LT3476EUHF-PBF [Linear]
High Current Quad Output LED Driver; 高电流四路输出LED驱动器
| 型号： | LT3476EUHF-PBF |
| 厂家： | 
Linear |
| 描述： | High Current Quad Output LED Driver |
| 文件： | 总12页 (文件大小：180K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LT3476
High Current
Quad Output LED Driver
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DESCRIPTIO
FEATURES
True Color PWMTM Dimming Delivers Up to 5000:1
The LT®3476 is a quad output DC/DC converter designed
to operate as a constant-current source for driving high
currentLEDs.Afixedfrequency,currentmodearchitecture
results in stable operation over a wide range of supply and
output voltages. A frequency adjust pin allows the user to
program switching frequency between 200kHz and 2MHz
to optimize efficiency and external component size.
■
Dimming Ratio (In Boost Configuration)
■
LED Current Regulation with High-Side Sense
■
VADJ Pin Accurately Sets LED Current Sense
Threshold Over Range 10mV to 120mV
■
Four Independent Driver Channels with 1.5A, 36V
Internal NPN Switches
■
Frequency Adjust Pin: 200kHz to 2MHz
The LT3476 senses output current at the high side of
the LED. High side current sensing is the most flexible
scheme for driving LEDs, allowing buck, boost or buck-
boost configurations. Each current monitor threshold is
trimmed to within 2.5% at the full scale of 105mV. With
an external sense resistor, the user programs the output
current range of each channel. Each of the four regulators
is independently operated by that channel’s PWM signal.
This PWM feature allows precise adjustment of the color
mixing or dimming ratio of the LED source. Dimming
ratios up to 1000:1 can be achieved.
■
High Efficiency Conversion = Up to 96%
■
Open LED Protection
■
Low Quiescent Current
22mA in Active Mode
<10μA in Shutdown Mode
■
Wide V Range: 2.8V to 16V
IN
■
Thermally Enhanced, 38-Lead, 5mm × 7mm
QFN Package
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APPLICATIO S
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are
the property of their respective owners.
■
RGGB Lighting
■
Automotive and Avionic Lighting
■
TFT LCD Backlighting
Constant-Current Sources
■
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TYPICAL APPLICATIO
100W Quad 1A × 8 LED Driver
1000:1 PWM Dimming at 100Hz
PV
IN
33V
CAP1
100mΩ
LED1
CAP2
100mΩ
LED2
CAP3
100mΩ
LED3
CAP4
100mΩ
LED4
PWM
5V/DIV
2.2μF
× 4
UP TO
8 LEDS
1A
1A
1A
1A
0.22μF
0.22μF
0.22μF
0.22μF
I
LED
500mA/DIV
3476 TA02
5μs/DIV
10μH
10μH
10μH
10μH
1.05V
SW1
SW2
SW3
SW4
REF
VADJ1-4
4.99k
100k
CAP1-4
LED1-4
V
IN
2.8V TO 16V
LT3476
V
IN
VC1-4
PWM1-4
SHDN
2.2μF
PWM1-4
SHDN
R
T
GND
21k
1nF
3476 TA01
3476fa
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LT3476
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W W U W
PI CO FIGURATIO
ABSOLUTE AXI U RATI GS
(Note 1)
TOP VIEW
V ............................................................................16V
IN
PWM1-4, SHDN........................................................16V
SW1-4, LED1-4, CAP1-4...........................................36V
38 37 36 35 34 33 32
VC1
LED1
CAP1
CAP2
LED2
1
2
3
4
5
6
7
8
9
31 NC
30 NC
REF, R , V
, V 1-4................................................2V
T
ADJ1-4
C
Operating Temperature Range (Note 2).... –40°C to 85°C
Maximum Junction Temperature........................... 125°C
Storage Temperature Range................... –65°C to 125°C
Lead Temperature ................................................. 300°C
SW1
29
28
SW1
27 SW2
SW2
R
T
26
39
GND
REF
LED3
CAP3
25 SW3
24 SW3
23 SW4
22 SW4
21 NC
CAP4 10
LED4 11
VC4 12
20
NC
13 14 15 16 17 18 19
UHF PACKAGE
38-LEAD (5mm × 7mm) PLASTIC QFN
T
JMAX
= 125°C, θ = 34°C/W
JA
EXPOSED PAD (PIN 39) IS GND
(MUST BE SOLDERED TO PCB)
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ORDER I FOR ATIO
LEAD FREE FINISH
LT3476EUHF#PBF
LEAD BASED FINISH
LT3476EUHF
TAPE AND REEL
LT3476EUHF#TRPBF
TAPE AND REEL
LT3476EUHF#TR
PART MARKING
3476
PACKAGE DESCRIPTION
TEMPERATURE RANGE
–40°C to 85°C
38-Lead (5mm × 7mm) Plastic QFN
PACKAGE DESCRIPTION
PART MARKING
3476
TEMPERATURE RANGE
–40°C to 85°C
38-Lead (5mm × 7mm) Plastic QFN
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
PWM1-4 = 3.3V, VADJ1-4 = REF, CAP1-4 = 5V, unless otherwise noted.
The ● denotes the specifications which apply over the full operating
temperature range, (Note 3) otherwise specifications are at TA = 25°C. SW1-4 = 5V, VIN = 3.3V, SHDN = 3.3V, RT = 21k to GND,
PARAMETER
Operating Range
CONDITIONS
MIN
TYP
MAX
UNITS
V
2.8
16
V
IN
Full-Scale LED Current Monitor Threshold
Over CAP1-4/LED1-4 Operating Range
102
100
105
12
107
108
mV
mV
●
●
One-Tenth Scale LED Current Monitor Threshold
CAP1-4/LED1-4 Operating Range
REF Output Voltage
V
= 100mV
8
16
36
mV
V
ADJ1-4
2.2
10μA ≥ I ≥ –200μA
1.032
1.050
0.003
0.1
1.063
V
REF
REF Line Regulation
2.8V ≤ V ≤ 16V
%/V
IN
Quiescent Current in Shutdown
SHDN = 0V
10
μA
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LT3476
ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating
temperature range, (Note 3) otherwise specifications are at TA = 25°C. SW1-4 = 5V, VIN = 3.3V, SHDN = 3.3V, RT = 21k to GND,
PWM1-4 = 3.3V, VADJ1-4 = REF, CAP1-4 = 5V, unless otherwise noted.
PARAMETER
CONDITIONS
PWM1-4 = 0V
VC1-4 = 0V
MIN
TYP
5.5
22
MAX
UNITS
mA
Quiescent Current Idle
Quiescent Current Active (Not Switching)
Switching Frequency
30
mA
R = 8.25k
1700
850
160
2000
1000
200
2300
1150
240
kHz
kHz
kHz
T
R = 21k
T
R = 140k
T
Nominal R Pin Voltage
1.26
V
T
Maximum Duty Cycle
R = 8.25k (2MHz)
76
90
98
%
%
%
T
R = 21k (1MHz)
84
T
R = 140k (200kHz)
T
V
Input Bias Current
Current Out of Pin
PWM1-4 = 0V
–10
–20
20
0
100
20
nA
nA
μS
MΩ
A
ADJ1-4
VC1-4 Idle Input Bias Current
EAMP GM (ΔI /ΔV
)
CAP-LED
200
3
VC
VC Output Impedance
SW1-4 Current Limit
Static Test
1.5
2
2.5
5
SW1-4 V
I
= 1.3A to GND
SW
350
0.1
35
mV
μA
V
CESAT
SW1-4 Leakage Current
SHDN = 0V
CAP1-4 Overvoltage Protect Threshold
CAP1-4/LED1-4 Idle Input Bias Current
CAP1-4/LED1-4 Input Bias Current
SHDN Input Low Voltage
33.5
PWM1-4 < 0.4V, CAP = LED = 5V
CAP = LED = 5V
100
0.4
nA
μA
V
70
16
50
SHDN Input High Voltage
1.5
1.5
V
SHDN Pin Current
30
μA
V
PWM1-4 Input Low Voltage
PWM1-4 Input High Voltage
PWM1-4 Pin Current
0.4
V
100
μA
Note 3: The LT3476E is guaranteed to meet specifications from 0°C to
85°C. Specifications over the –40°C to 85°C operating temperature range
are assured by design, characterization and correlation with statistical
process controls.
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2: This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
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LT3476
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TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C, unless otherwise noted.
V(CAP-LED) Threshold vs VADJ
Current Limit vs Duty Cycle
Oscillator Frequency vs RT
150
120
90
60
30
0
10000
1000
100
2.5
2
T
= 25°C
A
TYPICAL
MINIMUM
1.5
1
0.5
0
1
10
100
0
0.3
0.6
V
0.9
(V)
1.2
1.5
1000
0
20
40
60
80
100
R
(kΩ)
T
ADJ
DUTY CYCLE (%)
3476 G03
3476 G01
3476 G02
V(CAP-LED) Threshold
vs Temperature, VADJ = VREF
Switch Current Limit
vs Temperature
Oscillator Frequency
vs Temperature
1150
1100
1050
1000
108
107
106
105
2.5
2
R
= 21k
T
1.5
1
950
900
850
104
103
102
0.5
0
55
TEMPERATURE (°C)
105
–45
–20
5
30
80
55
TEMPERATURE (°C)
105
–45
–20
5
30
80
55
TEMPERATURE (°C)
105
–45
–20
5
30
80
3476 G06
3476 G05
3476 G04
Reference Voltage
V(CAP-LED) Threshold vs V(CAP)
Quiescent Current
1.065
1.060
1.055
1.050
1.045
1.040
25
20
15
10
5
108
107
106
105
104
103
102
PWM 1-4 = 3.6V
V
= 1.05V
ADJ
V
= GND, NOT SWITCHING
= 25°C
C
A
T
PWM 1-4 = 0V
0
4
8
12
0
16
55
TEMPERATURE (°C)
105
20
(V)
25
30
35
–45
–20
5
30
80
0
5
10
15
V
V
(V)
IN
CAP
3476 G09
3476 G08
3476 G07
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LT3476
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PI FU CTIO S
VC1, VC4, VC3, VC2, (Pins 1, 12, 13, 38): Error Amplifier
PWM4, PWM3, PWM2, PWM1, (Pins 16, 17, 34, 35):
Signal low turns off the channel—disables the main
switch, reduces quiescent supply current to the channel,
CompensationPin.WhenPWMislow,V pinfloatsexternal
C
compensation capacitor to save state for next cycle.
and causes the V pin for the channel to become high
C
LED1, LED2, LED3, LED4, (Pins 2, 5, 8, 11): Non-
Inverting Input of Current Sense Error Amplifier. Connect
directly to LED current sense resistor terminal. Switcher
impedance.
SHDN (Pin 18): Shutdown Pin. Higher than 1.5V turns
the device on.
will regulate this node to a voltage of 0.1• V below the
ADJ
CAP node. Also connected to CAP node through external
sense resistor and to anode of LED string. Do not allow
this pin to float independently of corresponding CAP input
pin. In applications where the LED current is low and the
NC (Pins 19, 20, 21, 30, 31, 32): Not Used. Connect to
GND (Pin 39) for better heat dissipation.
SW4,SW3,SW2,SW1,(Pins22,23,24,25,26,27,28,29):
SwitchPin.Connecttoexternalinductorandanodeofexternal
Schottky rectifier. Minimize area of SW trace and use a
GND plane to reduce EMI. Adjacent pins of same name
are internally connected.
PV changes widely, connect the output filter capacitor
IN
to LEDn.
CAP1, CAP2, CAP3, CAP4, (Pins 3, 4, 9, 10): Inverting
input of current sense error amplifier. Connect directly to
other terminal of LED current sense resistor. Also con-
nected to output filter capacitor and cathode of external
Schottkyrectifier.CAPgreaterthantheovervoltageprotect
threshold will inhibit switching.
V
(Pin 33): Input Supply Pin. Must be locally by-
IN
passed.
GND (Pin 39): Signal and Power GND. Solder Exposed
Pad directly to ground plane. The Exposed Pad metal of
the package provides both electrical contract to ground
and good thermal contact to the printed circuit board.
It must be soldered to the circuit board for proper
operation.
R (Pin 6): Oscillator Programming Pin. Place resistor
T
connected to GND to program oscillator frequency.
REF: (Pin 7): Reference Output Pin. Connect to V
pin
ADJ
to get full-scale LED current. Connect to resistor dividers
to program V pins to values lower than 1.05V. Bypass
ADJ
to local GND with 0.1μF capacitor.
V
, V
, V
, V
, (Pins 14, 15, 36, 37): LED
ADJ4 ADJ3 ADJ2 ADJ1
CurrentAdjustmentPin.Setsvoltageacrossexternalsense
resistor between CAPn and LEDn. Connect directly to
REF for full-scale threshold of 105mV, or use signal vales
between GND and REF to modulate LED current. V pin
input range is 1.25V maximum.
ADJ
3476fa
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LT3476
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BLOCK DIAGRA
C
BYP
2.2μF
C
FILT
EXTERNAL COMPONENTS
BUCK MODE
R (EXT)
SNS
0.1Ω
0.1μF
10μH
PV
IN
33V
CAP
3, 4, 9, 10
SW
22-29
LED
2, 5, 8, 11
LED ARRAY
+
–
35V
R
SET
A1
ERROR
OVERVOLTAGE
DETECT
2kΩ
AMPLIFIER
V
25k
ADJ
1.25V
14, 15,
36, 37
+
+
–
THERMAL
PWM
Q3
LIMIT
A4
145°C
Q2
DRIVER
–
+
PWM
16, 17,
34, 35
MAIN
Q1
R
Q
A2
SWITCH
S
R
SET1
PWM
COMPARATOR
IDLE MODE
20kΩ
+
V
C
R
SW
0.02Ω
1, 12, 13, 38
∑
A3
–
V
IN
V
IN
3V
RAMP
GENERATOR
33
CURRENT SENSE
AMPLIFIER
ISRC
300μA
REF
7
200kHz
to 2MHz
OSCILLATOR
LT3476 CHANNEL
–
+
V1
Q4
NC
1.05V
19, 20, 21
30, 31, 32
SHUTDOWN
R
T
SHDN
6
18
3476 BD
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OPERATIO
TheLT3476isaconstant-frequency, currentmoderegula-
tor with an internal power switch. Operation can be best
understood by referring to the Block Diagram. At the
start of each oscillator cycle, the SR latch is set, which
turns on the Q1 power switch. A voltage proportional to
the switch current is added to a stabilizing ramp and the
resulting sum is fed into the positive terminal of the PWM
comparator, A2. When this voltage exceeds the level at
the negative input of A2, the SR latch is reset, turning off
the power switch. The level at the negative input of A2 is
set by the error amplifier A1, and is simply an amplified
version of the difference between the voltage across the
The current regulated in R
can be adjusted by chang-
SNS
ing the voltage across R using the V input pin. The
SET
ADJ
amplifier A4 regulates current in Q3 to produce a voltage
across R equal to V . This current flowing through
SET
ADJ
transistor Q3 also produces a voltage across R
one-
SET
tenth the magnitude of the V
input and level shifted
ADJ
to the CAP input. The voltage across R
is limited to
SET
125mV (typ) by the separate 1.25V input on A4.
TheaveragecurrentregulatedinR canalsobeadjusted
SNS
for dimming using the PWM pin. When the PWM pin is
low, switching is disabled and the error amplifier is turned
off so that it does not drive the V pin. Also, all internal
C
internal resistor R and the voltage across the external
SET
loads on the V pin are disabled so that the charge state
C
current sense resistor R . In this manner, the error
of the V pin will be saved on the external compensation
SNS
C
amplifier sets the correct peak switch current level to
capacitor. This feature reduces transient recovery time
because when the PWM input again transitions high, the
demand current for the switch returns to the value just
regulate the current through R . If the error amplifier’s
SNS
output increases, more current is delivered to the output;
if it decreases, less current is delivered.
before PWM last transitioned low.
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LT3476
APPLICATIONS INFORMATION
Layout Hints
protection feature to adequately protect the switch, it is
important that the CAP input sample a voltage at or near
the highest voltage reached by the SW node. As a result,
this OVP function will not provide adequate protection
from open load events in isolated power configurations
such as the 1:1 flyback, since input and output voltage
magnitudes must be summed to obtain the voltage seen
by the switch.
The high speed operation of the LT3476 demands careful
attentiontoboardlayout.Severalitemsareworthyofnote.
The exposed pad of the package is the only GND terminal
of the IC and is also important to thermal management
for the IC, so it is crucial to achieve a good electrical and
thermal contact between the exposed pad and the ground
plane of the board. Also, the Schottky rectifier and the
capacitor between GND at the cathode of the Schottky
are in the high frequency switching path where current
flow is discontinuous. These elements should be placed
so as to minimize the path between SW and the GND of
the IC. To reduce EMI, it is important to minimize the area
of the SW trace. Use a GND plane under SW to minimize
interplane coupling to sensitive signals. To obtain good
currentregulationaccuracyandeliminatesourcesofchan-
nel-to-channel coupling, the CAP and LED inputs of each
channel of the LT3476 should be run as separate lines
back to the terminals of the appropriate sense resistor.
Since there is a small DC input bias current (~50μA) to
the LED and CAP inputs, resistance in series with these
inputs should be minimized, otherwise there will be an
35V
V(CAP)
LED
20V
DISCONNECT
HERE
I(SW)
1A/DIV
0A
3476 F01
20μs/DIV
Figure 1. LED Disconnect Transient
Setting the Switching Frequency
The switching frequency of the LT3476 is set by an ex-
ternal resistor connected between the R pin and GND.
T
offset. Finally, the bypass capacitor on the V supply to
IN
Do not leave this pin open. Also, do not load this pin
with a capacitor. A resistor must always be connected
for proper operation. See Table 1 below or see the Oscil-
the LT3476 should be placed as close as possible to the
V terminal of the device.
IN
lator Frequency vs R graph in the Typical Performance
T
Open-Circuit Protection/Overvoltage Lockout
Characteristics for resistor values and corresponding
The LT3476 has independent internal overvoltage/open-
circuit protection (OVP) for all four converters, sensed
through their respective CAP inputs. The purpose of the
OVP feature is to protect the main switch of the device
from damage. In the boost configuration, if the LEDs are
disconnected from the circuit or fail open, the converter
output voltage at CAP is clamped at the OVP voltage of
35V (typ). Figure 1 shows the transient response of the
step-up converter application with LED1 disconnected.
With LED1 disconnected, the converter switches at cur-
rent limit as the output ramps up to OVP. Upon reaching
the OVP clamp voltage, the converter will switch with a
reduced current limit to regulate the converter output
voltage at the OVP clamp. In the buck mode application
shown in the Block Diagram, should the external supply
for CAP exceed the OVP clamp, then switching will be
inhibited for the converter. In order for the overvoltage
switching frequencies.
Table 1. Switching Frequency vs RT
SWITCHING FREQUENCY (kHz)
R (kΩ)
T
200
400
140
61.9
21
1000
1200
2000
16.2
8.25
In general, a lower switching frequency should be used
whereeitherveryhighorverylowswitchdutycycleopera-
tion is required, or higher efficiency is desired. Selection
of a higher switching frequency will allow use of smaller
value external components and yield a smaller solution
size and profile. Also for high frequency PWM dimming,
a higher switching frequency (shorter switching period)
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LT3476
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APPLICATIO S I FOR ATIO
will give better dimming control since for turning on the
switch, the state of the PWM pin is sampled only during a
narrow time slot at the beginning of each switch period.
Input Capacitor Selection
For proper operation, it is necessary to place a bypass
capacitor to GND close to the V pin of the LT3476. A
IN
1μF, or greater, capacitor with low ESR should be used.
Inductor Selection
A ceramic capacitor is usually the best choice.
The inductors used with the LT3476 should have a satura-
tioncurrentratingof2.5Aorgreater. Forbestloopstability
results, theinductorvalueselectedshouldprovidearipple
currentof350mAormore. Forbuck(step-down)orboost
(step-up) configurations, and using a 21kΩ resistor on
In the buck configuration, the capacitor at the input to the
power converter has large pulsed currents due to the cur-
rent returned through the Schottky diode when the switch
is off. For best reliability, this capacitor should have low
ESR and ESL and meet the ripple current requirement,
R (T ~ 1μs), inductor values from 4.7μH to 10μH are
T
SW
recommended for most applications. In the buck mode,
the inductor value can be estimated using the formula:
IRMS = ISW
•
(1− D) •D
(
)
DBUCK • TSW(μS) • (VCAP − VLED
)
where D is the switch duty cycle. A 2.2μF ceramic type
capacitor placed close to the Schottky and the ground
plane is usually sufficient for each channel.
L(μH) =
,
ΔI
VLED
VCAP
DBUCK
=
Output Capacitor Selection
V
is the voltage across the LED string and V
is the
Theselectionofoutputfiltercapacitordependsontheload
andtheconverterconfiguration,i.e.,step-uporstep-down.
For LED applications, the equivalent resistance of the LED
is typically low, and the output filter capacitor should be
sized to attenuate the current ripple from the inductor to
35mA or less. The following equation is useful to estimate
the required capacitor value:
LED
CAP
input voltage to the converter. In the boost mode, the
inductor value can be estimated using the formula:
DBOOST • TSW(μS) • V
IN
L(μH) =
,
ΔI
VCAP − V
IN
DBOOST
=
TSW
CFILT = 2 •
RLED
VCAP
V
is the input voltage and V
is the voltage across
IN
CAP
the LED string. Table 2 below provides some suggested
A typical filter capacitor value for R
= 5Ω and T
=
SW
LED
components and vendors.
1μs is 0.47μF. For loop stability, consider the output pole
is at the frequency where closed loop gain should be
unity, so the dominant pole for loop compensation will
Table 2. Inductors
VALUE
(μH)
IRMS
(A)
DCR
(Ω)
HEIGHT
(mm)
be established by the capacitor at the V input.
PART NUMBER
Sumida
C
For the LED boost applications, to achieve the same LED
ripple current the required filter capacitor value is about
five times larger than the value calculated above due to
the pulsed nature of the source current. A 2.2μF ceramic
typecapacitorplacedclosetotheSchottkyandtheground
CDRH6D38-100
CDRH5D28-5R3
CDRH73-100
Toko
10
5.3
10
2.0
0.028
0.028
0.072
4.0
3.0
3.4
1.90
1.68
plane of the I is usually sufficient for each channel.
D63CB
10
1.49
2.08
0.042
0.026
3.5
3.5
C
D63CB
4.7
As the output capacitor is subject to high ripple current,
ceramic capacitors are recommended due to their low
ESR and ESL at high frequency.
Cooper-ET
SD25-4R7
4.7
1.80
0.047
2.5
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LT3476
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APPLICATIO S I FOR ATIO
Ceramic type capacitors using X7R dielectric are best for
temperature and DC bias stability of the capacitor value.
All ceramic capacitors exhibit loss of capacitance value
with increasing DC voltage bias, so it may be necessary to
choose a higher value capacitor or larger case size to get
the required capacitance at the operating voltage. Always
check that the voltage rating of the capacitor is sufficient.
Table 3 shows some recommended capacitor vendors.
capacitor that is 1:1000 the value of the compensation
capacitor. In the buck configuration, an additional tech-
nique is available. The filter capacitor between the CAP
node and the LED bottom (see the Typical Application on
the first page) can be moved to between the LED top and
the LED bottom. This circuit change places the inductor
ripple current through the sense resistor, which improves
pulse-skipping behavior. There is usually less than 1%
impact to the current regulation point.
Table 3. Low-ESR Surface Mount Capacitors
VENDOR
Taiyo-Yuden
AVX
TYPE
SERIES
X5R, X7R
X5R, X7R
X5R, X7R
Diode Selection
Ceramic
Ceramic
Ceramic
The Schottky rectifier conducts current during the interval
when the switch is turned off. Select a diode with V rated
R
Murata
for the maximum SW voltage. For boost circuits that may
use the output disconnect feature, the diode should be
rated for at least 40V. It is not necessary that the forward
current rating of the diode equal the switch current limit.
Compensation Design
The LT3476 uses an internal transconductance error
amplifier whose V output compensates the control loop.
The average current I through the diode is a function
F
C
of the switch duty cycle, so select a diode with forward
The external inductor, output capacitor, and compensa-
tion resistor and capacitor determine the loop stability.
The inductor and output capacitor are chosen based on
performance, size and cost. The compensation resistor
current rating of I = 1.5A • (1-D). If using the PWM fea-
F
ture for dimming, it may also be important to consider
diode leakage from the output (especially at hot) during
the PWM low interval. Table 4 has some recommended
component vendors.
and capacitor at V are selected to optimize control loop
C
stability. The component values shown in the typical ap-
plications circuits yield stable operation over the given
range of input-to-output voltages and load currents. For
most buck applications, a small filter capacitor (1μF or
less) across the load is desirable. In this case, a 10nF
Table 4. Schottky Diodes
V
I
V AT 1A
(mV)
R
AVE
F
PART NUMBER
On Semiconductor
MBRM140
(V)
(A)
compensation capacitor at V is usually quite adequate.
A compensation resistor of 5kΩ placed between the V
40
1
550
C
Diodes Inc.
C
output and the compensation capacitor minimizes chan-
nel-to-channel interaction by reducing transient recovery
time. The boost configuration will have a larger output
capacitor, 2.2μF to 10μF.
DFLS140L
40
40
1
1
550
530
B140 HB
Philips Semiconductor
PMEG4010EJ
40
1
540
The following circuit techniques involving the compensa-
tion pin may be helpful where there is a large variation in
programmed LED current, or a large input supply range is
expected.Atlowdutycycles(T lessthan350ns)andlow
ON
average inductor current (less than 500mA), the LT3476
may start to skip switching pulses to maintain output
regulation. Pulse-skipping mode is usually less desirable
because it leads to increased ripple current in the LED.
To improve the onset of pulse-skipping behavior, place a
capacitor between the SW node and the compensation
3476fa
9
LT3476
U
W U U
APPLICATIO S I FOR ATIO
Programming the LED Current
Dimming Control
The LED Current is programmed using an external sense
resistor in series with the load. This method allows flex-
ibility in driving the load (i.e., sensing one of several
parallel strings) while maintaining good accuracy. The
There are two methods to control the current source for
dimming using the LT3476. The first method, popular
with LED applications, uses the PWM pin to modulate the
current source between zero and full current to achieve
a precisely programmed average current. To make this
method of current control more accurate, during the qui-
escent phase the switch demand current is stored on the
V
input sets the voltage regulation threshold across
ADJ
the external sense resistor between 10mV and 120mV.
A 1.05V reference output (REF) is provided to drive the
V node. This feature minimizes recovery time when the
V
pins either through a resistor divider, or connected
C
ADJ
PWMsignalgoeshigh.TheminimumPWMon-oroff-time
will depend on the choice of operating frequency through
directly to REF to give the full-scale threshold of 105mV.
A DAC may also be used to drive the V pins. The V
ADJ
ADJ
the R input pin. For best current accuracy, the minimum
T
pins should not be left open. If the V input is connected
ADJ
PWM low or high time should be at least ten switching
cycles. This guideline has two reasons: first to allow the
outputtoreachsteadystatebeforeshuttingoff,andsecond
because the oscillator is not synchronized to the PWM
signal and there may be as much as one switching cycle
delay from PWM going high to the start of switching. This
delay, however, does not apply to the negative transition
of the PWM signal. The minimum PWM low/high time can
be reduced to five switching cycles if a disconnect switch
is used in the LED current path.
to a voltage higher than 1.25V, the default regulation
threshold across CAP and LED is 125mV (typ). The V
ADJ
pin can also be used in conjunction with a PTC thermistor
to provide overtemperature protection for the LED load
as shown in Figure 2.
1.05V
V
REF
20k
25k
V
ADJ
1-4
The second method of dimming control uses the V pin
ADJ
to linearly adjust the current sense threshold during the
PWM high state. The LED current programming feature
augments the PWM dimming control, possibly increasing
total dimming range by a factor of ten.
470
PTC
3476 F01
Figure 2. Overtemperature Protect Circuit
3476fa
10
LT3476
U
TYPICAL APPLICATIO
5V to 25V Step-Up/Step-Down Driver for 2 Series 350mA LEDs
PV
IN
5V TO 25V
350mA
350mA
10μH
350mA
10μH
350mA
10μH
10μH
LED1
300mΩ
LED2
300mΩ
LED3
LED4
300mΩ
300mΩ
CAP1
CAP2
CAP3
CAP4
2.2μF
2.2μF
2.2μF
2.2μF
SW1
SW2
SW3
SW4
REF
VADJ1-4
1.05V
CAP1-4
LED1-4
V
IN
LT3476
GND
2.8V TO
16V
V
IN
VC1-4
PWM1-4
PWM1-4
SHDN
R
T
SHDN
47k
47nF
2.2μF
3476 TA03
U
PACKAGE DESCRIPTIO
UHF Package
38-Lead Plastic QFN (5mm × 7mm)
(Reference LTC DWG # 05-08-1701)
PIN 1 NOTCH
R = 0.30 TYP
OR 0.35 × 45°
CHAMFER
3.15 ± 0.10
(2 SIDES)
0.75 ± 0.05
5.00 ± 0.10
(2 SIDES)
37 38
0.00
– 0.05
0.70
± 0.05
0.40
±0.10
PIN 1
TOP MARK
(SEE NOTE 6)
5.50
1
2
± 0.05
(2 SIDES)
4.10
± 0.05
(2 SIDES)
3.15
7.00
5.15
± 0.05
± 0.10
± 0.10
(2 SIDES)
(2 SIDES)
(2 SIDES)
PACKAGE
OUTLINE
0.25 ± 0.05
0.50 BSC
5.15 ± 0.05 (2 SIDES)
6.10 ± 0.05 (2 SIDES)
7.50 ± 0.05 (2 SIDES)
0.40
±0.10
0.200 REF
0.25 ± 0.05
R = 0.115
TYP
RECOMMENDED SOLDER PAD LAYOUT
0.50 BSC
0.200 REF
0.00 – 0.05
0.75
± 0.05
BOTTOM VIEW—EXPOSED PAD
(UH) QFN 0205
NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE
OUTLINE M0-220 VARIATION WHKD
2. DRAWING NOT TO SCALE
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE
3. ALL DIMENSIONS ARE IN MILLIMETERS
3476fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
11
LT3476
U
TYPICAL APPLICATIO
Quad Boost 100mA × 8LED Driver
PV
IN
2.8V TO
5V
4.7μH
4.7μH
2.2μF
4.7μH
4.7μH
CAP1 CAP2
CAP3 CAP4
1Ω
1Ω
1Ω
1Ω
2.2μF
2.2μF
2.2μF
2.2μF
LED1
LED2
LED3
LED4
UP TO
8 LEDS
100mA
100mA
100mA
100mA
1.05V
SW1
SW2
SW3
SW4
REF
VADJ1-4
5k
CAP1-4
LED1-4
V
IN
LT3476
GND
V
2.8V TO
5V
IN
100k
VC1-4
PWM1-4
SHDN
2.2μF
PWM1-4
SHDN
R
T
21k
47nF
3476 TA04
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
= 5V, V
LT1618
LTC3454
LTC3474
LT3475
LT3477
LT3479
1.5A, Constant-Current, 1.4MHz, Boost
Converter
V
= 18V, V
= 36V, Dimming = Analog/PWM, I < 1μA, MS10
OUT(MAX) SD
IN(MIN)
IN(MAX)
Package
1A Synchronous Buck-Boost LED Driver
V
= 2.7V, V
= 5.5V, V
= 5.5V, Dimming = 4-Levels of Adj, I < 1μA,
OUT(MAX) SD
IN(MIN)
IN(MAX)
DFN-10 Package
1A (I ), 36V, 2MHz, Step-Down LED
V
= 4V, V
= 36V, V
= 13.5V, 400:1 True Color PWM, I < 1μA,
OUT(MAX) SD
LED
IN(MIN)
IN(MAX)
Driver
TSSOP-16E Package
Dual 1.5A (I ), 36V, 2MHz, Step-Down
V
= 4.0V, V
= 36V, V
= 13.5V, Dimming = 3000:1 True Color PWM,
LED
IN(MIN)
IN(MAX)
OUT(MAX)
OUT(MAX)
OUT(MAX)
LED Driver
I
< 1μA, TSSOP-20E Package
SD
3A, 42V, 3.5MHz Boost, Buck-Boost, Buck
LED Driver
V
= 2.5V, V
= 25V, V
= 40V, Dimming = Analog/PWM, I < 1μA, QFN
SD
IN(MIN)
IN(MAX)
and TSSOP-20E Packages
3A, Full-Featured DC/DC Converter with
Soft-Start and Inrush Current Protection
V
= 2.5V, V = 24V, V
= 40V, Dimming = Analog/PWM, I < 1μA, DFN
IN(MIN)
IN(MAX)
SD
and TSSOP Packages
3476fa
LT 0707 REV A • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
12
●
●
© LINEAR TECHNOLOGY CORPORATION 2006
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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