MAX16821 [MAXIM]
High-Power Synchronous HBLED Drivers with Rapid Current Pulsing;
| 型号： | MAX16821 |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | High-Power Synchronous HBLED Drivers with Rapid Current Pulsing |
| 文件： | 总24页 (文件大小：1078K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Not Recommended for New Designs.
Refer to MAX20078 for New Designs.
EVALUATION KIT AVAILABLE
Click here for production status of specific part numbers.
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
General Description
Features
● Up to 30A Output Current
The MAX16821A, MAX16821B, and MAX16821C pulse-
width-modulation (PWM) LED driver controllers provide
high output-current capability in a compact package with a
minimum number of external components. The
MAX16821A–MAX16821C are suitable for use in synchro-
nous and nonsynchronous step-down (buck), boost, buck-
boost, SEPIC, and Cuk LED drivers. A logic input (MODE)
allows the devices to switch between synchronous buck
and boost modes of operation. These devices are the first
high-power drivers designed specifically to accommodate
common-anode HB LEDs.
● True-Differential Remote Output Sensing
● Average Current-Mode Control
● 4.75V to 5.5V or 7V to 28V Input Voltage Range
● 0.1V/0.03V LED Current-Sense Options Maximize
Efficiency (MAX16821B/MAX16821C)
● Thermal Shutdown
● Nonlatching Output-Overvoltage Protection
● Low-Side Buck Mode with or without Synchronous
Rectification
● High-Side Buck and Low-Side Boost Mode with or
without Synchronous Rectification
The ICs offer average current-mode control that enable
the use of MOSFETs with optimal charge and on-
resistance figure of merit, thus minimizing the need for
external heatsinking even when delivering up to 30A of
LED current.
● 125kHz to 1.5MHz Programmable/Synchronizable
Switching Frequency
● Integrated 4A Gate Drivers
● Clock Output for 180° Out-of-Phase Operation for
Second Driver
The differential sensing scheme provides accurate control
of the LED current. The ICs operate from a 4.75V to 5.5V
● -40°C to +125°C Operating Temperature Range
supply range with the internal regulator disabled (V
connected to IN). These devices operate from a 7V to 28V
input supply voltage with the internal regulator enabled.
Ordering Information
CC
PART
TEMP RANGE
-40°C to +125°C
-40°C to +125°C
-40°C to +125°C
PIN-PACKAGE
28 TQFN-EP*
28 TQFN-EP*
28 TQFN-EP*
MAX16821AATI+
MAX16821BATI+
MAX16821CATI+
The MAX16821A–MAX16821C feature a clock output
with 180° phase delay to control a second out-of-phase
LED driver to reduce input and output filter capacitor size
and to minimize ripple currents. The wide switching fre-
quency range (125kHz to 1.5MHz) allows the use of small
inductors and capacitors.
+Denotes lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
Simplified Diagram
Additional features include programmable overvoltage
protection and an output enable function.
7V TO 28V
Applications
● Front Projectors/Rear-Projection TVs
● Portable and Pocket Projectors
● LCD TVs and Display Backlight
C1
IN
Q1
EN
DH
V
LED
L1
I.C.
MAX16821
Q2
DL
C2
Q3
OVI
CSP
R1
PGND
CLP
Typical Operating Circuit and Selector Guide appears at
end of data sheet.
HIGH-FREQUENCY
PULSE TRAIN
19-0881; Rev 4; 4/18
MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Absolute Maximum Ratings
IN to SGND ...........................................................-0.3V to +30V
BST to SGND........................................................-0.3V to +35V
BST to LX................................................................-0.3V to +6V
All Other Pins to SGND............................ -0.3V to (V
+ 0.3V)
CC
Continuous Power Dissipation (T = +70°C)
A
28-Pin TQFN 5mm x 5mm (derate 34.5mW/°C
DH to LX.......................................-0.3V to (V
- V ) + 0.3V
above +70°C) ...........................................................2758mW
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
BST
LX
DL to PGND .............................................-0.3V to (V
+ 0.3V)
to SGND..........................................................-0.3V to +6V
DD
V
V
CC
, V
to PGND.................................................-0.3V to +6V
CC DD
SGND to PGND....................................................-0.3V to +0.3V
Current .....................................................................300mA
V
CC
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.
Electrical Characteristics
(V
= 5V, V
= V , T = T = -40°C to +125°C, unless otherwise noted. Typical values are at T = +25°C.) (Note 1)
CC
DD
CC
A
J
A
PARAMETER
Input-Voltage Range
Quiescent Supply Current
SYMBOL
CONDITIONS
Internal LDO on
Internal LDO off (V
MIN
7
TYP
MAX
28
UNITS
V
IN
V
connected to V
)
IN
4.75
5.50
5.5
CC
I
V
= V
or SGND, no switching
2.7
mA
Q
EN
CC
LED CURRENT REGULATOR
V
= V
= 4.75V to 5.5V, f
= 500kHz
IN
CC
SW
0.594
0.594
0.098
0.098
0.028
0.028
0.600
0.600
0.100
0.100
0.030
0.030
1024
0.606
0.606
0.102
0.102
0.032
0.032
(MAX16821A)
V
= 7V to 28V, f
= 500kHz
SW
IN
(MAX16821A)
V
= V
= 4.75V to 5.5V,
IN
CC
Differential Set Value
f
= 500kHz (MAX16821B)
SW
(V
+ to V
-)
V
SENSE
SENSE
V
= 7V to 28V, f
= 500kHz
(Note 2)
IN
SW
(MAX16821B)
V
= V
= 4.75V to 5.5V,
IN
CC
f
= 500kHz (MAX16821C)
SW
V
= 7V to 28V, f
= 500kHz
IN
SW
(MAX16821C)
Clock
Cycles
Soft-Start Time
t
SS
STARTUP/INTERNAL REGULATOR
Undervoltage Lockout
V
CC
UVLO
V
rising
falling
4.1
4.3
4.5
V
CC
(UVLO)
UVLO Hysteresis
V
V
200
mV
V
CC
V
Output Voltage
= 7V to 28V, I = 0 to 60mA
SOURCE
4.85
5.10
5.30
3
CC
IN
MOSFET DRIVER
Output Driver Impedance
Low or high output, I
= 20mA
1.1
4
Ω
A
SOURCE/SINK
Output Driver Source/Sink
Current
I
, I
DH DL
Nonoverlap Time
t
C
= 5nF
35
ns
NO
DH/DL
Maxim Integrated
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Electrical Characteristics (continued)
(V
= 5V, V
= V , T = T = -40°C to +125°C, unless otherwise noted. Typical values are at T = +25°C.) (Note 1)
CC
DD
CC
A
J
A
PARAMETER
OSCILLATOR
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
kHz
Switching Frequency Range
125
120
495
1515
-5
1500
130
547
1725
+5
R = 500kΩ
125
521
T
Switching Frequency
f
R = 120kΩ
kHz
SW
T
R = 39.9kΩ
1620
T
120kΩ < R ≤ 500kΩ
T
Switching Frequency Accuracy
%
40kΩ ≤ R ≤ 120kΩ
-8
+8
T
CLKOUT Phase Shift with
Respect to DH (Rising Edges)
f
= 125kHz, MODE connected
SW
180
180
to SGND
degrees
CLKOUT Phase Shift with
Respect to DL (Rising Edges)
f = 125kHz, MODE connected
to V
CC
SW
CLKOUT Output-Voltage Low
CLKOUT Output-Voltage High
SYNC Input High Pulse Width
V
I
I
= 2mA
0.4
V
V
OL
SINK
V
= 2mA
4.5
OH
SOURCE
t
200
ns
SYNC
SYNC Input Clock High
Threshold
V
2
V
V
SYNCH
SYNC Input Clock Low
Threshold
V
0.4
500
0.4
SYNCL
I
SYNC_
OUT
SYNC Pullup Current
V
= 0V
250
µA
V
RT/SYNC
V
SYNC_
OFF
SYNC Power-Off Level
INDUCTOR CURRENT LIMIT
Average Current-Limit Threshold
Reverse Current-Limit Threshold
Cycle-by-Cycle Current Limit
Cycle-by-Cycle Overload
V
CSP to CSN
CSP to CSN
CSP to CSN
26.4
27.5
-2.0
60
33.0
mV
mV
mV
ns
CL
V
CLR
V
to V
= 75mV
260
CSP
CSN
CURRENT-SENSE AMPLIFIER
CSP to CSN Input Resistance
R
4
kΩ
CS
Common-Mode Range
V
V
= 7V to 28V
0
5.5
V
CMR(CS)
IN
Input Offset Voltage
Amplifier Voltage Gain
3dB Bandwidth
V
0.1
34.5
4
mV
V/V
OS(CS)
A
V(CS)
f
MHz
3dB
CURRENT-ERROR AMPLIFIER (TRANSCONDUCTANCE AMPLIFIER)
Transconductance
Open-Loop Gain
g
550
50
µS
dB
m
A
VL(CE)
Maxim Integrated
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Electrical Characteristics (continued)
(V
= 5V, V
= V , T = T = -40°C to +125°C, unless otherwise noted. Typical values are at T = +25°C.) (Note 1)
CC
DD
CC
A
J
A
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
LED CURRENT SIGNAL DIFFERENTIAL VOLTAGE AMPLIFIER (DIFF)
V
CMR
(DIFF)
Common-Mode Voltage Range
DIFF Output Voltage
0
1.0
V
V
V
V
= V = 0V
SENSE-
0.6
CM
SENSE+
MAX16821A
-3.7
-1.5
+3.7
+1.5
1.008
6.1
Input Offset Voltage
V
mV
OS(DIFF)
MAX16821B/MAX16821C
MAX16821A
0.992
5.85
18.5
1
6
Amplifier Voltage Gain
A
MAX16821B
V/V
V(DIFF)
MAX16821C
20
21.5
MAX16821A, C
MAX16821B, C
MAX16821C, C
MAX16821A
= 20pF
= 20pF
= 20pF
1.7
1600
550
100
60
MHz
kHz
DIFF
DIFF
DIFF
3dB Bandwidth
f
3dB
50
30
10
SENSE+ to SENSE- Input
Resistance
R
MAX16821B
kΩ
VS
MAX16821C
20
OUTV AMPLIFIER
Gain-Bandwidth Product
3dB Bandwidth
V
V
= 2V
= 2V
4
1
MHz
MHz
µA
OUTV
OUTV
Output Sink Current
Output Source Current
Maximum Load Capacitance
30
80
µA
50
135
1
pF
OUTV to (CSP - CSN) Transfer
Function
4mV ≤ C – C ≤ 32mV
132.5
137.7
V/V
mV
SP
SN
Input Offset Voltage
VOLTAGE-ERROR AMPLIFIER (EAOUT)
Open-Loop Gain
A
70
3
dB
MHz
µA
VOLEA
Unity-Gain Bandwidth
EAN Input Bias Current
f
GBW
I
V
= 2V
-0.2
905
+0.03
+0.2
940
B(EA)
EAN
Error Amplifier Output
Clamping Voltage
V
CLAMP
(EA)
With respect to V
930
mV
CM
INPUTS (MODE AND OVI)
MODE Input-Voltage High
MODE Input-Voltage Low
MODE Pulldown Current
OVI Trip Threshold
2
V
V
0.8
6
4
5
µA
V
OVP
1.244
1.276
200
0.2
1.308
TH
OVI Hysteresis
OVI
mV
µA
HYS
OVI Input Bias Current
I
V
= 1V
OVI
OVI
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Electrical Characteristics (continued)
(V
= 5V, V
= V , T = T = -40°C to +125°C, unless otherwise noted. Typical values are at T = +25°C.) (Note 1)
CC
DD
CC
A
J
A
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
2.562
16.5
UNITS
ENABLE INPUT (EN)
EN Input-Voltage High
EN Input Hysteresis
EN Pullup Current
EN rising
2.437
13.5
2.5
0.28
15
V
V
I
µA
EN
THERMAL SHUTDOWN
Thermal Shutdown
165
20
°C
°C
Thermal-Shutdown Hysteresis
Note 1: All devices are 100% production tested at +25°C. Limits over temperature are guaranteed by design.
Note 2: Does not include an error due to finite error amplifier gain. See the Voltage-Error Amplifier section.
Typical Operating Characteristics
(V = 12V, V
= V
= 5V, T = +25°C, unless otherwise noted.)
IN
DD
CC A
V
CC
LOAD REGULATION vs. V
SUPPLY CURRENT (IQ) vs. FREQUENCY
SUPPLY CURRENT vs. TEMPERATURE
IN
5.5
5.4
5.3
5.2
5.1
5.0
4.9
4.8
4.7
4.6
4.5
10
9
8
7
6
5
4
3
2
1
0
70
65
60
55
50
45
40
EXTERNAL CLOCK
NO DRIVER LOAD
V
IN
= 24V
V
IN
= 24V
V
IN
= 12V
V
IN
= 12V
V
IN
= 5V
V
IN
= 7V
V
C
= 12V
IN
= 22nF
DH/DL
0
15 30 45 60 75 90 105 120 135 150
LOAD CURRENT (mA)
100 300 500 700 900 1100 1300 1500
FREQUENCY (kHz)
-40
-15
10
35
60
85
V
CC
TEMPERATURE (°C)
DRIVER RISE TIME
vs. DRIVER LOAD CAPACITANCE
DRIVER FALL TIME
vs. DRIVER LOAD CAPACITANCE
HIGH-SIDE DRIVER (DH) SINK
AND SOURCE CURRENT
MAX16821A toc06
200
180
160
140
120
100
80
100
80
60
40
20
0
C
= 22nF
= 12V
LOAD
V
IN
2A/div
DL
60
DH
DH
40
DL
15
20
0
0
5
10
20
25
0
5
10
15
20
25
100ns/div
LOAD CAPACITANCE (nF)
LOAD CAPACITANCE (nF)
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Typical Operating Characteristics (continued)
(V = 12V, V
= V
= 5V, T = +25°C, unless otherwise noted.)
IN
DD
CC A
LOW-SIDE DRIVER (DL) SINK
AND SOURCE CURRENT
HIGH-SIDE DRIVER (DH) FALL TIME
HIGH-SIDE DRIVER (DH) RISE TIME
MAX16821A toc09
MAX16821A toc07
MAX16821A toc08
C
= 22nF
LOAD
= 12V
C
V
= 22nF
= 12V
V
= 12V
LOAD
IN
V
DH RISING
IN
IN
3A/div
2V/div
2V/div
100ns/div
40ns/div
40ns/div
FREQUENCY vs. R
LOW-SIDE DRIVER (DL) FALL TIME
T
LOW-SIDE DRIVER (DL) RISE TIME
MAX16821A toc11
MAX16821A toc10
10,000
C
= 22nF
= 12V
V
IN
= 12V
C
V
= 22nF
= 12V
LOAD
LOAD
V
IN
IN
2V/div
1000
2V/div
100
40ns/div
40ns/div
30 70 110 150 190230 270 310 350 390 430 470 510 550
R (k)
T
SYNC, CLKOUT, AND DL WAVEFORMS
FREQUENCY vs. TEMPERATURE
SYNC, CLKOUT, AND DH WAVEFORMS
MAX16821A toc15
MAX16821A toc14
260
V
IN
= 12V
258
256
254
252
250
RT/SYNC
5V/div
0V
RT/SYNC
5V/div
0V
MODE = V
CC
MODE = SGND
CLKOUT
5V/div
0V
CLKOUT
5V/div
0V
248
246
244
242
DL
5V/div
0V
DH
5V/div
0V
240
1s/div
0
5
10
15
20
25
30
35
1s/div
TEMPERATURE (°C)
Maxim Integrated
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Pin Description
PIN
1
NAME
PGND
N.C.
FUNCTION
Power-Supply Ground
2, 7
3
No Connection. Not internally connected.
Low-Side Gate-Driver Output
DL
Boost-Flying Capacitor Connection. Reservoir capacitor connection for the high-side MOSFET driver
supply. Connect a ceramic capacitor between BST and LX.
4
5
6
BST
LX
High-Side MOSFET Source Connection
High-Side Gate-Driver Output
DH
Signal Ground. SGND is the ground connection for the internal control circuitry. Connect SGND and
PGND together at one point near the IC.
8, 22, 25
SGND
CLKOUT
MODE
EN
Oscillator Output. If MODE is low, the rising edge of CLKOUT phase shifts from the rising edge of DH by
180°. If MODE is high, the rising edge of CLKOUT phase shifts from the rising edge of DL by 180°.
9
Buck/Boost Mode Selection Input. Drive MODE low for low-side buck mode operation. Drive MODE high
for boost or high-side buck mode operation. MODE has an internal 5µA pulldown current to ground.
10
11
Output Enable. Drives EN high or leave unconnected for normal operation. Drive EN low to shut down
the power drivers. EN has an internal 15µA pullup current.
Switching Frequency Programming. Connect a resistor from RT/SYNC to SGND to set the internal
oscillator frequency. Drive RT/SYNC to synchronize the switching frequency with an external clock.
12
RT/SYNC
Inductor Current-Sense Output. OUTV is an amplifier output voltage proportional to the inductor current.
13
14
OUTV
I.C.
The voltage at OUTV = 135 x (V
- V
).
CSP
CSN
Internally Connected. Connect to SGND for proper operation.
Overvoltage Protection. When OVI exceeds the programmed output voltage by 12.7%, the low-side and
the high-side drivers are turned off. When OVI falls 20% below the programmed output voltage, the
drivers are turned on after power-on reset and soft-start cycles are completed.
15
OVI
16
17
18
CLP
EAOUT
EAN
Current-Error-Amplifier Output. Compensate the current loop by connecting an RC network to ground.
Voltage-Error-Amplifier Output. Connect EAOUT to the external gain-setting network.
Voltage-Error-Amplifier Inverting Input
Differential Remote-Sense Amplifier Output. DIFF is the output of a precision amplifier with SENSE+
and SENSE- as inputs.
19
20
DIFF
CSN
Current-Sense Differential Amplifier Negative Input. The differential voltage between CSN and CSP is
amplified internally by the current-sense amplifier (Gain = 34.5) to measure the inductor current.
Current-Sense Differential Amplifier Positive Input. The differential voltage between CSP and CSN is
amplified internally by the current-sense amplifier (Gain = 34.5) to measure the inductor current.
21
23
24
CSP
Differential LED Current-Sensing Negative Input. Connect SENSE- to the negative side of the LED
current- sense resistor or to the negative feedback point.
SENSE-
Differential LED Current-Sensing Positive Input. Connect SENSE+ to the positive side of the LED
current- sense resistor, or to the positive feedback point.
SENSE+
IN
26
27
28
Supply Voltage Input. Connect IN to V , for a 4.75V to 5.5V input supply range.
CC
Internal +5V Regulator Output. V
ceramic capacitors.
is derived from V . Bypass V
to SGND with 4.7µF and 0.1µF
CC
IN
CC
V
V
CC
DD
Low-Side Driver Supply Voltage
Exposed Pad. EP is internally connected to SGND. Connect EP to a large-area ground plane for
effective power dissipation. Connect EP to SGND. Do not use as a ground connection.
—
EP
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Undervoltage Lockout (UVLO)
Detailed Description
The MAX16821A–MAX16821C include UVLO and a
2048 clock-cycle power-on-reset circuit. The UVLO rising
threshold is set to 4.3V with 200mV hysteresis. Hysteresis
at UVLO eliminates chattering during startup. Most of the
internal circuitry, including the oscillator, turns on when the
input voltage reaches 4V. The MAX16821A–MAX16821C
draw up to 3.5mA of quiescent current before the input
voltage reaches the UVLO threshold.
The MAX16821A, MAX16821B, and MAX16821C are
high-performance average current-mode PWM control-
lers for high-power and high-brightness LEDs (HB LEDs).
The average current-mode control technique offers inher-
ently stable operation, reduces component derating and
size by accurately controlling the inductor current. The
devices achieve high efficiency at high currents (up to
30A) with a minimum number of external components.
A logic input (MODE) allows the LED driver to switch
between buck and boost modes of operation.
Soft-Start
The MAX16821A–MAX16821C include an internal soft-
start for a glitch-free rise of the output voltage. After 2048
power-on-reset clock cycles, a 0.6V reference voltage
connected to the positive input of the internal error ampli-
fier ramps up to its final value after 1024 clock cycles.
Soft-start reduces inrush current and stress on system
components. During soft-start, the LED current will ramp
monotonically towards its final value.
The MAX16821A–MAX16821C feature a CLKOUT output
180° out-of-phase with respect to either the high-side
or low-side driver, depending on MODE’s logic level.
CLKOUT provides the drive for a second out-of-phase
LED driver for applications requiring reduced input capac-
itor ripple current while operating another LED driver.
The MAX16821A–MAX16821C consist of an inner aver-
age current regulation loop controlled by an outer loop.
The combined action of the inner current loop and outer
voltage loop corrects the LED current errors by adjusting
the inductor current resulting in a tightly regulated LED
current. The differential amplifier (SENSE+ and SENSE-
inputs) senses the LED current using a resistor in series
with the LEDs and produces an amplified version of the
sense voltage at DIFF. The resulting amplified sensed
voltage is compared against an internal 0.6V reference at
the error amplifier input.
Internal Oscillator
The internal oscillator generates a clock with the fre-
quency inversely proportional to the value of R (see
T
the Typical Operating Circuit). The oscillator frequency is
adjustable from 125kHz to 1.5MHz range using a single
resistor connected from RT/SYNC to SGND. The fre-
quency accuracy avoids the overdesign, size, and cost
of passive filter components like inductors and capaci-
tors. Use the following equation to calculate the oscillator
frequency:
Input Voltage
The MAX16821A–MAX16821C operate with a 4.75V to
5.5V input supply range when the internal LDO is disabled
For 120kΩ ≤ R ≤ 500kΩ:
T
10
6.25 x10
f
=
SW
(Hz)
(V
connected to IN) or a 7V to 28V input supply range
CC
R
T
when the internal LDO is enabled. For a 7V to 28V input
voltage range, the internal LDO provides a regulated 5V
For 40kΩ ≤ R ≤ 120kΩ:
T
output with 60mA of sourcing capability. Bypass V
to
CC
SGND with 4.7µF and 0.1µF low-ESR ceramic capacitors.
The MAX16821A–MAX16821C’s input pro-
vides supply voltage for the low-side and the high-
side MOSFET drivers. Connect V to V using an
10
6.40 x10
f
=
(Hz)
SW
V
DD
R
T
The oscillator also generates a 2V
ramp signal for the
P-P
DD
CC
PWM comparator and a 180° out-of-phase clock signal
at CLKOUT to drive a second out-of-phase LED current
regulator.
R-C filter to isolate the analog circuits from the MOSFET
drivers. The internal LDO powers up the MAX16821A–
MAX16821C. For applications utilizing a 5V input volt-
age, disable the internal LDO by connecting IN and V
CC
Synchronization
together. The 5V power source must be in the 4.75V to
5.5V range of for proper operation of the MAX16821A–
MAX16821C.
The MAX16821A–MAX16821C synchronize to an exter-
nal clock connected to RT/SYNC. The application of an
external clock at RT/SYNC disables the internal oscillator.
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
V
CC
MAX16821A
MAX16821B
MAX16821C
EN
IN
0.5 x V
CC
UVLO
POR
+5V LDO
TEMP SEN
V
CC
TO INTERNAL CIRCUIT
I.C.
CLP
CSP
CSN
A
V
= 34.5
V
CM
A
V
= 4
g
m
V
LOW
CLAMP
OUTV
BST
V
HIGH
CLAMP
DH
S
R
Q
Q
LX
V
PWM
COMPARATOR
MUX
CLK
DD
RT/SYNC
CLKOUT
OSCILLATOR
DL
PGND
2 x f
S
RAMP
GENERATOR
V
TH
DIFF
SENSE-
V
CM
DIFF
AMP
SENSE+
EAOUT
EAN
MODE
ERROR
AMP
OVP
COMPARATOR
0.12 x V
REF
V
REF
= 0.6V
SOFT-
START
ENABLE
UVLO
V
CM
OVI
SGND
Figure 1. Internal Block Diagram
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Once the MAX16821A–MAX16821C are synchronized to
an external clock, the external clock cannot be removed if
reliable operation is to be maintained.
The differential current-sense amplifier (CSA) provides
a 34.5V/V DC gain. The typical input offset voltage of
the current-sense amplifier is 0.1mV with a 0 to 5.5V
common-mode voltage range (V = 7V to 28V). The cur-
IN
Control Loop
rent-sense amplifier senses the voltage across R . The
S
The MAX16821A–MAX16821C use an average current-
mode control scheme to regulate the output current
(Figure 2). The main control loop consists of an inner
current regulation loop for controlling the inductor current
and an outer current regulation loop for regulating the
LED current. The inner current regulation loop absorbs
the double pole of the inductor and output capacitor com-
bination reducing the order of the outer current regulation
loop to that of a single-pole system. The inner current
maximum common-mode voltage is 3.2V when V = 5V.
IN
Inductor Peak-Current Comparator
The peak-current comparator provides a path for fast
cycle-by-cycle current limit during extreme fault condi-
tions, such as an inductor malfunction (Figure 3). Note the
average current-limit threshold of 27.5mV still limits the
output current during short-circuit conditions. To prevent
inductor saturation, select an inductor with a saturation
current specification greater than the average current limit.
The 60mV threshold for triggering the peak-current limit is
twice the full-scale average current-limit voltage threshold.
The peak-current comparator has only a 260ns delay.
regulation loop consists of a current-sense resistor (R ),
S
a current-sense amplifier (CSA), a current-error amplifier
(CEA), an oscillator providing the carrier ramp, and a
PWM comparator (CPWM) (Figure 2). The MAX16821A–
MAX16821C outer LED-current control loop consists of
a differential amplifier (DIFF), a reference voltage, and a
voltage-error amplifier (VEA).
Current-Error Amplifier
The MAX16821A–MAX16821C include a transconduc-
tance current-error amplifier with a typical g of 550µS
m
Inductor Current-Sense Amplifier
and 320µA output sink and source capability. The current-
C
CZ
R
CF
C
CP
C
F
R
IN
R
F
DIFF
EAN
EAOUT
CSN
CSP
CA
CLP
V
IN
SENSE+
SENSE-
CEA
LED
STRING
L
DIFF
VEA
CPWM
DRIVER
C
OUT
R
LS
V
REF
R
S
MODE = SGND
Figure 2. MAX16821A–MAX16821C Control Loop
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
error amplifier output (CLP) is connected to the inverting
input of the PWM comparator. CLP is also externally
accessible to provide frequency compensation for the
inner current regulation loop (Figure 2). Compensate CEA
so the inductor current negative slope, which becomes
the positive slope to the inverting input of the PWM com-
parator, is less than the slope of the internally generated
voltage ramp (see the Compensation section). In applica-
tions without synchronous rectification, the LED driver
can be turned off and on instantaneously by shorting or
opening the CLP to ground.
soon as the ramp signal exceeds the CLP voltage, thus
terminating the ON cycle. See Figure 3.
Differential Amplifier
The differential amplifier (DIFF) allows LED current sens-
ing (Figure 2). It provides true-differential LED current
sensing, and amplifies the sense voltage by a factor of 1
(MAX16821A), 6 (MAX16821B), and 20 (MAX16821C),
while rejecting common-mode voltage errors. The VEA
provides the difference between the differential ampli-
fier output (DIFF) and the desired LED current-sense
voltage. The differential amplifier has a bandwidth of
1.7MHz (MAX16821A), 1.6MHz (MAX16821B), and
550kHz (MAX16821C). The difference between SENSE+
and SENSE- is regulated to +0.6V (MAX16821A), +0.1V
(MAX16821B), or +0.03V (MAX16821C).
PWM Comparator and R-S Flip-Flop
An internal PWM comparator sets the duty cycle by
comparing the output of the current-error amplifier to a
2V
ramp signal. At the start of each clock cycle, an
P-P
R-S flip-flop resets and the high-side driver (DH) turns on
if MODE is connected to SGND, and DL turns on if MODE
Voltage-Error Amplifier (VEA)
The VEA sets the gain of the voltage control loop, and
determines the error between the differential amplifier
is connected to V . The comparator sets the flip-flop as
CC
60mV
PEAK-CURRENT
COMPARATOR
CLP
CSP
A
V
= 34.5
CSN
IN
g
= 550S
m
PWM
COMPARATOR
MODE = GND
BST
DH
LX
S
R
Q
RAMP
CLK
V
DD
DL
Q
PGND
SHDN
Figure 3. MAX16821A–MAX16821C Phase Circuit
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
output and the internal reference voltage. The VEA output
clamps to 0.93V relative to the internal common- mode
voltage (V ). Average current-mode control limits the
CM
average current sourced by the converter during a fault
condition. When a fault condition occurs, the VEA output
clamps to +0.93V with respect to the common-mode volt-
age (0.6V) to limit the maximum current sourced by the
voltage, V
(+0.6V), limiting the average maximum
CM
current. The maximum average current-limit threshold is
equal to the maximum clamp voltage of the VEA divided by
the gain (34.5) of the current-sense amplifier. This results
in accurate settings for the average maximum current.
converter to I
= 0.0275 / R .
LIMIT
S
Overvoltage Protection
MOSFET Gate Drivers
The OVP comparator compares the OVI input to the
overvoltage threshold. The overvoltage threshold is typi-
cally 1.127 times the internal 0.6V reference voltage
The high-side (DH) and low-side (DL) drivers drive the
gates of external n-channel MOSFETs. The drivers’ 4A
peak sink- and source-current capability provides ample
drive for the fast rise and fall times of the switching
MOSFETs. Faster rise and fall times result in reduced
cross-conduction losses. Size the high-side and low-side
MOSFETs to handle the peak and RMS currents during
overload conditions. The driver block also includes a logic
circuit that provides an adaptive nonoverlap time to pre-
vent shoot-through currents during transition. The typical
nonoverlap time is 35ns between the high-side and low-
side MOSFETs.
plus V
(0.6V). A detected overvoltage event trips the
CM
comparator output turning off both high-side and low-side
MOSFETs. Add an RC delay to reduce the sensitivity of
the overvoltage circuit and avoid unnecessary tripping of
the converter (Figure 4). After the OVI voltage falls below
1.076V (typ.), high-side and low-side drivers turn on only
after a 2048 clock-cycle POR and a 1024 clock-cycle soft-
start have elapsed. Disable the overvoltage function by
connecting OVI to SGND.
BST
The MAX16821A–MAX16821C provide power to the low-
side and high-side MOSFET drivers through V . A boot-
DD
strap capacitor from BST to LX provides the additional
C
OVI
boost voltage necessary for the high-side driver. V
sup-
DD
plies power internally to the low-side driver. Connect a
0.47µF low-ESR ceramic capacitor between BST and LX
RA
OVI
V
OUT
and a Schottky diode from BST to V
.
DD
RB
Protection
MAX16821A
MAX16821B
MAX16821C
The MAX16821A–MAX16821C include output overvolt-
age protection (OVP). During fault conditions when the
load goes to high impedance (output opens), the control-
ler attempts to maintain LED current. The OVP disables
the MAX16821A–MAX16821C whenever the output volt-
age exceeds the OVP threshold, protecting the external
circuits from undesirable voltages.
DIFF
EAN
R
IN
R
F
EAOUT
Current Limit
The error amplifier (VEA) output is clamped between
-0.050V and +0.93V with respect to common-mode
Figure 4. Overvoltage-Protection Input Delay
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
to the output. The output voltage cannot go below the
input voltage in this configuration. Resistor R1 senses
the inductor current and resistor R2 senses the LED
current. The outer LED current regulation loop programs
the average current in the inductor, thus achieving tight
LED current regulation.
Applications Information
Boost LED Driver
F
igure 5 shows the MAX16821A–MAX16821C configured
as a synchronous boost converter with MODE connected
to V . During the on-time, the input voltage charges
CC
the inductor. During the off-time, the inductor discharges
V
CC
R4
V
LED
ON/OFF
C3
12
R9
V
IN
R3
13
7V TO 28V
C2
R10
14
9
10
11
8
L1
I.C.
OUTV RT/SYNC EN
MODE CLKOUT SGND
N.C.
C11
15
7
6
5
4
3
2
1
OVI
V
LED
C10
R8
R7
Q2
DH
LX
16 CLP
C9
Q1
EAOUT
EAN
17
18
C4
R5
MAX16821A
MAX16821B
MAX16821C
C1
C8
BST
DL
LED
STRING
R5
19 DIFF
20 CSN
21 CSP
R2
N.C.
D1
PGND
SGND SENSE- SENSE+ SGND
24 25
IN
V
V
DD
CC
26
22
23
27
28
V
IN
C6
C5
C7
Figure 5. Synchronous Boost LED Driver (Output Voltage Not to Exceed 28V)
Maxim Integrated
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
output to the input. This effectively removes the boost-
only restriction of the regulator in Figure 5, allowing the
voltage across the LED to be greater or less than the
input voltage. LED current-sensing is not ground-refer-
enced, so a high-side current-sense amplifier is used to
measure current.
Input-Referenced Buck-Boost LED Driver
The circuit in Figure 6 shows a step-up/step-down regula-
tor. It is similar to the boost converter in Figure 5 in that
the inductor is connected to the input and the MOSFET
is essentially connected to ground. However, rather than
going from the output to ground, the LEDs span from the
V
CC
R4
V
LED
ON/OFF
V
IN
C3
12
R8
7V TO 28V
LED
R3
13
R2
STRING
1 TO 6
LEDS
C2
C2
R9
L1
14
9
10
11
8
I.C.
OUTV RT/SYNC EN
MODE CLKOUT SGND
N.C.
C11
D1
15
7
6
5
4
3
2
1
OVI
V
LED
C10
Q1
R7
R6
DH
LX
16 CLP
V
CC
C9
EAOUT
EAN
17
18
C1
RS+
RS-
MAX16821A
MAX16821B
MAX16821C
OUT
C8
BST
DL
R5
19 DIFF
20 CSN
21 CSP
N.C.
PGND
SGND SENSE- SENSE+ SGND
24 25
IN
V
V
DD
CC
26
22
23
27
28
V
IN
C6
C5
C7
Figure 6. Typical Application Circuit for an Input-Referred Buck-Boost LED Driver (7V to 28V Input)
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
with L2 to provide current to recharge C1 and supplies
the load current. Since the voltage waveform across L1
and L2 are exactly the same, it is possible to wind both
inductors on the same core (a coupled inductor). Although
voltages on L1 and L2 are the same, RMS currents can
be quite different so the windings may require a different
gauge wire. Because of the dual inductors and segment-
ed energy transfer, the efficiency of a SEPIC converter is
lower than the standard buck or boost configurations. As
in the boost driver, the current-sense resistor connects to
ground, allowing the output voltage of the LED driver to
exceed the rated maximum voltage of the MAX16821A–
MAX16821C.
SEPIC LED Driver
Figure 7 shows the MAX16821A–MAX16821C configured
as a SEPIC LED driver. While buck topologies produce an
output always lower than the input, and boost topologies
produce an output always greater than the input, a SEPIC
topology allows the output voltage to be greater than,
equal to, or less than the input. In a SEPIC topology, the
voltage across C3 is the same as the input voltage, and
L1 and L2 have the same inductance. Therefore, when
Q1 turns on (on-time), the currents in both inductors (L1
and L2) ramp up at the same rate. The output capacitor
supports the output voltage during this time. When Q1
turns off (off-time), L1 current recharges C3 and combines
V
CC
R4
V
LED
ON/OFF
V
IN
C2
12
R8
7V TO 28V
R3
13
R9
L1
14
9
10
11
8
V
LED
I.C.
OUTV RT/SYNC EN
MODE CLKOUT SGND
N.C.
C10
C3
D1
15
7
6
5
4
3
2
1
OVI
C9
C8
R7
R6
Q1
DH
LX
16 CLP
EAOUT
EAN
17
18
LED
STRING
C1
L2
MAX16821A
MAX16821B
MAX16821C
C7
BST
DL
R5
19 DIFF
20 CSN
21 CSP
R2
N.C.
R1
PGND
SGND SENSE- SENSE+ SGND
24 25
IN
V
V
DD
CC
26
22
23
27
28
V
IN
C5
C4
C6
Figure 7. Typical Application Circuit for a SEPIC LED Driver
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
current-loop inductor, current is sensed by resistor R1.
To regulate the LED current, R2 creates a voltage that
the differential amplifier compares to 0.6V. Capacitor C1
is small and helps reduce the ripple current in the LEDs.
Omit C1 in cases where the LEDs can tolerate a higher
ripple current. The average current-mode control scheme
converts the input voltage to a current source feeding the
LED string.
Low-Side Buck Driver
with Synchronous Rectification
In Figure 8, the input voltage goes from 7V to 28V and,
because of the ground-based current-sense resistor, the
output voltage can be as high as the input. The synchro-
nous MOSFET keeps the power dissipation to a mini-
mum, especially when the input voltage is large compared
to the voltage on the LED string. For the inner average
V
CC
R4
V
LED
ON/OFF
C3
12
R9
R3
13
V
IN
R10
7V TO 28V
14
9
10
11
8
I.C.
OUTV RT/SYNC EN
MODE CLKOUT SGND
N.C.
C11
15
7
6
5
4
3
2
1
OVI
C2
C10
R9
R7
Q1
DH
LX
16 CLP
V
LED
C9
L1
EAOUT
EAN
17
18
C4
MAX16821A
MAX16821B
MAX16821C
R5
C8
R6
BST
DL
LED
STRING
Q2
19 DIFF
20 CSN
21 CSP
C1
D2
N.C.
R2
R1
PGND
SGND SENSE- SENSE+ SGND
24 25
IN
V
V
DD
CC
26
22
23
27
28
V
IN
C6
C5
C7
Figure 8. Application Circuit for a Low-Side Buck LED Driver
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
current-sense amplifier, U2. The voltage appearing across
resistor R11 becomes the average inductor current-sense
voltage for the inner average current loop. To regulate
the LED current, R2 creates a voltage that the differential
amplifier compares to its internal reference. Capacitor
C1 is small and is added to reduce the ripple current in
the LEDs. In cases where the LEDs can tolerate a higher
ripple current, capacitor C1 can be omitted.
High-Side Buck Driver
with Synchronous Rectification
In Figure 9, the input voltage goes from 7V to 28V, the
LED load is connected from the positive side to the
current-sense resistor (R1) in series with the inductor,
and MODE is connected to V . For the inner average
current-loop inductor, current is sensed by resistor R1
and is then transferred to the low side by the high-side
CC
V
CC
R4
ON/OFF
C3
12
R3
13
V
IN
7V TO 28V
14
9
10
11
8
I.C.
OUTV RT/SYNC EN
MODE CLKOUT SGND
N.C.
C11
15
7
6
5
4
3
2
1
OVI
LED
STRING
C2
Q1
I.C.
C10
C9
R8
R7
C1
DH
LX
16 CLP
L1
EAOUT
EAN
17
18
R1
V
CC
C4
MAX16821A
MAX16821B
MAX16821C
R5
C8
R6
BST
DL
RS+
RS-
D1
U2
OUT
R2
Q2
19 DIFF
20 CSN
21 CSP
R11
N.C.
PGND
SGND SENSE- SENSE+ SGND
24 25
IN
V
V
DD
CC
26
22
23
27
28
V
IN
C6
C5
C7
Figure 9. Application Circuit for a High-Side Buck LED Driver
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Inductor Selection
Switching MOSFETs
The switching frequency, peak inductor current, and
allowable ripple at the output determine the value and
size of the inductor. Selecting higher switching frequen-
cies reduces inductance requirements, but at the cost
of efficiency. The charge/discharge cycle of the gate
and drain capacitance in the switching MOSFETs cre-
ate switching losses worsening at higher input voltages,
since switching losses are proportional to the square of
the input voltage. The MAX16821A–MAX16821C operate
up to 1.5MHz.
When choosing a MOSFET for voltage regulators, con-
sider the total gate charge, RDS(ON), power dissipation,
and package thermal impedance. The product of the
MOSFET gate charge and on-resistance is a figure of
merit, with a lower number signifying better performance.
Choose MOSFETs optimized for high-frequency switching
applications. The average current from the MAX16821A–
MAX16821C gate-drive output is proportional to the total
capacitance it drives from DH and DL. The power dissi-
pated in the MAX16821A–MAX16821C is proportional to
the input voltage and the average drive current. The gate
charge and drain capacitance losses (CV2), the cross-
conduction loss in the upper MOSFET due to finite rise/fall
time, and the I2R loss due to RMS current in the MOSFET
Choose inductors from the standard high-current, surface-
mount inductor series available from various manufactur-
ers. Particular applications may require custom-made
inductors. Use high-frequency core material for custom
R
account for the total losses in the MOSFET.
DS(ON)
inductors. High ∆I causes large peak-to-peak flux excur-
L
Estimate the power loss (PD
) in the high-side and
MOS_
sion increasing the core losses at higher frequencies. The
low-side MOSFETs using the following equations:
high-frequency operation coupled with high ∆I reduces
L
the required minimum inductance and makes the use of
planar inductors possible.
PD = Q × V × f
+
DD SW
(
)
MOS_HI
G
V
×I
× t + t × f
(
2
)
IN LED R F SW
The following discussion is for buck or continuous boost-
mode topologies. Discontinuous boost, buck-boost, and
SEPIC topologies are quite different in regards to compo-
nent selection. Use the following equations to determine
the minimum inductance value:
+
2
R
×I
RMS−HI
DS(ON)
where Q , R
, t , and t are the upper-switching
R F
G
DS(ON)
Buck regulators:
MOSFET’s total gate charge, on-resistance, rise time,
and fall time, respectively.
V
− V
× V
(
)
INMAX
V
LED LED
L
=
(
MIN
× f
× ∆I
L
D
INMAX SW
2
2
I
=
I
+ I
+ I
×I
×
RMS−HI
VALLEY
PK
VALLEY PK
3
Boost regulators:
For the buck regulator, D is the duty cycle, I
=
VALLEY
V
− V
× V
)
LED
V
INMAX INMAX
L
=
(I
- ∆I / 2) and I = (I
+ ∆I / 2).
OUT
L
MOS_LO
=
PK
OUT
L
MIN
× f
× ∆I
L
LED SW
2
PD
= Q × V
× f
+ R
×I
DS(ON)
(
)
G
DD SW
RMS−LO
1− D
where V
is the total voltage across the LED string.
LED
(
)
The average current-mode control feature of the
MAX16821A–MAX16821C limits the maximum peak
inductor current and prevents the inductor from saturat-
ing. Choose an inductor with a saturating current greater
than the worst-case peak inductor current. Use the follow-
ing equation to determine the worst-case current in the
average current-mode control loop.
2
2
I
I
+ I
+ I
×I
×
)
RMS−LO
(
VALLEY
PK
VALLEY PK
3
Input Capacitors
The discontinuous input-current waveform of the buck
converter causes large ripple currents in the input capaci-
tor. 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
V
∆I
CL
2
CL
I
=
+
LPEAK
R
S
ripple is comprised of ∆V (caused by the capacitor dis-
Q
charge) and ∆V
(caused by the ESR of the capacitor).
ESR
where R is the sense resistor and V = 0.030V. For the
S
CL
buck converter, the sense current is the inductor current and
for the boost converter, the sense current is the input current.
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Use low-ESR ceramic capacitors with high ripple-current
capability at the input. In the case of the boost topology
where the inductor is in series with the input, the ripple
current in the capacitor is the same as the inductor ripple
and the input capacitance is small.
Select a 5% lower value of R to compensate for any
S
parasitics associated with the PCB. Select a non-inductive
resistor with the appropriate wattage rating. In the case of
the boost configuration, the MAX16821A–MAX16821C
accurately limits the maximum input current. Use the
following equation to calculate the current-sense resistor
value:
Output Capacitors
The function of the output capacitor is to reduce the out-
put ripple to acceptable levels. The ESR, ESL, and the
bulk capacitance of the output capacitor contribute to the
output ripple. In most of the applications, the output ESR
and ESL effects can be dramatically reduced by using
low-ESR ceramic capacitors. To reduce the ESL effects,
connect multiple ceramic capacitors in parallel to achieve
the required bulk capacitance.
0.0264
R
=
SENSE
I
IN
where I is the input current.
IN
Compensation
The main control loop consists of an inner current loop
(inductor current) and an outer LED current regulation
loop. The MAX16821A–MAX16821C use an average
current-mode control scheme to regulate the LED current
(Figure 2). The VEA output provides the controlling volt-
age for the current source. The inner current loop absorbs
the inductor pole reducing the order of the LED current
loop to that of a single-pole system. The major consider-
ation when designing the current control loop is making
certain that the inductor downslope (which becomes an
upslope at the output of the CEA) does not exceed the
internal ramp slope. This is a necessary condition to avoid
subharmonic oscillations similar to those in peak current
mode with insufficient slope compensation. This requires
that the gain at the output of the CEA be limited based on
the following equation:
In a buck configuration, the output capacitance, C
calculated using the following equation:
, is
OUT
(V
− V
) × V
INMAX
LED LED
C
≥
OUT
2
∆V × 2 ×L × V
× f
R
INMAX SW
where ∆V is the maximum allowable output ripple.
R
In a boost configuration, the output capacitance, C
is calculated as:
,
OUT
(V
− V
) × 2 ×I
LED
INMIN LED
C
≥
OUT
∆V × V
× f
R
LED SW
where I
is the output current.
LED
In a buck-boost configuration, the output capacitance,
is:
Buck:
C
OUT
V
× f
×L
× g
RAMP SW
R
≤
CF
2 × V
×I
LED LED
A
×R × V
V LED
S
m
C
≥
OUT
∆V × (V
+ V
) × f
R
LED
INMIN SW
where V
= 2V, g = 550µS, A = 34.5V/V, and V
m V LED
RAMP
where V
is the voltage across the load and I
is
is the voltage across the LED string.
LED
LED
the output current.
The crossover frequency of the inner current loop is given
by:
Average Current Limit
The average current-mode control technique of the
MAX16821A–MAX16821C accurately limits the maximum
output current in the case of the buck configuration. The
MAX16821A–MAX16821C sense the voltage across the
R
V
IN
2× π ×L
S
f
=
×
× 34.5× g ×R
m CF
C
V
RAMP
For adequate phase margin place the zero formed by
and C at least 3 to 5 times below the crossover
frequency. The pole formed by R
be required in most applications but can be added to
minimize noise at a frequency at or above the switching
frequency.
sense resistor and limit the peak inductor current (I
)
L-PK
R
CF
CZ
accordingly. The on-cycle terminates when the current-
sense voltage reaches 26.4mV (min). Use the following
equation to calculate the maximum current-sense resistor
value:
and C
may not
CF
CP
0.0264
I
R
=
SENSE
LED
Maxim Integrated
│ 19
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Boost:
For adequate phase margin at crossover, place the zero
formed by R
crossover frequency. The pole formed by R
and C
at least 3 to 5 times below the
CF
CZ
V
× f
×L
RAMP SW
and C
CP
R
≤
CF
CF
A
×R × (V
− V ) × g
is added to eliminate noise spikes riding on the current
waveform and is placed at the switching frequency.
V
LED
IN
m
S
The crossover frequency of the inner current loop is
given by:
PWM Dimming
Even though the MAX16821A–MAX16821C do not
have a separate PWM input, PWM dimming can be
easily achieved by means of simple external circuitry. See
Figures 10 and 11.
R
V
LED
S
f
=
×
× 34.5× g ×R
C
m
CF
V
2× π ×L
RAMP
V
CC
R4
V
LED
ON/OFF
C3
R9
R3
V
IN
R10
7V TO 28V
14
13
12
9
10
11
8
I.C.
OUTV RT/SYNC EN
MODE CLKOUT SGND
N.C.
C11
R9
15
7
6
5
4
3
2
1
OVI
C2
C10
C9
Q1
DH
LX
16 CLP
V
LED
R7
C8
L1
EAOUT
EAN
17
18
C4
MAX16821A
MAX16821B
MAX16821C
R5
BST
DL
LED
STRING
PWM DIM
R6
Q2
Q3
19 DIFF
20 CSN
21 CSP
D2
N.C.
R2
R1
PGND
SGND SENSE- SENSE+ SGND
24 25
IN
V
V
DD
CC
26
22
23
27
28
V
IN
C6
C5
C7
Figure 10. Low-Side Buck LED Driver with PWM Dimming
Maxim Integrated
│ 20
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
V
CC
V
LED
R4
V
CC
R8
R9
ON/OFF
V
IN
R10
C3
12
7V TO 28V
R3
13
Q5
C2
PWM DIM
Q4
L1
14
9
10
11
8
V
LED
I.C.
OUTV RT/SYNC EN
MODE CLKOUT SGND
N.C.
C11
D1
15
7
6
5
4
3
2
1
OVI
C10
R7
R6
Q1
DH
LX
16 CLP
LED
STRING
C9
C1
EAOUT
EAN
17
18
MAX16821A
MAX16821B
MAX16821C
PWM DIM
PWM DIM
R2
C8
BST
DL
Q3
Q2
R5
19 DIFF
20 CSN
21 CSP
N.C.
R1
PGND
SGND SENSE- SENSE+ SGND
24 25
IN
V
V
DD
CC
26
22
23
27
28
V
IN
C6
C5
C7
Figure 11. Boost LED Driver with PWM Dimming
where Q and Q are the total gate charge of the low-
Power Dissipation
Calculate power dissipation in the MAX16821A–
MAX16821C as a product of the input voltage and the
G1
G2
side and high-side external MOSFETs at V
= 5V, I
GATE
Q
is the supply current, and f
of the LED driver.
is the switching frequency
SW
total V
regulator output current (I ). I
includes
CC
CC
CC
quiescent current (I ) and gate-drive current (I ):
Use the following equation to calculate the maximum
power dissipation (P ) in the chip at a given ambient
Q
DD
DMAX
P
= V x I
IN CC
D
temperature (T ):
A
I
= I + [f
x (Q + Q )]
SW G1 G2
CC
Q
P
= 34.5 x (150 – T ) mW
A
DMAX
Maxim Integrated
│ 21
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
PCB Layout
Use the following guidelines to layout the LED driver.
Selector Guide
DIFFERENTIAL
SET VALUE
(VSENSE+ - VSENSE-)
(V)
DIFFERENTIAL
AMP
GAIN (V/V)
1) Place the IN, V , and V
bypass capacitors close to
DD
CC
PART
the MAX16821A–MAX16821C.
2) Minimize the area and length of the high-current switch-
ing loops.
MAX16821A
MAX16821B
MAX16821C
0.60
0.10
0.03
1
6
3) Place the necessary Schottky diodes that are con-
nected across the switching MOSFETs very close to the
respective MOSFET.
20
4) Use separate ground planes on different layers of
the PCB for SGND and PGND. Connect both of
these planes together at a single point and make this
connection under the exposed pad of the MAX16821A–
MAX16821C.
Pin Configuration
TOP VIEW
5) Run the current-sense lines CSP and CSN very close
to each other to minimize the loop area. Run the sense
lines SENSE+ and SENSE- close to each other. Do
not cross these critical signal lines with power circuitry.
Sense the current right at the pads of the current-sense
resistors. The current-sense signal has a maximum
amplitude of 27.5mV. To prevent contamination of this
signal from high dv/dt and high di/dt components and
traces, use a ground plane layer to separate the power
traces from this signal trace.
21 20 19 18 17 16 15
14
13
SGND 22
I.C.
*EP
OUTV
SENSE- 23
12 RT/SYNC
24
25
26
27
28
SENSE+
SGND
IN
MAX16821A
MAX16821B
MAX16821C
EN
11
10
9
MODE
CLKOUT
SGND
V
V
CC
DD
+
6) Place the bank of output capacitors close to the load.
8
7) Distribute the power components evenly across the
board for proper heat dissipation.
1
2
3
4
5
6
7
8) Provide enough copper area at and around the switch-
ing MOSFETs, inductor, and sense resistors to aid in
thermal dissipation.
TQFN
*EP = EXPOSED PAD.
9) Use 2oz or thicker copper to keep trace inductances
and resistances to a minimum. Thicker copper con-
ducts heat more effectively, thereby reducing thermal
impedance. Thin copper PCBs compromise efficiency
in applications involving high currents.
Chip Information
PROCESS: BiCMOS
Maxim Integrated
│ 22
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Typical Operating Circuit
V
CC
V
LED
R4
ON/OFF
R9
C3
12
R3
13
V
IN
R10
7V TO 28V
14
9
10
11
8
I.C.
OUTV RT/SYNC EN
MODE CLKOUT SGND
N.C.
C11
15
7
6
5
4
3
2
1
OVI
C2
C10
R9
Q1
DH
LX
16 CLP
V
LED
L1
C9
R7
C8
EAOUT
EAN
17
18
C4
MAX16821A
MAX16821B
MAX16821C
R5
BST
DL
LED
STRING
R6
Q2
19 DIFF
20 CSN
21 CSP
C1
D2
N.C.
R2
R1
PGND
SGND SENSE- SENSE+ SGND
24 25
IN
V
V
DD
CC
26
22
23
27
28
V
IN
C6
C5
C7
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
DOCUMENT NO.
21-0140
28 TQFN-EP
T2855+8
Maxim Integrated
│ 23
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MAX16821A/MAX16821B/
MAX16821C
High-Power Synchronous HBLED
Drivers with Rapid Current Pulsing
Revision History
REVISION REVISION
PAGES
CHANGED
DESCRIPTION
NUMBER
DATE
7/07
3/09
1/10
4/14
0
1
2
3
Initial release
—
3, 4
—
Updated Electrical Characteristics table.
—
No /V OPNs; removed automotive references from Applications section
1
Added label above part number indicating parts are not recommended for new
designs and to refer to the MAX20078
4
4/18
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.
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.
2018 Maxim Integrated Products, Inc.
│ 24
相关型号：
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2MHz, High-Brightness LED Drivers with Integrated MOSFET and High-Side Current Sense
MAXIM
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