LINER LTC3452 Synchronous buck-boost main/camera white led driver Datasheet

LTC3452
Synchronous Buck-Boost
MAIN/CAMERA White LED Driver
U
FEATURES
■
■
■
■
■
■
■
■
■
■
■
■
DESCRIPTIO
High Efficiency: ≥85% Over Entire Li-Ion Battery
Range
Wide VIN Range: 2.7V to 5.5V
Independent MAIN/CAMERA Current Control
Up to 425mA Continuous Output Current
Internal Soft-Start
Open/Shorted LED Protection
PWM Brightness Control
LED Current Matching Typically <2.5%
Constant Frequency 1MHz Operation
Low Shutdown Current: 6.5µA
Overtemperature Protection
Small Thermally Enhanced 20-Lead
(4mm × 4mm) QFN Package
The LTC®3452 is a synchronous buck-boost DC/DC converter optimized for driving two banks of white LEDs from
a single Li-Ion battery input. Five parallel LEDs can be
driven at up to 25mA each in the low power LED bank,
while two LEDs can be driven at up to 150mA each (or a
single LED at 300mA) in the high power LED bank. The
regulator operates in either synchronous buck, synchronous boost or buck-boost mode, depending on input
voltage and LED maximum forward voltage. Optimum
efficiency is achieved by sensing which LED requires the
largest forward voltage drop at its programmed current,
and regulating the common output rail for lowest dropout.
Efficiency of 85% can be achieved over the entire usable
range of a Li-Ion battery (2.7V to 4.2V).
Cell Phones
Digital Cameras
PDAs
Portable Devices
Maximum LED current for each LED display is programmable with a single external resistor. Dual enable pins
allow for PWM brightness control in the low power bank
and independent on/off control for the high current bank
(optimal for LED camera flash). In shutdown, the supply
current is only 6.5µA.
U
APPLICATIO S
■
■
■
■
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
A high constant operating frequency of 1MHz allows the
use of a small external inductor. The LTC3452 is offered
in a low profile (0.75mm) thermally enhanced 20-lead
(4mm × 4mm) QFN package.
U
TYPICAL APPLICATIO
5 × 20mA White LED Display + 200mA Camera Light Driver
L
4.7µH
+
Torch and Flash Mode Efficiency
2.2µF
VIN
PVIN SW1
SW2
VOUT
LEDH1
ENH
4.7µF
D1
CAM
95
93
200mA
91
LEDH2
ISETH
EFFICIENCY (%)
VIN
SINGLE
Li-Ion CELL
2.7V TO 4.2V
D2
6.19k
LEDL1
1MHz
BUCK/BOOST
VC
LEDL2
0.1µF
LEDL3
ENL
LEDL4
LTC3452
LEDL5
ISETL
0mA TO
20mA
D3
0mA TO
20mA
D4
0mA TO
20mA
D5
GND
PGND
EXPOSED PAD
89
TORCH MODE AT 100mA
87
85
83
FLASH MODE AT 200mA
81
0mA TO
20mA
79
D6
77
0mA TO
20mA
75
2.7
10.2k
GND
TA = 25°C
(V
– VLEDx) • ILEDx
EFFICIENCY = Σ OUT
VIN • IIN
MAIN DISPLAY LED BACKLIGHT
D1: AOT 2015
D2 TO D6: NICHIA NSCW100
L: COILCRAFT DO3314-472
3452 TA01a
3.1
3.5
3.9 4.3
VIN (V)
4.7
5.1
5.5
3452 TA01b
3452f
1
LTC3452
W W
W
AXI U
U
ABSOLUTE
RATI GS
U
U
W
PACKAGE/ORDER I FOR ATIO
(Note 1)
VIN, PVIN, SW1, SW2, VOUT Voltage ........... – 0.3V to 6V
LEDL1 to LEDL5 Voltage ... – 0.3V to (VOUT + 0.3V) or 6V
LEDH1, LEDH2 Voltage ..... – 0.3V to (VOUT + 0.3V) or 6V
VC, ENL, ENH,
ISETL, ISETH Voltage ............ – 0.3V to (VIN + 0.3V) or 6V
LEDL1 to LEDL5 Current ....................................... 50mA
LEDH1, LEDH2 Current ....................................... 250mA
Operating Temperature Range (Note 2) .. – 40°C to 85°C
Junction Temperature (Note 3) ............................ 125°C
Storage Temperature Range ................ – 65°C to 125°C
VOUT
SW2
PGND
SW1
PVIN
TOP VIEW
20 19 18 17 16
15 VC
VIN 1
14 ENH
ENL 2
13 ISETH
21
ISETL 3
7
8
9 10
GND
LEDH1
6
LEDL5
11 GND
LEDL4
12 LEDH2
LEDL2 5
LEDL3
LEDL1 4
UF PACKAGE
20-LEAD (4mm × 4mm) PLASTIC QFN
TJMAX = 125°C, θJA = 40°C/W
EXPOSED PAD (PIN 21) IS GND, MUST BE SOLDERED TO PCB
ORDER PART NUMBER
LTC3452EUF
UF PART MARKING
3452
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VIN = PVIN = VOUT = 3.6V unless otherwise specified. (Note 2)
PARAMETER
CONDITIONS
Input DC Supply Current
Normal Operation
Shutdown
UVLO
MIN
●
Input Supply Voltage (VIN)
TYP
MAX
UNITS
5.5
V
0.6
6.5
3
1
18
5
mA
µA
µA
2.0
1.87
2.3
V
V
0.54
1.2
V
2.7
2.7V ≤ VIN ≤ 5.5V, RISETL = RISETH = 51.1k, ILEDx = 0 (Note 4)
2.7V ≤ VIN ≤ 5.5V, VENL = VENH = 0V
VIN < UVLO Threshold
VIN Rising
VIN Falling
●
ENL,H DC Threshold for Normal Operation (VIH)
2.7V ≤ VIN ≤ 5.5V, VENL,H Rising
●
ENL,H DC Threshold for Shutdown (ILEDx = 0)
(VIL)
2.7V ≤ VIN ≤ 5.5V, VENL,H Falling
●
0.2
ENL,H Input Current (IIH, IIL)
2.7V ≤ VIN ≤ 5.5V
●
–1
ENL PWM Frequency
2.7V ≤ VIN ≤ 5.5V (Note 5)
●
10
ISETL,H Servo Voltage
RISETL = RISETH = 20k
●
788
780
800
800
812
812
mV
mV
●
730
714
768
768
806
806
mA/mA
mA/mA
1
6
Undervoltage Lockout Threshold
LEDHx Output Current Ratio (ILEDHx/ISETH)
1.6
ILEDHx = 100mA, VLEDHx = 300mV
LEDHx Output Current Matching
(Max – Min)/[(Max + Min)/2] • 100%, ILEDHx = 100mA,
VLEDHx = 300mV, 2.7V ≤ VIN ≤ 5.5V
LEDHx Pin Voltage
ILEDHx = 100mA
0.52
V
1
µA
kHz
250
%
mV
3452f
2
LTC3452
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VIN = PVIN = VOUT = 3.6V unless otherwise specified. (Note 2)
PARAMETER
CONDITIONS
LEDLx Output Current Ratio (ILEDLx/ISETL)
(Note 6)
ILEDLx|MAX = 20mA, VLEDLx = 300mV
PWM Duty Cycle = 6%
MIN
TYP
MAX
UNITS
●
1.8
1.75
2
2
2.16
2.21
mA/mA
mA/mA
●
3.66
3.56
4
4
4.28
4.38
mA/mA
mA/mA
●
7.32
7.12
8
8
8.56
8.76
mA/mA
mA/mA
●
14.72
14.32
16
16
17.04
17.44
mA/mA
mA/mA
●
29.44
28.64
32
32
33.92
34.56
mA/mA
mA/mA
●
58.88
57.92
64
64
67.2
68.16
mA/mA
mA/mA
●
117.12
114.56
128
128
134.4
137.6
mA/mA
mA/mA
●
234.24
229.12
256
245
268.8
272.64
mA/mA
mA/mA
2.5
8
PWM Duty Cycle = 19%
PWM Duty Cycle = 31%
PWM Duty Cycle = 44%
PWM Duty Cycle = 56%
PWM Duty Cycle = 69%
PWM Duty Cycle = 81%
PWM Duty Cycle = 94%
LEDLx Output Current Matching
(Max – Min)/[(Max + Min)/2] • 100%, ILEDLx = 20mA,
VLEDLx = 300mV
LEDLx Pin Voltage
ILEDLx = 20mA
Regulated Maximum VOUT
VLEDLx = VLEDHy = 0V
PMOS Switch RON
Switches A and D at 100mA
210
mΩ
NMOS Switch RON
Switches B and C at 100mA
205
mΩ
Forward Current Limit
Switch A
Reverse Current Limit
Switch D
PMOS Switch Leakage
Switches A and D
NMOS Switch Leakage
Switches B and C
Oscillator Frequency
Soft-Start Time
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: The LTC3452E is guaranteed to meet specifications from 0°C to
70°C. Specifications over the –40°C to 85°C operating temperature range
are assured by design, characterization and correlation with statistical
process controls.
%
130
●
4.35
1000
4.5
1600
mV
4.75
V
2400
mA
200
–1
–1
0.9
mA
µA
1
µA
1
1
1.1
MHz
µs
650
Note 3: TJ is calculated from the ambient temperature TA and power
dissipation PD according to the following formula:
TJ = TA + (PD • θJA°C/W).
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 5: Do not exceed 50kHz PWM frequency in the application.
Note 6: This parameter is tested in a setup which forces conditions
equivalent to those programmed by the indicated duty cycle.
3452f
3
LTC3452
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Shutdown Current vs Temperature
Undervoltage Lockout Threshold
vs Temperature
Shutdown Current vs VIN
12
20
2.5
TA = 25°C
10
14
12
10
8
VIN = 5.5V
VIN = 4.2V
6
4
2
0
–55 –35 –15
8
6
4
VIN = 3.6V
VIN = 2.7V
2.3
UVLO THRESHOLD (V)
16
SHUTDOWN CURRENT (µA)
SHUTDOWN CURRENT (µA)
18
0
2.7
3.1
3.5
3.9 4.3
VIN (V)
4.7
1000
ENABLE THRESHOLDS (mV)
ENABLE THRESHOLDS (mV)
700
600
VIH
500
VIL
400
300
200
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
3452 G03
ISETL,H Servo Voltage
vs Temperature
812
900
808
800
804
700
600
VIH
VIL
500
796
792
788
300
784
200
2.7
3.1
3.5
3.9 4.3
VIN (V)
4.7
5.1
5.5
VIN = 3.6V
RISETL = 10.2k
RISETH = 4.99k
800
400
5 25 45 65 85 105 125
TEMPERATURE (°C)
780
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
3452 G05
3452 G04
3452 G06
Maximum Regulated VOUT
vs Temperature
ISETL,H Servo Voltage vs VIN
812
1.5
–55 –35 –15
5.5
TA = 25°C
VIN = 3.6V
800
5.1
VISETL,H (mV)
1200
900
VIN FALLING
Enable Thresholds vs VIN
Enable Thresholds vs Temperature
1000
1.9
3452 G02
3452 G01
1100
VIN RISING
1.7
2
5 25 45 65 85 105 125
TEMPERATURE (°C)
2.1
4.60
TA = 25°C
4.58
808
VIN = 3.6V
4.56
804
VOUT (V)
VISETL,H (mV)
4.54
800
796
792
4.52
4.50
4.48
4.46
788
4.44
784
780
2.7
4.42
3.1
3.5
3.9 4.3
VIN (V)
4.7
5.1
5.5
3452 G07
4.40
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
3452 G08
3452f
4
LTC3452
U W
TYPICAL PERFOR A CE CHARACTERISTICS
PMOS RDS(ON) vs Temperature
Oscillator Frequency
vs Temperature
NMOS RDS(ON) vs Temperature
325
325
300
300
1050
1040
VIN = 2.7V
1030
275
250
225
VIN = 5.5V
VIN = 4.2V
VIN = 2.7V
250
FREQUENCY (kHz)
VIN = 3.6V
RDS(ON) (mΩ)
RDS(ON) (mΩ)
275
200
VOUT = 3V
VIN = 3.6V
225
200
VIN = 5.5V
VIN = 4.2V
VIN = 5.5V
1020
1010
VIN = 4.2V
1000
980
175
175
150
150
960
125
–55 –35 –15
125
–55 –35 –15
950
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
970
5 25 45 65 85 105 125
TEMPERATURE (°C)
VIN = 3.6V
990
VIN = 2.7V
5 25 45 65 85 105 125
TEMPERATURE (°C)
3452 G11
3452 G10
Output Voltage Ripple
(Front Page Application)
3452 G12
Start-Up Transient
CH1, VOUT
1V/DIV
CH2, ILED
300mA FINAL VALUE
CH3, ENH
1V/DIV
VIN = 3V
VOUT = 3.1V
ILED = 100mA
3452 G13
VIN = 3.6V
ILEDH = 300mA
3452 G14
3452f
5
LTC3452
U
U
U
PI FU CTIO S
VIN (Pin 1): Signal Voltage Input Supply Pin (2.7V ≤ VIN ≤
5.5V). Recommended bypass capacitor to GND is 2.2µF
ceramic or larger. Connect to PVIN (Pin 20).
ENL (Pin 2): Enable Input Pin and PWM Brightness Control
for Low Power LED Bank. Active high. For constant IMAXL
operation, connect the ENL pin to VIN (or any voltage
>1.2V). For ENL voltage <0.2V, all low power bank LED
current source outputs are Hi-Z (if both ENL and ENH are
<0.2V, the part is in shutdown and the input supply current
drops to ~6µA). For brightness control between zero
current and IMAXL, drive the ENL pin with a PWM waveform
of frequency ≥10kHz. The low power LED bank currents
will be equal to a percentage of IMAXL as given in Table 1.
The ENL pin is high impedance and should not be floated.
ISETL (Pin 3): Low Power LED Bank Current Programming
Pin. A resistor to ground programs each low power bank
current source output maximum to ILEDLx|MAX = 256 •
(0.8V/RISETL). Enabled by ENL (Pin 2). PWM brightness
control also via ENL.
ISETH (Pin 13): High Power LED Bank Current Programming Pin. A resistor to ground programs each high power
bank current source output to ILEDHx = 768(0.8V/RISETH).
Enabled by ENH (Pin 14).
ENH (Pin 14): Enable Input Pin for High Power LED Bank.
Active high. The ENH pin is high impedance and should not
be floated.
VC (Pin 15): Compensation Point for the Internal Error
Amplifier Output. Recommended compensation capacitor
to GND is 0.1µF ceramic or larger.
VOUT (Pin 16): Buck-Boost Output Pin. Recommended
bypass capacitor to GND is 4.7µF ceramic.
SW2 (Pin 17): Switching Node Pin. Connected to internal
power switches C and D. External inductor connects
between SW1 and SW2. Recommended value is 4.7µH.
PGND (Pin 18): Power Ground Pin. Connect to GND (Pins
9 and 11).
SW1 (Pin 19): Switching Node Pin. Connected to internal
power switches A and B. External inductor connects
between SW1 and SW2. Recommended value is 4.7µH.
LEDL1 to LEDL5 (Pins 4 to 8): Individual Low Dropout
Current Source Outputs for Low Power LED Bank Current
Biasing. Connect each low power LED between VOUT and
an individual LEDLx pin. Unused LEDLx outputs should be
connected to VOUT.
PVIN (Pin 20): Power Voltage Input Supply Pin. Connect to
VIN (Pin 1).
GND (Pins 9 and 11): Signal Ground Pins. Connect
together and to PGND (Pin 18) and Exposed Pad ground
(Pin 21).
Exposed Pad (Pin 21): Heat Sink Ground. Connect to GND
(Pins 9 and 11) and solder to PCB ground for electrical
contact and rated thermal performance.
LEDH1, LEDH2 (Pins 10, 12): Individual Low Dropout
Current Source Outputs for High Power LED Bank Current
Biasing. Connect each high power LED between VOUT and
an individual LEDHx pin. Unused LEDHx outputs should be
connected to VOUT.
3452f
6
LTC3452
W
BLOCK DIAGRA
VIN
2.7V TO 5.5V
VOUT
VIN
PVIN
SW1
20
1
SW2
19
VOUT
17
16
4
UNDERVOLTAGE
LOCKOUT
UV
OVERTEMPERATURE
PROTECTION
OT
BANDGAP
REFERENCE
SWITCH
A
SWITCH
D
SWITCH
B
SWITCH
C
GATE
DRIVERS
AND
ANTI-CROSSFORWARD CONDUCTION
CURRENT
LIMIT
1.23V
+
5
LEDL2
LED
DETECT
REVERSE
CURRENT
LIMIT
6
+
–
–
1600mA
LEDL1
LED
DETECT
LEDL3
LED
DETECT
LOW
POWER
LED
BANK
200mA
7
AB PWM
COMPARATOR
CD PWM
COMPARATOR
LOGIC
OT
UV
–
8
–
LEDL5
LED
DETECT
1MHz
OSCILLATOR
VC
LEDL4
LED
DETECT
+
+
15
MAIN
ERROR AMP
–
VBIAS
VFB
SAFETY
ERROR AMP
1.23V
–
VOUT
327k
+
+
123k
SOFT-START
CLAMP
1.23V
800mV
ISETL
+
LOW POWER
LED CURRENT
SETTING AMP
IMAXL
256
RISETL
8 LEVELS
EXPONENTIAL
BRIGHTNESS
CONTROL
–
3
SDL
10
SDL
ENL 2
SHUTDOWN
CIRCUIT
ENH 14
800mV
ISETH
+
LEDH1
LED
DETECT
SHUTDOWN
HIGH
POWER
LED
BANK
12
HIGH POWER
LED CURRENT
SETTING AMP
SDH
LEDH2
LED
DETECT
IMAXH
768
13
RISETH
–
SDH
9
GND
11
GND
18
PGND
21
EXPOSED
PAD
3452 BD
3452f
7
LTC3452
U
OPERATIO
Buck-Boost DC-DC Converter
Buck Mode (VIN > VOUT)
The LTC3452 employs an LTC proprietary buck-boost
DC/DC converter to generate the output voltage required
to drive the LEDs. This architecture permits high-efficiency, low noise operation at input voltages above, below
or equal to the output voltage by properly phasing four
internal power switches. The error amp output voltage on
the VC pin determines the duty cycle of the switches. Since
the VC pin is a filtered signal, it provides rejection of
frequencies well below the factory trimmed switching
frequency of 1MHz. The low RDS(ON), low gate charge
synchronous switches provide high frequency pulse width
modulation control at high efficiency. Schottky diodes
across synchronous rectifier switch B and synchronous
rectifier switch D are not required, but if used, do provide
a lower voltage drop during the break-before-make time
(typically 20ns), which improves peak efficiency by typically 1% to 2% at higher loads.
In buck mode, switch D is always on and switch C is always
off. Referring to Figure 2, when the control voltage VC is
above voltage V1, switch A begins to turn on each cycle.
During the off time of switch A, synchronous rectifier
switch B turns on for the remainder of the cycle. Switches
A and B will alternate conducting similar to a typical
synchronous buck regulator. As the control voltage increases, the duty cycle of switch A increases until the
maximum duty cycle of the converter in buck mode
reaches DCBUCK|max given by:
Figure 1 shows a simplified diagram of how the four
internal power switches are connected to the inductor, VIN
= PVIN, VOUT and GND. Figure 2 shows the regions of
operation of the buck-boost as a function of the control
voltage VC. The output switches are properly phased so
transitions between regions of operation are continuous,
filtered and transparent to the user. When VIN approaches
VOUT, the buck-boost region is reached where the conduction time of the four switch region is typically 150ns.
Referring to Figures 1 and 2, the various regions of
operation encountered as VC increases will now be
described.
DCBUCK|max = 100% – DC4SW
where DC4SW equals the duty cycle in % of the “four
switch” range.
DC4SW = (150ns • f) • 100%
where f is the operating frequency in Hz.
Beyond this point the “four switch” or buck-boost region
is reached.
Buck-Boost or Four-Switch Mode (VIN ≈ VOUT)
Referring to Figure 2, when the control voltage VC is above
voltage V2, switch pair AD continue to operate for duty
cycle DCBUCK|max, and the switch pair AC begins to phase
in. As switch pair AC phases in, switch pair BD phases out
accordingly. When the VC voltage reaches the edge of the
buck-boost range at voltage V3, switch pair AC completely
phases out switch pair BD and the boost region begins at
75%
DMAX
BOOST
PVIN
VOUT
20
16
SW1
SW2
19
17
NMOS B
A ON, B OFF
BOOST REGION
PWM CD SWITCHES
DMIN
BOOST
PMOS D
PMOS A
V4 (≈2.1V)
DMAX
BUCK
V3 (≈1.65V)
FOUR SWITCH PWM
BUCK/BOOST REGION
V2 (≈1.55V)
D ON, C OFF
PWM AB SWITCHES BUCK REGION
NMOS C
3452 F01
Figure 1. Simplified Diagram of Internal Power Switches
V1 (≈0.9V)
0%
DUTY
CYCLE
3452 F02
CONTROL
VOLTAGE, VC
Figure 2. Switch Control vs Control Voltage, VC
3452f
8
LTC3452
U
OPERATIO
duty cycle DC4SW. The input voltage VIN where the four
switch region begins is given by:
VIN =
VOUT
1 – (150ns • f)
and the input voltage VIN where the four switch region
ends is given by:
[
]
VIN = VOUT • 1 – (150ns • f)
Boost Mode (VIN < VOUT)
In boost mode, switch A is always on and switch B is
always off. Referring to Figure 2, when the control voltage
VC is above voltage V3, switches C and D will alternate
conducting similar to a typical synchronous boost regulator. The maximum duty cycle of the converter is limited to
88% typical and is reached when VC is above V4.
Forward Current Limit
If the current delivered from VIN through PMOS switch A
exceeds 1600mA (typical), switch A is shut off immediately. Switches B and D are turned on for the remainder of
the cycle in order to safely discharge the forward inductor
current at the maximum rate possible.
Reverse Current Limit
If the current delivered from VOUT backwards through
PMOS switch D exceeds 200mA (typical), switch D is shut
off immediately. Switches A and C are turned on for the
remainder of the cycle in order to safely discharge the
reverse inductor current at the maximum rate possible.
Undervoltage Lockout
To prevent operation of the power switches at high RDS(ON),
an undervoltage lockout is incorporated on the LTC3452.
When the input supply voltage drops below approximately
1.9V, the four power switches and all control circuitry are
turned off except for the undervoltage block, which draws
only a few microamperes.
Overtemperature Protection
If the junction temperature of the LTC3452 exceeds 130°C
for any reason, all four switches are shut off immediately.
The overtemperature protection circuit has a typical hysteresis of 11°C.
Soft-Start
The LTC3452 includes an internally fixed soft-start which
is active when powering up or coming out of shutdown.
The soft-start works by clamping the voltage on the VC
node and gradually releasing it such that it requires 650µs
to linearly slew from 0.9V to 2.1V. This has the effect of
limiting the rate of duty cycle change as VC transitions
from the buck region through the buck-boost region into
the boost region. Once the soft-start times out, it can only
be reset by entering shutdown, or by an undervoltage or
overtemperature condition.
Main Error Amp
The main error amplifier is a transconductance amplifier
with source and sink capability. The output of the main
error amplifier drives a capacitor to GND at the VC pin. This
capacitor sets the dominant pole for the regulation loop.
(See the Applications Information section for selecting the
capacitor value.) The error amp gets its feedback signal
from a proprietary circuit which monitors all 7 LED current
sources to determine which LED to close the regulation
loop on.
Safety Error Amp
The safety error amplifier is a transconductance amplifier
with sink only capability. In normal operation, it has no
effect on the loop regulation. However, if any of the LED
pins open-circuits, the output voltage will keep rising, and
safety error amp will eventually take over control of the
regulation loop to prevent VOUT runaway. The VOUT threshold at which this occurs is approximately 4.5V.
3452f
9
LTC3452
U
OPERATIO
LED Current Setting Amps
The maximum forward current per LED for all LEDs in a
given bank is programmed by a single external resistor to
ground at the corresponding ISETL,H pin according to the
following formulas:
⎛ 0.8 ⎞
⎛ 0.8 ⎞
IMAXL = 256⎜
⎟
⎟ , IMAXH = 768⎜
⎝ RISETH ⎠
⎝ RISETL ⎠
For operation at currents below IMAXL in the low power
bank, refer to the Exponential Brightness Control section
and also to external circuit options given in the Applications Section. For operation at currents below IMAXH in the
high power bank, refer only to the external circuit options
given in the Applications Section.
implemented results in “smoother” brightness and dimming control as perceived by the human eye, which is
logarithmic in nature.
Table 1. Low Power Bank Brightness Control
ENL DUTY CYCLE (% LOGIC HIGH)
LEDLx CURRENT
0% (Logic Low)
0 (Shutdown)
0% < Duty Cycle < 12.5%
1/128 • IMAXL
12.5% < Duty Cycle < 25%
1/64 • IMAXL
25% < Duty Cycle < 37.5%
1/32 • IMAXL
37.5% < Duty Cycle < 50%
1/16 • IMAXL
50% < Duty Cycle < 62.5%
1/8 • IMAXL
62.5% < Duty Cycle < 75%
1/4 • IMAXL
75% < Duty Cycle < 87.5%
1/2 • IMAXL
87.5% < Duty Cycle ≤ 100%
IMAXL
Shutdown Circuit
LED Current Sources
The shutdown circuit monitors the voltages at the ENL,H
pins. Logic high on either/both inputs enables the part and
logic low on both puts the part in shutdown. Since the ENL
pin doubles as a PWM input for LED brightness control, an
output filter in the shutdown circuit is employed to prevent
the part from toggling in and out of shutdown for normal
PWMing of the ENL input when ENH is low. If ENH is low,
the LTC3452 is enabled immediately after a rising edge at
the ENL pin, but waits 200µs (typical) after a falling edge
to enter shutdown. Consequently, a minimum PWM frequency is required for smooth brightness control at currents below IMAXL. The recommended PWM frequency is
10kHz to 50kHz.
Each LED pin is driven by a current source specifically
designed for low dropout. The LTC3452 employs a proprietary architecture that determines which of the seven
LEDs requires the largest forward voltage drop at its
programmed current, and then generates a feedback
voltage based on this one for closing the buck-boost
regulation loop. This results in the lowest output voltage
required for regulating all of the LEDs and thus the highest
LED power efficiency. The voltage present at the LED pin
of the “controlling LED” will be typically 130mV at 20mA
(low power bank) or 250mA at 100mA (high power bank)
of current.
LED Detect Circuit
Exponential Brightness Control
(Low Power LED Bank Only)
The LTC3452 implements an exponential brightness control function for the low power LED bank only in which the
LEDLx current is a function of the PWM duty cycle at the
ENL pin. The LED current will be equal to a fraction of
IMAXL as given in Table 1. As the duty cycle (that the PWM
waveform is logic high) increases linearly, the LED current will increase exponentially from 1/128th IMAXL to
128/128ths IMAXL in seven binary steps. The function
If fewer than five LED outputs in the low power bank and/
or fewer than two LED outputs in the high power bank are
required, unused outputs should be connected to VOUT.
Each LED pin has an internal LED detect circuit that
disables the output current source to save power if an
output is not needed. A small current is employed to detect
the presence of an LED at startup. This current is typically
10µA for the low power bank and 30µA for the high power
bank.
3452f
10
LTC3452
U
W
U U
APPLICATIO S I FOR ATIO
COMPONENT SELECTION
Input Capacitor Selection
Inductor Selection
Since the VIN pin is the supply voltage for the IC it is
recommended to place at least a 2.2µF, low ESR bypass
capacitor to ground. See Table 3 for a list of component
suppliers.
The high frequency operation of the LTC3452 allows the
use of small surface mount inductors. The inductor current ripple is typically set to 20% to 40% of the maximum
average inductor current. For a given ripple the inductance
term in boost mode is:
L>
VIN(MIN)2 •
( VOUT – VIN(MIN) ) • 100%
f • IOUT(MAX ) • %Ripple • VOUT 2
Table 3. Capacitor Vendor Information
SUPPLIER
WEB SITE
AVX
www.avxcorp.com
Sanyo
www.sanyovideo.com
Taiyo Yuden
www.t-yuden.com
TDK
www.component.tdk.com
and in buck mode is:
L>
(
)
VOUT • VIN(MAX ) – VOUT • 100%
f • IOUT(MAX ) • %Ripple • VIN(MAX )
where:
f = operating frequency, Hz
%Ripple = allowable inductor current ripple, %
VIN(MIN) = minimum input voltage, V
VIN(MAX) = maximum input voltage, V
VOUT = output voltage, V
IOUT(MAX) = maximum output load current
For high efficiency, choose an inductor with a high frequency core material, such as ferrite, to reduce core loses.
The inductor should have low ESR (equivalent series
resistance) to reduce the I2R losses, and must be able to
handle the peak inductor current without saturating. Molded
chokes or chip inductors usually do not have enough core
to support peak inductor currents >1A. To minimize radiated noise, use a toroid, pot core or shielded bobbin
inductor. For the white LED application, a 4.7µH inductor
value is recommended. See Table 2 for a list of component
suppliers.
Table 2. Inductor Vendor Information
SUPPLIER
WEB SITE
Coilcraft
www.coilcraft.com
Cooper/Coiltronics
www.cooperet.com
Murata
www.murata.com
Sumida
www.japanlink.com/sumida
Vishay-Dale
www.vishay.com
Output Capacitor Selection
The bulk value of the capacitor is set to reduce the ripple
due to charge into the capacitor each cycle. The steady
state ripple due to charge is given by:
%Ripple _ Boost =
%Ripple _ Buck =
(
)
IOUT(MAX ) • VOUT – VIN(MIN) • 100
COUT • VOUT 2 • f
%
( VIN(MAX) – VOUT ) • 100 %
8 • VIN(MAX ) • f 2 • L • COUT
where COUT = output filter capacitor, F
The output capacitance is usually many times larger in
order to handle the transient response of the converter.
For a rule of thumb, the ratio of the operating frequency to
the unity-gain bandwidth of the converter is the amount
the output capacitance will have to increase from the
above calculations in order to maintain the desired transient response.
The other component of ripple is due to the ESR (equivalent series resistance) of the output capacitor. Low ESR
capacitors should be used to minimize output voltage
ripple. For surface mount applications, Taiyo Yuden, TDK,
AVX ceramic capacitors, AVX TPS series tantalum capacitors or Sanyo POSCAP are recommended. For the white
LED application, a 4.7µF capacitor value is recommended.
See Table 3 for a list of component suppliers.
3452f
11
LTC3452
U
W
U U
APPLICATIO S I FOR ATIO
Optional Schottky Diodes
Schottky diodes across the synchronous switches B and
D are not required, but provide a lower drop during the
break-before-make time (typically 20ns) of the NMOS to
PMOS transition, improving efficiency. Use a Schottky
diode such as an MBRM120T3 or equivalent. Do not use
ordinary rectifier diodes, since the slow recovery times
will compromise efficiency.
The unity-gain frequency of the error amplifier with the
Type I compensation is given by:
fUG =
gm
2 • π • CVC
where gm is the error amp transconductance (typically
1/5.2k) and CVC is the external capacitor to GND at the
VC pin. For the white LED application, a 0.1µF or greater
capacitor value is recommended.
Closing the Feedback Loop
The LTC3452 incorporates voltage mode PWM control.
The control to output gain varies with operation region
(Buck, Boost, Buck/Boost), but is usually no greater than
15. The output filter exhibits a double pole response
given by:
fFILTER _ POLE =
1
Hz
2 • π • L • COUT
where COUT is the output filter capacitor.
The output filter zero is given by:
fFILTER _ ZERO =
1
2 • π • RESR • COUT
Hz
where RESR is the capacitor equivalent series resistance.
A troublesome feature in Boost mode is the right-half
plane zero (RHP), and is given by:
2
fRHPZ
VIN
=
Hz
2 • π • IOUT • L • VOUT
The loop gain is typically rolled off before the RHP zero
frequency.
A simple Type I compensation network can be incorporated to stabilize the loop but at a cost of reduced bandwidth and slower transient response. To ensure proper
phase margin, the loop is required to be crossed over a
decade before the LC double pole.
Paralleling LED Outputs for Higher Current
Two or more LED output pins can be connected together
in parallel to achieve higher output current in fewer than 7
LEDs. For a very high power LED such as a LumiLED, all
7 outputs can be connected in parallel for maximum total
output current, as shown in the back page application of
this data sheet.
Maximum LED Current
As described in the Operation section, the maximum
output LED currents are equal to:
⎛ 0.8 V ⎞
IMAXL = 256⎜
⎟
⎝ RISETL ⎠
and
⎛ 0.8 V ⎞
IMAXH = 768⎜
⎟
⎝ RISETH ⎠
Since the maximum LED current for the low power bank is
25mA, this sets a minimum limit on RISETL of:
⎛ 0.8 V ⎞
RMINL = 256 ⎜
= 8192Ω
⎝ 25mA ⎟⎠
Similarly, for the high power bank:
⎛ 0.8 V ⎞
= 4096Ω
RMINH = 768 ⎜
⎝ 150mA ⎟⎠
In addition, since the maximum continuous output current
for the buck-boost is limited to 425mA, this may impose
higher resistor value minimums if all outputs are used.
3452f
12
LTC3452
U
W
U U
APPLICATIO S I FOR ATIO
Although the LTC3452 can safely provide this current
continuously, the external LED(s) may not be rated for this
high a level of continuous current. Higher current levels in
a single LED are generally reserved for pulsed applications, such as LED camera flash. This is accomplished by
programming a high current with one or both of the RISET
resistors and pulsing the appropriate enable pin or pins as
shown in the back page application.
VIN
Varying LED Brightness Linearly
Continuously variable LED brightness control can be
achieved by interfacing directly to one or both of the ISET
pins. Figure 3 shows four such methods employing a
voltage DAC, a current DAC, a simple potentiometer or a
PWM input applied to the ISETL pin for controlling the low
power bank LED currents. These four techniques can be
similarly applied to the ISETH pin for controlling the high
power bank LED currents.
VOUT
ENL
VIN
VOUT
ENL
LEDL1
LTC3452
ISETL
VOLTAGE
DAC
ISETL
LEDL5
0.8V – VDAC
ILED = 256
RSET
RSET ≥ RMINL
LEDL1
LTC3452
ILED = 256 • IDAC
0.8V
RMINL
IDAC ≤
CURRENT
DAC
VDAC
LEDL5
(3a)
VIN
(3b)
VOUT
ENL
VIN
VOUT
ENL
LEDL1
LTC3452
ISETL
ISETL
LEDL5
0.8V
ILED = 256
RMINL + RPOT
RMINL
LEDL1
LTC3452
RSET
100
RSET ≥ RMINL
LEDL5
ILED = 256
VPWM
RPOT
= 256
1µF
0.8V – VPWM
RSET
0.8V – (DC% • VDVCC)
RSET
DVCC
fPWM ≥ 10kHz
(3c)
(3d)
3452 F03
Figure 3. Additional Brightness Control Methods: (3a) Using Voltage DAC,
(3b) Using Current DAC, (3c) Using Potentiometer, (3d) Using PWM Input
3452f
13
LTC3452
U
W
U U
APPLICATIO S I FOR ATIO
Unused Outputs
If fewer than 7 LED pins are to be used, unused LEDx pins
should be connected to VOUT. The LTC3452 senses which
current source outputs are not being used and shuts off
the corresponding output currents to save power. A small
trickle current (10µA: low power bank, 30µA: high power
bank) is still applied to unused outputs to detect if a white
LED is later switched in and also to distinguish unused
outputs from used outputs during start-up.
LED Failure Modes
If an individual LED fails as a short circuit, the current
source biasing it is shut off to save power. This is the same
operation as described previously (if the output were
initially designated unused at power-up by connecting its
LEDx pin to VOUT). Efficiency is not materially affected.
If an individual LED fails as an open circuit, the control loop
will initially attempt to regulate off of its current source
feedback signal, since it will appear to be the one requiring
the largest forward voltage drop to run at its programmed
current. This will drive VOUT higher. As the open circuited
LED will never accept its programmed current, VOUT must
be voltage-limited by means of a secondary control loop.
The LTC3452 limits VOUT to 4.5V in this failure mode. The
other LEDs will still remain biased at the correct programmed current but the overall circuit efficiency will
decrease.
3452f
14
LTC3452
U
PACKAGE DESCRIPTIO
UF Package
20-Lead Plastic QFN (4mm × 4mm)
(Reference LTC DWG # 05-08-1710)
0.70 ±0.05
4.50 ± 0.05
3.10 ± 0.05
2.45 ± 0.05
(4 SIDES)
PACKAGE OUTLINE
0.25 ±0.05
0.50 BSC
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
BOTTOM VIEW—EXPOSED PAD
4.00 ± 0.10
(4 SIDES)
0.75 ± 0.05
R = 0.115
TYP
PIN 1 NOTCH
R = 0.30 TYP
19 20
0.38 ± 0.10
PIN 1
TOP MARK
(NOTE 6)
1
2
2.45 ± 0.10
(4-SIDES)
(UF20) QFN 10-04
0.200 REF
0.00 – 0.05
0.25 ± 0.05
0.50 BSC
NOTE:
1. DRAWING IS PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220
VARIATION (WGGD-1)—TO BE APPROVED
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm 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
3452f
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3452
U
TYPICAL APPLICATIO
4 × 20mA White LED Display + 2 × 150mA Camera Light Driver
L
4.7µH
VIN
3V TO 5.5V
2.2µF
VIN
PVIN SW1
SW2
150mA
150mA
D1
D2
VOUT
4.7µF
ENH
ENH
ISETH
LEDH1
4.02k
LEDH2
D3
LOW
POWER
LED
BANK
LEDL1, 20mA
1MHz
BUCK/BOOST
VC
0.1µF
ENL
D4
LEDL2, 20mA
D5
LEDL3, 20mA
ENL
LEDL4, 20mA
LTC3452
D6
LEDL5, UNUSED
ISETL
10.2k
GND
GND
PGND
EXPOSED PAD
D1, D2: AOT 2015
D3-D6: NICHIA NSCW100
L: COILCRAFT D03314-472
3452 TA02a
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1618
Constant Current, Constant Voltage 1.4MHz, High Efficiency
Boost Regulator
VIN: 1.6V to 18V, VOUT(MAX) = 34V, IQ = 1.8mA, ISD = <1µA,
MS10 Package/EDD Package
LT1930/LT1930A 1A (ISW), 1.2MHz/2.2MHz, High Efficiency Step-Up
DC/DC Converter
VIN: 2.6V to 16V, VOUT(MAX) = 34V, IQ = 4.2mA/5.5mA, ISD = <1µA,
ThinSOT Package
LT1932
Constant Current, 1.2MHz, High Efficiency White LED
Boost Regulator
VIN: 1V to 10V, VOUT(MAX) = 34V, IQ = 1.2mA, ISD = <1µA,
ThinSOT Package
LT1937
Constant Current, 1.2MHz, High Efficiency White LED
Boost Regulator
VIN: 2.5V to 10V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD = <1µA,
ThinSOT Package/SC70 Package
LTC3205
High Efficiency, Multi-Display LED Controller
VIN: 2.8V to 4.5V, VOUT(MAX) = 6V, IQ = 50µA, ISD = <1µA,
QFN-24 Package
LTC3215
700mA Low Noise High Current LED Charge Pump
VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300µA, ISD = <2.5µA,
DFN Package
LTC3216
1A Low Noise High Current LED Charge Pump with
Independent Flash/Torch Current
VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300µA, ISD = <2.5µA,
DFN Package
LTC3440/
LTC3441
600mA/1.2A IOUT, 2MHz/1MHz, Synchronous Buck-Boost
DC/DC Converter
VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 25µA/50µA, ISD = <1µA,
MS-10 Package/DFN Package
LTC3443
600mA/1.2A IOUT, 600kHz, Synchronous Buck-Boost
DC/DC Converter
VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 28µA, ISD = <1µA,
DFN Package
LTC3453
500mA Synchronous Buck-Boost High Power White
LED Driver
VIN: 2.7V to 5.5V, VOUT(MAX) = 4.5V, IQ = 600µA, ISD = 6µA,
QFN-16 Package
LTC3454
1A Synchronous Buck-Boost High Power White
LED Driver
VIN: 2.7V to 5.5V, VOUT(MAX) = 5.15V, IQ = 825µA, ISD = 0µA,
DFN Package
LT3465/LT3465A Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD = <1µA,
Boost Regulator with Integrated Schottky Diode
ThinSOT Package
LT3466
Dual Constant Current, 2MHz, High Efficiency White LED
Boost Regulator with Integrated Schottky Diode
VIN: 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD = <16µA,
DFN Package
LT3479
3A, Full Featured DC/DC Converter with Soft-Start and
Inrush Current Protection
VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 6.5mA, ISD = <1µA,
DFN Package/TSOPP Package
3452f
16
Linear Technology Corporation
LT 0406 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006