGND............................................................-1.0V to +70V
BST to AGND.........................................................-0.3V to +75V
BST, DH to LX..........................................................-0.3V to +6V
DH to AGND...........................................................-0.3V to +75V
DL to AGND........................................................... -0.3V to +VCC
VCC to AGND................. -0.3V to lower of (VIN + 0.3V) and +6V
CSP, CSN to AGND.............................................. -2.5V to +VCC
OUT, FLT, IOUTV to AGND......................................-0.3V to +6V
PGND to AGND.....................................................-0.3V to +0.3V
REFI......................................................................-0.3V to +2.5V
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these
or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect
device reliability.
Package Thermal Characteristics (Note 1)
16 TQFN-EP
Junction-to-Ambient Thermal Resistance (θJA)...........48°C/W
Junction-to-Case Thermal Resistance (θJC)..................7°C/W
16 TSSOP-EP
Junction-to-Ambient Thermal Resistance (θJA)...........39°C/W
Junction-to-Case Thermal Resistance (θJC)..................3°C/W
Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-layer
board. For detailed information on package thermal considerations, refer to www.maximintegrated.com/thermal-tutorial.
Electrical Characteristics
(VIN = VDIM = 14V, TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
INPUT SUPPLY
Input Voltage Range
VIN
Quiescent Current
IQ
Shutdown Current
ISHDN
IN connected to VCC
4.5
65
4.5
5.5
V
VDIM = 5V, VIN = 65V
2
4
mA
VDIM = 0V, VIN = 12V
8
15
VDIM = 0V, VIN = 65V
12
30
5
5.15
V
0.07
0.15
V
V
µA
VCC REGULATOR
5.5V < VIN < 65V; IVCC = 1mA
6V < VIN < 25V; IVCC = 10mA
4.85
Output Voltage
VCC
Dropout Voltage
VCC DROP
VIN = 4.5V, IVCC = 5mA
VCC UVLO Rising
VCC UVLOR
Rising
3.8
4.1
4.4
VCC UVLO Falling
VCC UVLOFALL
Falling
3.55
3.8
4.0
Short-Circuit Current Limit
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IVCC_SC
VCC shorted to AGND
80
V
mA
Maxim Integrated │ 2
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Electrical Characteristics (continued)
(VIN = VDIM = 14V, TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
DH and DL DRIVERS
DH Sourcing Resistance
RDH_SRC
DH = high, TA = -40°C to +125°C
2.5
5.0
Ω
DH Sinking Resistance
RDH_SINK
DH = low, TA = -40°C to +125°C
1.0
2.0
Ω
DL Sourcing Resistance
RDL_SRC
DL = low, TA = -40°C to +125°C
2.5
5.0
Ω
DL Sinking Resistance
RDL_SINK
DL = low, TA = -40°C to +125°C
1.5
3.0
Ω
DH-to-DL Dead Time
DH fall to DL rise
20
ns
DL-to-DH Dead Time
DL fall to DH rise
20
ns
ON-TIME CONTROL/OVERVOLTAGE PROTECTION/SHORT-FAULT INDICATOR
Minimum On-Time
tON_MIN
Programmed On-Time
Maximum On-Time
VOUT = 1V, R1 = 50kΩ, C1 = 1nF
tON_MAX
TON Pulldown Resistance
TON Threshold to DH Falling
Delay
tD-ON
OUT Overvoltage Threshold
VTH_OVP
OUT Overvoltage Hysteresis
Short-Fault Threshold
80
OUTV_SHF
110
ns
4.55
µs
tON = AGND, VOUT = 1V
24
µs
VIN = 65V, R1 > 20kΩ
15
30
65
OUT rising
2.9
3.0
Ω
ns
3.1
V
OUT falling
0.02
V
Output falling, VOUT is lower than
threshold
50
mV
CS = 0V
200
ns
65
ns
OFF-TIME CONTROL
Minimum Off-Time
CS Comparator Propagation
Delay
Linear Range of Pulse Doubler
0
Maximum Off-Time
5
42
µs
µs
ANALOG DIMMING INPUT
1.2
V
VCS < 5mV
0.165
0.18
0.195
V
REFICLMP
IREFI sink = 1µA
1.274
1.3
1.326
V
REFIIN
VREFI = 0 to 2V
0
20
200
nA
Current-Sense Amplifier Offset
0.18
0.2
0.22
V
Current-Sense Gain
4.9
5.0
5.05
V/V
REFI Input Voltage Range
REFI Zero-Current Threshold
REFI Clamp Voltage
REFI Input Bias Current
0.2
REFIRNG
REFIZC_VTH
CURRENT-SENSE AMPLIFIER
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Maxim Integrated │ 3
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Electrical Characteristics (continued)
(VIN = VDIM = 14V, TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
PWM DIMMING
DIM Rising Threshold
DIMVTHR
DIM rising
DIM Falling Threshold
DIMVTHF
DIM falling
DIM Rising-to-DL Rising Delay
tDIM_RIS
DIM rising
DIM Shutdown Detect Timer
tSHDW
DIM low duration to enter shutdown
mode
2.0
V
0.8
40
180
200
V
ns
220
ms
CURRENT MONITOR
Current Monitor Amplifier Gain
Offset Voltage
V(CSP - CSN) < 200mV
VTHDIML
5
V/V
0.2
V
FAULT FLAG
FLT Output Voltage
FLT Leakage Current
LED Open-Fault REFI Range
LED Open-Fault Threshold
FLTV
FLTLGK
ISINK is 1mA after fault
VFLT = 5.5V
LOFREFI_RNG
LOFTH
0.05
VIOUTV is lower than the threshold
when DIM is high
0.3
V
1
µA
300
325
350
mV
10
25
40
%
THERMAL SHUTDOWN
Thermal-Shutdown Threshold
TSHUTDOWN
Thermal-Shutdown Hysteresis
THYS
Temperature rising
165
°C
15
°C
Note 2: Limits are 100% tested at TA = +25°C and TA = +125°C. Limits over the operating temperature range and relevant supply
voltage range are guaranteed by design and characterization.
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Maxim Integrated │ 4
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Typical Operating Characteristics
Data taken on Typical Operating Circuit
VCC VOLTAGEvs. TEMPERATURE
5.050
NO LOAD SUPPLY CURRENT
vs. TEMPERATURE
toc01
5.040
5.010
5.000
4.990
4.980
12V SUPPLY
24V SUPPLY
36V SUPPLY
48V SUPPLY
4.970
4.960
-40
15
10
12V SUPPLY
24V SUPPLY
36V SUPPLY
48V SUPPLY
5
-10
20
50
80
AMBIENT TEMPERATURE (°C)
SWITCHING FREQUENCY (Hz)
SUPPLY CURRENT (mA)
VCC VOLTAGE (V)
5.020
0
110
-40
-10
20
50
80
AMBIENT TEMPERATURE (°C)
1.60E-07
1.00E-07
ON TIME (s)
OFF TIME (s)
1.10E-07
-40
-10
20
50
80
-10
20
50
80
AMBIENT TEMPERATURE (°C)
12V SUPPLY
24V SUPPLY
36V SUPPLY
48V SUPPLY
7.00E-08
110
110
toc05
6.00E-08
-40
UVLO vs. TEMPERATURE
-10
20
50
80
AMBIENT TEMPERATURE (°C)
110
EFFICIENCY vs. LEDCURRENT
toc06
4.4
toc07
100
4.3
90
4.2
80
4.1
EFFICIENCY (%)
UVLO VOLTAGE (V)
-40
9.00E-08
AMBIENT TEMPERATURE (°C)
4.0
3.9
70
60
50
3.8
RISING
3.7
3.6
12V SUPPLY
24V SUPPLY
36V SUPPLY
48V SUPPLY
8.00E-08
12V SUPPLY
24V SUPPLY
36V SUPPLY
48V SUPPLY
1.30E-07
1.20E-07
760000
1.20E-07
1.70E-07
1.40E-07
770000
MIN ON TIMEvs. TEMPERATURE
toc04
1.50E-07
780000
740000
110
toc03
790000
750000
MIN OFF TIME vs. TEMPERATURE
1.80E-07
SWITCHING FREQUENCY
vs. TEMPERATURE
800000
20
5.030
4.950
toc02
-40
-10
20
50
80
AMBIENT TEMPERATURE (°C)
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FALLING
110
2 LED; 12V IN
40
30
2 LED; 24V IN
0
0.5
1
1.5
2
LED CURRENT (A)
Maxim Integrated │ 5
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Typical Operating Characteristics (continued)
Data taken on Typical Operating Circuit
LED CURRENT vs. REFI VOLTAGE
LED CURRENT vs. REFI VOLTAGE
toc10
2.00
2.00
1.50
1.50
1.50
1.00
1.00
0.50
2 LED; 12V IN
0.50
LED CURRENT (A)
2.00
LED CURRENT (A)
LED CURRENT (A)
LED CURRENT vs. REFI VOLTAGE
toc09
toc08
0.00
0
0.2
0.4
0.6
0.8
1
REFI VOLTAGE (V)
0.50
+125°C, 12V IN
2 LED; 24V IN
1.2
0.00
1.4
0
0.2
0.4
0.6
0.8
1
1.2
0.00
1.4
0.4
0.6
0.8
1
1.2
1.4
LED CURRENT vs. SUPPLY VOLTAGE
toc11
toc12
1.04
1.03
1.02
LED CURRENT (A)
80
EFFICIENCY (%)
0.2
REFI VOLTAGE (V)
90
70
60
ILED = 1A
50
2 LED
40
15
25
1.00
0.99
0.98
0.97
0.96
1 LED
5
1.01
2 LED
0.95
0.94
35
SUPPLY VOLTAGE (V)
1 LED
10
0
20
40
30
SUPPLY VOLTAGE (V)
LED CURRENT vs. REFI VOLTAGE
EFFICIENCY vs. LED CURRENT
toc13
100
toc14
2.40
90
2.00
80
LED CURRENT (A)
EFFICIENCY (%)
0
REFI VOLTAGE (V)
100
70
60
50
1.60
1.20
0.80
0.40
12 LED; 60V
SUPPLY, +25°C
40
30
+90°C, 60V IN
-40°C, 60V IN
-40°C, 12V IN
EFFICIENCY vs. SUPPLY VOLTAGE
30
1.00
0.00
0
0.5
1
LED CURRENT (A)
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1.5
2
12 LED; 60V SUPPLY,
+25°C
0
0.2
0.4
0.6
0.8
1
1.2
1.4
REFI VOLTAGE (V)
Maxim Integrated │ 6
MAX20078
Synchronous Buck,
High-Brightness LED Controller
CSN
13
PGND
14
VCC
15
DL
16
FLT
IOUTV
AGND
TOP VIEW
CSP
Pin Configurations
12
11
10
9
MAX20078
+
7
OUT
6
TON
5
DIM
4
LX
BST
REFI
IN
3
2
DH
1
8
AGND
IOUTV
FLT
CSP
CSN
VCC
DL
TOP VIEW
PGND
TQFN-EP/TQFN-EP (SW)
3mm x 3mm
16 15 14 13 12 11 10
9
MAX20078
EP
DH
LX
IN
5
6
7
8
REFI
4
OUT
3
DIM
2
TON
1
BST
+
TSSOP-EP
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Maxim Integrated │ 7
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Pin Description
PIN
NAME
FUNCTION
TQFN
TSSOP
1
1
BST
High-Side Power Supply for High-Side Gate Drive. Connect a 0.1μF ceramic capacitor
from BST to LX.
2
2
DH
Connect to Gate of High-Side n-Channel MOSFET of Buck LED Driver. Use series resistor
to limit current slew rate and mitigate EMI noise if required.
3
3
LX
Switching Node of Buck LED Driver. Connect to one end of output inductor.
4
4
IN
Bias Supply Input. Connect a 4.5V to 65V supply to IN. Bypass to ground with a 2.2µF
ceramic capacitor.
5
5
DIM
Connect DIM to an External PWM Signal for PWM Dimming
6
6
TON
Connect a Resistor to the Input Supply and Capacitor to AGND to Set Switching
Frequency
7
7
OUT
Connect a Resistor-Divider from OUT to the Output Voltage. This pin has the scaled-down
measurement of the output voltage.
8
8
REFI
Analog Dimming-Control Input. Connect an analog voltage from 0 to 1.2V for analog
dimming of LED current.
9
9
AGND
Analog Ground Connection
10
10
IOUTV
Analog Voltage Indication of Inductor Current. Bypass to ground with a 1µF ceramic
capacitor.
11
11
FLT
Open-Drain Fault Output. See the Fault Indicator (FLT) section for information.
12
12
CSP
Connect to source of external MOSFET that is driven by DL. Connect a resistor from this
pin to CSN to sense the current in the MOSFET.
13
13
CSN
Connect Directly to the Other End of the Current-Sense Resistor. This end is also
connected to the power-ground plane.
14
14
PGND
15
15
VCC
16
16
DL
Connect to Gate of Low-Side n-Channel MOSFET of Buck LED Driver. Use series resistor
to limit current slew rate and mitigate EMI noise if required.
—
—
EP
Exposed Pad. Connect EP to a large-area contiguous-copper ground plane for effective
power dissipation. Do not use as the main IC ground connection. EP must be connected
to AGND.
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Power-Ground Connection
5V Regulator Output. Connect a 2.2μF ceramic capacitor to AGND from VCC for stable
operation.
Maxim Integrated │ 8
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Block Diagram
IN
VCCOK
BG
VCC
POK
LDO
INUVLO
VCCOK
BST
MAX20078
GND
DEAD TIME
AND LEVEL
SHIFT UP
REFI
DH
LX
CSN
0.2V
CSA
PWM
TTL
Q
R
Q
VCC
DL
DEAD TIME
DL ENABLE
PULSE
DOUBLER
CSP
S
LOGIC
200ms LOW-STATE
TIME COUNTER
SHUTDOWN MODE
CSA
x1
IOUTV
TON_RESET
TON
N
TON_RESET
FLT
OUT
N
3.0V
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Maxim Integrated │ 9
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Detailed Description
includes a current monitor that provides an analog
voltage proportional to the inductor current. The device
has a fault flag that indicates open and shorts across
the output. Protection features include inductor currentlimit protection, over-voltage protection, and thermal
shutdown. The MAX20078 is available in a space-saving
(3mm x 3mm), 16-pin TQFN or a 16-pin TSSOP package
and is specified to operate over the -40°C to +125°C
automotive temperature range.
The MAX20078 is a high-voltage, synchronous n-channel
MOSFET controller for high-current buck LED drivers. The
device uses a proprietary average current-mode-control
scheme to regulate the inductor current. This control
method does not need any control-loop compensation
while maintaining nearly constant switching frequency.
Inductor current sense is achieved by sensing the current
in the bottom synchronous n-channel MOSFET. It does
not require any current sense at high voltages. The device
operates over a wide 4.5V to 65V input range. The device
is designed for high-frequency operation and can operate
at switching frequencies as high as 1MHz. The highand low-side gate drivers have peak source and sink
current capability of 2A. The driver block also includes
a logic circuit that provides an adaptive nonoverlap
time to prevent shoot-through currents during transition.
The device includes both analog and PWM dimming.
The device includes a 5V VCC regulator capable of
delivering 10mA to external circuitry. The device also
New Average Current-Mode-Controlled
Architecture
The device uses a new average current-modecontrol scheme to regulate the current in the output
inductor of the buck LED driver. The inductor current is
not directly sensed. The device senses the current in the
bottom synchronous switch. See Figure 1 for the location
of the current-sense resistor (RCS). In a buck converter,
operating in continuous-conduction mode, when the top
switch is turned off the current in the inductor also flows in
INPUT
CIN
CVCC
R1
D1
BST
VCC
DH
IN
Q1
CBST
MAX20078
TON
C1
PWM
LED CURRENT CONTROL
FAULT FLAG
CURRENT MONITOR
L1
LX
COUT
DL
DIM
Q2
R2
LED1
R3
LEDn
CSP
REFI
RCS
FLT
CSN
IOUTV
PGND
AGND
OUT
Figure 1. Application Circuit Using the MAX20078
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Maxim Integrated │ 10
MAX20078
Synchronous Buck,
High-Brightness LED Controller
IP
Iav
IV
IV
DL ON
DH ON
t
tpw
t
Figure 2. Inductor Current Waveform in One Full Switching Cycle
fSW
ILED
AVERAGE LED
CURRENT
t
VDIM
t1
t2
t
Figure 3. Operation During PWM Dimming When PWM Signal is Applied at PWM Pin
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Maxim Integrated │ 11
MAX20078
Synchronous Buck,
High-Brightness LED Controller
the bottom switch or diode. This peak current is Ip. When
the bottom switch is turned off and the top switch is then
turned on the current in the switch is the same as the
current in the inductor, and it is Iv. The average current in
the inductor is given by Iav = 0.5(Ip + Iv). Iav is the same
as the output current, Io. If the bottom switch current is
sensed at exactly half of the bottom switch period, the
current in the switch would be Iav. A pulse doubler is used
to determine the on-time of the bottom switch:
tOFF = 2 x tPW
where tPW is the high-state pulse width of the internal
comparator in the device.
The on-time is determined based on the external
resistor (R1) connected between TON and the input
voltage, in combination with a capacitor (C1) between
R1 and AGND/PGND pins. The input voltage and the R1
resistor set the current sourced into the capacitor (C1),
which governs the ramp speed. The ramp threshold
is proportional to scaled-down feedback of the output
voltage at the OUT pin. The proportionality of VOUT is set
by an external resistor-divider (R2, R3) from VOUT.
tONVIN/R1 = C1 (VOUT x R3/(R3 + R2))
tON = KVOUT/VIN
VIN
where K = C1R3R1/(R3 + R2)
In the case of a buck converter tONVIN is also given by:
tON = VOUT/VINfSW
where fSW is the switching frequency.
Based on that, the switching frequency in case of the new
average current-mode-controlled architecture is given by:
fSW = 1/K or fSW = (R3 + R2)/(C1R3R1)
In the actual application, there are slight variations in
switching frequency due to the voltage drops in the
switches and the inductor, the propagation delay from the
TON input to the LX switching node, and the nonlinear
current charging the TON capacitor. These effects have
been ignored in the calculations for switching frequency.
Analog Dimming
The device has an analog dimming-control input (REFI).
The voltage at REFI sets the LED current level when
VREFI ≤ 1.2V. For VREFI > 1.3V, REFI is clamped to 1.3V
(typ). The maximum withstand voltage of this input is 2V.
The LED current is guaranteed to be at zero when the
REFI voltage is at or below 0.18V. The LED current can
be linearly adjusted from zero to full scale for the REFI
voltage in the range of 0.2V to 1.2V.
CIN
D1
BST
IN
R1
CVCC
C1
VCC
DH
TON
LX
Q1
CBST
L1
MAX20078
PWM
DIM
REFI
REFI
FAULT
CURRENT
MONITOR
COUT
DL
R2
LED1
Q2
ON/OFF
CONTROL OF
LED1
CSP
R3
FLT
RCS
CSN
IOUTV
AGND
PGND
LEDn
ON/OFF
CONTROL OF
LEDn
OUT
Figure 4. Dimming of Individual LEDs in the Entire String
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Maxim Integrated │ 12
MAX20078
PWM Dimming
The DIM pin functions as the PWM dimming input of the
LEDs. The DIM pin can be driven with a PWM signal
that controls the dimming operation of the device. When
the DIM signal is high, the switching of the synchronous
MOSFETs in the buck LED driver is enabled, but when
DIM goes low, both the high- and low-side MOSFETs are
turned off. The LED current waveform is shown in Figure
3. The device goes into shutdown mode if the DIM input is
below the ON threshold minus the hysteresis for 210ms.
In shutdown mode, the input current is less than 5μA (typ).
Dimming by Shorting Individual LEDs
in the String
Extremely fast dimming of individual LEDs in the string
can be done by applying a shorting FET across each
LED, as shown in Figure 4. This application is used in
matrix lighting where individual LEDs in the string are
controlled by a shorting MOSFET across each LED.
Each LED in the string can be turned on and off without
any impact on the brightness of the other LEDs in the
string by this method. If required, the entire string can
be shorted at the same time while still maintaining
current regulation in the inductor with minimal overshoot
or undershoot. The rise and fall times of the currents in
each LED are extremely fast. With this method, only the
speed of the parallel-shunt MOSFET limits the dimming
frequency and dimming duty cycle. Minimize the output
capacitor (COUT) to minimize current spikes due to the
discharge of this capacitor into the LEDs when the
shorting FETs are turned on. In some applications, this
capacitor can be completely eliminated.
5V Regulator
A regulated 5V output is provided for driving the gates of
the external MOSFETs and other external circuitry with a
current up to 10mA. Bypass VCC to AGND/PGND with a
minimum of 2.2μF ceramic capacitor, positioned as close
as possible to the device. In certain applications when an
external regulated 5V supply is available, the IN and VCC
pins can be connected together and the regulated 5V can
be applied directly to VCC saving the power dissipation in
the internal regulator of the device.
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Synchronous Buck,
High-Brightness LED Controller
Overvoltage Protection
The device has programmable overvoltage protection by
using the resistor-divider at the OUT pin. The overvoltage
setpoint is defined by:
VOVP_ON =
3.0 (R2 + R3)
R3
If the output voltage reaches VOVP_ON, the DH and DL
pins are pulled low to prevent damage to the LEDs or the
rest of the circuit. The OVP circuit has a fixed hysteresis
of 20mV before the driver attempts to switch again.
High-Side Gate-Drive Supply
The high-side MOSFET is turned on by closing an
internal switch between BST and DH and transferring the
bootstrap capacitor’s (at BST) charge to the gate of the
high-side MOSFET. This charge refreshes when the highside MOSFET turns off and the LX voltage drops down
to ground potential, taking the negative terminal of the
capacitor to the same potential. At this time, the bootstrap
diode recharges the positive terminal of the bootstrap
capacitor. The selected n-channel high-side MOSFET
determines the appropriate boost capacitance values
(CBST in the Typical Operating Circuit), according to the
following equation:
CBST = QG/∆VBST
where QG is the total gate charge of the high-side
MOSFET and ∆VBST is the voltage variation allowed
on the high-side MOSFET driver after turn-on. Choose
∆VBST such that the available gate-drive voltage is not
significantly degraded (e.g., ∆VBST = 100mV to 300mV)
when determining CBST. Use a Schottky diode when
efficiency is most important, as this maximizes the gatedrive voltage. If the quiescent current at high temperature
is important, it may be necessary to use a low-leakage
switching diode. The boost capacitor should be a lowESR ceramic capacitor. A minimum value of 100nF works
in most cases. A minimum value of 220nF is recommended when using a Schottky diode.
Maxim Integrated │ 13
MAX20078
Current Monitor
The device includes a current monitor on the IOUTV pin.
The IOUTV voltage is an analog voltage indication of the
inductor current when DIM is high. The current-sense
signal on the bottom MOSFET across RCS is inverted
and amplified by a factor of 5 by an inverting amplifier
inside the device. An added offset voltage of 0.2V is also
added to this voltage. This amplified signal goes through
a sample and hold switch. The sample and hold switch is
controlled by the DL signal. The sample and hold switch
is turned on only when DL is high and is off when DL is
low. This provides a signal on the output of the sample
and hold that is a true representation of the inductor
current when DIM is high. The sample and hold signal
passes through an RC filter and then the buffered output
is available on the IOUTV pin. The voltage on the IOUTV
pin is given by:
VIOUTV = ILED x RCS x 5 + 0.2V
where ILED is the LED current, which is the same
as the average inductor current when DIM is high.
VIOUTV indicates the same voltage when DIM goes low
that was indicated by VIOUTV when DIM was high prior to
it going low.
Thermal Shutdown
Internal thermal-shutdown circuitry is provided to protect
the device in the event the maximum junction temperature is exceeded. The threshold for thermal shutdown is
165°C with a 15°C hysteresis (both values typical). During
thermal shutdown, the low- and high-side gate drivers are
disabled.
Fault Indicator (FLT)
The device features an active-low, open-drain fault indicator (FLT). The FLT pin goes low under the following
conditions.
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Synchronous Buck,
High-Brightness LED Controller
Short-Circuit Condition Across the LED String
When the LED string is shorted and the OUT pin voltage
goes below the short threshold of 50mV for more than
1.2ms, the FLT pin goes low. During PWM dimming, the
short detection is reported on the FLT pin only when DIM
is high. Once the FLT is asserted when the DIM is high, it
stays asserted until the fault condition is removed.
Open LED Detection
When the LED string is opened and the IOUTV pin
voltage drops to lower than 75% of the targeted voltage
for more than 1.2ms, the FLT pin goes low. During PWM
dimming, the open detection is reported on the FLT pin
only when DIM is high. Once the FLT is asserted when
the DIM is high, it stays asserted until the fault condition
is removed. The LED open detection works only when the
REFI pin is greater than 325mV.
Overvoltage Detection
When the voltage on the OUT pin exceeds the overvoltage threshold of 3V for more than 1.2ms, the FLT
pin goes low. During PWM dimming, the over-voltage
detection is reported on the FLT pin only when DIM is
high. Once the FLT is asserted when the DIM is high, it
stays asserted till the fault condition is removed.
Thermal Shutdown
When the junction temperature of the IC exceeds the
thermal shutdown threshold of 165°C, the FLT pin goes low.
Shutdown Mode
When DIM pin is pulled low for more than 200ms, the
linear regulator generating the 5V on the VCC pin is
turned off for low power consumption. The FLT pin is also
pulled low in this condition.
Maxim Integrated │ 14
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Typical Operating Circuit
INPUT
CIN
2x2.2µF
D1
B180
CVCC
2.2µF
R1
47.5kΩ
R4
4.7Ω
BUK9Y107-80E
BST
VCC
DH
IN
TON
C1
470pF
PWM
LX
MAX20078
DIM
DL
Q1
CBST
0.22µF
L1
47µH
Q2
COUT
1µF
R2
453kΩ
LED1
R3
24.9kΩ
LEDn
BUK9Y107-80E
LED CURRENT CONTROL
FAULT FLAG
CURRENT MONITOR
CSP
REFI
0.1Ω
RCS
FLT
CSN
IOUTV
PGND
www.maximintegrated.com
AGND
OUT
Maxim Integrated │ 15
MAX20078
Applications Information
Switching Frequency
Switching frequency is selected based on the tradeoffs between efficiency, solution size/cost, and the
range of output voltage that can be regulated. Many
applications place limits on switching frequency due
to EMI sensitivity. The on-time of the MAX20078 can
be programmed for switching frequencies ranging
from 100kHz up to 1MHz. This on-time varies in
proportion to both input voltage and output voltage, as
described in the New Average Current-Mode-Controlled
Architecture section. However, in practice, the switching
frequency shifts in response to large swings in input or
output voltage. The maximum switching frequency is
limited only by the minimum on-time and minimum offtime requirements. The switching frequency (fSW) is given
by:
fSW = (R3 + R2)/(C1R3R1)
Programming the LED Current
The LED current can be programmed using the voltage
on REFI when VREFI ≤ 1.2V (analog dimming). The
current is given by:
ILED = (VREFI - 0.2)/(5 x RCS)
Inductor Selection
The peak inductor current, selected switching frequency,
and the allowable inductor current ripple determine
the value and size of the output inductor. Selecting
a higher switching frequency reduces the inductance
requirements, but at the cost of efficiency. The charge/
discharge cycle of the gate capacitance of the external
switching MOSFET’s gate and drain capacitance create
switching losses, which worsen at higher input voltages
since the switching losses are proportional to the square
of the input voltage. Choose inductors from the standard
high-current, surface-mount inductor series available from
various manufacturers. High inductor ripple current
causes large peak-to peak flux excursion, increasing the
core losses at higher frequencies.
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Synchronous Buck,
High-Brightness LED Controller
The peak-to-peak current-ripple values typically range
from ±10% to ±40% of DC current (ILED). Based on the
LED current-ripple specification and desired switching
frequency, the inductor value can be calculated as follows:
L = (VIN - VOUT) tON/∆ILED
where ∆ILED is the peak-to-peak inductor ripple.
It is important to ensure that the rated inductor saturation
current is greater than the worst-case operating current
(ILED+∆ILED/2) under the wide operating temperature
range.
Input Capacitor
The discontinuous input-current waveform of the buck
converter causes large ripple currents in the input
capacitor. The switching frequency, peak inductor current,
and the allowable peak-to-peak voltage ripple reflected
back to the source dictate the capacitance requirement.
The input ripple is comprised of ∆VQ (caused by the
capacitor discharge) and ∆VESR (caused by the ESR of
the capacitor). Use low-ESR ceramic capacitors with high
ripple-current capability at the input. A good starting point
for selection of CIN is to use an input-voltage ripple of 2%
to 10% of VIN. CIN_MIN can be selected as follows:
CIN_MIN = 2(ILED x tON)/∆VIN
where tON is the on-time pulse width per switching cycle.
When selecting a ceramic capacitor, special attention
must be paid to the operating conditions of the application.
Ceramic capacitors can lose one-half or more of their
capacitance at their rated DC-voltage bias and also lose
capacitance with extremes in temperature. Make sure to
check any recommended deratings and also verify if there
is any significant change in capacitance at the operating
input voltage and the operating temperature.
Switching MOSFET Selection
The device requires two external n-channel MOSFETs for
the switching regulator. The MOSFETs should have a voltage rating at least 20% higher than the maximum input
voltage to ensure safe operation during the ringing of the
Maxim Integrated │ 16
MAX20078
Synchronous Buck,
High-Brightness LED Controller
switch node. In practice, all switching converters have
some ringing at the switch node due to the diode parasitic capacitance and the lead inductance. The MOSFETs
should also have a current rating at least 50% higher
than the average switch current. The total losses of the
power MOSFETs in both high- and low-side MOSFETs
should be estimated once the MOSFETs are chosen.
Both n-channel MOSFETs must be logic-level types with
guaranteed on-resistance specifications at VGS = 4.5V.
The conduction losses at minimum input voltage should
not exceed MOSFET package thermal limits or violate the
overall thermal budget. Also, ensure that the conduction
losses plus switching losses at the maximum input voltage do not exceed package ratings or violate the overall
thermal budget. In particular, check that the dV/dt caused
by DH turning on does not pull up the DL gate through
its drain-to-gate capacitance. This is the most frequent
cause of cross-conduction problems.
Gate-charge losses are dissipated by the driver and do
not heat the MOSFET. Therefore, the power dissipation
in the device due to drive losses must be checked. Both
MOSFETs must be selected so that their total gate charge
is low enough; such that the IC can power both drivers
without overheating the device. The total power dissipated in the internal gate drivers of the device is given by:
PDRIVE = VCC x (QGTOTH + QGTOTL) x fSW
PCB Layout
For proper operation and minimum EMI, PCB layout
should follow the guidelines below:
1)
Large switched currents flow in the IN and AGND/
PGND pins and the input capacitor (CIN) of Figure 3.
The loop formed by the input capacitor should be as
small as possible by placing this capacitor as close as
possible to the IN and AGND/PGND pins. The input
capacitor, device, output inductor, and output capacitor
should be placed on the same side of the PCB, with the
connections made on the same layer.
2)
Place an unbroken ground plane on the layer closest
to the surface layer with the inductor, device, and the
input and output capacitors.
3)
The surface area of the LX and BST nodes should be
as small as possible to minimize emissions.
4)
The exposed pad on the bottom of the package must
be soldered to ground so that the pad is connected
to ground electrically and also acts as a heatsink
thermally. To keep thermal resistance low, extend the
ground plane as much as possible, and add thermal
vias under and near the device to additional ground
planes within the circuit board.
5)
In a synchronous rectifier, the high-speed gate-drive
signals can generate significant conducted and
radiated EMI. This noise can couple with highimpedance nodes of the IC and result in undesirable
operation. A small amount (4―10) of resistors (RDH
and RDL), in series with the gate-drive signals are
recommended to slow the slew rate of the LX node
and reduce the noise signature. They also improve
the robustness of the circuit by reducing the noise
coupling into sensitive nodes.
6)
The parasitic capacitance between switching node
and ground node should be minimized to reduce
common-mode noise. Other common layout
techniques, such as star ground and noise
suppression using local bypass capacitors, should
be followed to maximize noise rejection and minimize
EMI within the circuit.
7)
Place a capacitor (CBST) as close as possible to the
BST and LX pins.
where QGTOTL is the low-side MOSFET total gate charge
and QGTOTH is the high-side MOSFET total gate charge.
The power dissipated in the 5V regulator in the device due
to the gate drivers is given by:
PLG = (VIN - VCC) x (QGTOTH + QGTOTL) x fSW
Output Capacitor Selection
The function of the output capacitor is to reduce the
output ripple to acceptable levels. The ESR, ESL, and
the bulk capacitance of the output capacitor contribute
to the output ripple. In most applications, using low-ESR
ceramic capacitors can dramatically reduce the output
ESR and ESL effects. To reduce the ESL effects,
connect multiple ceramic capacitors in parallel to achieve
the required bulk capacitance.
The output capacitance (COUT) is calculated using the
following equation:
C OUT =
((VIN_MAX − VLED ) × VLED )
( ∆VR × 8 × L × VIN_MAX × fsw 2 )
where ∆VR is the maximum allowable voltage ripple.
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Maxim Integrated │ 17
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Typical Application Circuits
INPUT
CIN
CVCC
R1
D1
BST
VCC
DH
IN
Q1
CBST
MAX20078
TON
C1
PWM
LED CURRENT CONTROL
FAULT FLAG
CURRENT MONITOR
L1
LX
COUT
DL
DIM
Q2
R2
LED1
R3
LEDn
CSP
REFI
RCS
FLT
CSN
IOUTV
PGND
AGND
OUT
Figure 5. Typical Application Circuit for High-Beam, Low-Beam, Daytime-Running Lights, and Turn Indicators
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Maxim Integrated │ 18
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Typical Application Circuits (continued)
INPUT
CIN
CVCC
R1
D1
BST
VCC
DH
IN
Q1
CBST
MAX20078
TON
C1
ENABLE
LED CURRENT CONTROL
FAULT FLAG
CURRENT MONITOR
L1
LX
COUT
DL
DIM
Q2
R2
LED1
ON/OFF
CONTROL
OF LED1
LEDn
ON/OFF
CONTROL
OF LEDn
CSP
REFI
RCS
FLT
CSN
IOUTV
PGND
AGND
R3
OUT
Figure 6. Typical Application Circuit for Automotive Matrix Lighting
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Maxim Integrated │ 19
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Typical Application Circuits (continued)
INPUT
CIN
CVCC
D1
TO VCC PIN
R1
BST
VCC
DH
IN
Q1
REXT
CBST
MAX20078
TON
LX
C1
ENABLE
LED CURRENT CONTROL
FAULT FLAG
CURRENT MONITOR
TO DIM PIN
L1
CEXT
COUT
DL
DIM
Q2
R2
LED1
R3
LEDn
EXTERNAL
PWM
DIMMING
CSP
REFI
RCS
FLT
CSN
IOUTV
PGND
AGND
OUT
Figure 7. Typical Application Circuit For Head-Up Displays
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Maxim Integrated │ 20
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Ordering Information
Chip Information
TEMP
RANGE
PIN-PACKAGE
MAX20078ATE+
-40ºC to
+125ºC
16 TQFN-EP*
MAX20078ATE/V+
-40ºC to
+125ºC
16 TQFN-EP*
PART
MAX20078ATEY+
MAX20078ATE/VY+
MAX20078AUE+
MAX20078AUE/V+
-40ºC to
+125ºC
-40ºC to
+125ºC
16 TQFN-EP* (SW)
16 TQFN-EP* (SW)
-40ºC to
+125ºC
16 TSSOP-EP*
-40ºC to
+125ºC
16 TSSOP-EP*
PROCESS: CMOS
Package Information
For the latest package outline information and land patterns
(footprints), go to www.maximintegrated.com/packages. Note
that a “+”, “#”, or “-” in the package code indicates RoHS status
only. Package drawings may show a different suffix character, but
the drawing pertains to the package regardless of RoHS status.
PACKAGE
TYPE
PACKAGE
CODE
OUTLINE
NO.
LAND
PATTERN NO.
TQFN-EP
T1633+4C
21-0136
90-0031
TQFN-EP (SW)
T1633Y+4C
21-100108
90-100046
TSSOP-EP
U16E+4C
21-0108
90-0446
+Denotes a lead(Pb)-free/RoHS-compliant package.
/V denotes an automotive qualified part.
(SW) = Side wettable.
*EP = Exposed pad.
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Maxim Integrated │ 21
MAX20078
Synchronous Buck,
High-Brightness LED Controller
Revision History
REVISION
NUMBER
REVISION
DATE
PAGES
CHANGED
0
4/17
Initial release
1
6/17
Changed data sheet title from “Synchronous Buck Controller for High-Power HB
LED Drivers” to “Synchronous Buck, High-Brightness LED Controller”
2
9/17
Updated Benefits and Features, REFI in Absolute Maximum Ratings, Package
Thermal Characteristics, Shutdown Current and DIM Rising-to-DL Rising Delay
in Electrical Characteristics, TOC11 in Typical Operating Characteristics, total list
of steps in PCB Layout section; added one new variant (MAX20078ATE+) and
deleted three (MAX20078ATEV+T, MAX20078ATE/VY+T, MAX20078AUE/V+T**)
in Ordering Information; and updated POD and LPN in Package Information
3
1/18
Removed future product status from MAX20078AUE/V+ in Ordering Information
20
4
7/18
Added MAX20078ATEY+ and MAX20078ATUE+ to Ordering Information
20
5
4/19
Updated General Description and New Average Current-Mode-Controlled
Architecture and Fault Indicator (FLT) sections
1, 12, 14
DESCRIPTION
—
1–23
1, 2, 4, 6, 16, 20
For pricing, delivery, and ordering information, please visit Maxim Integrated’s online storefront at https://www.maximintegrated.com/en/storefront/storefront.html.
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses
are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits)
shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
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