SEMTECH SC656EVB

SC656
Backlight Driver for 7 LEDs with
Charge Pump and PWM Control
POWER MANAGEMENT
Features
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Description
Input supply voltage range — 2.9V to 5.5V
Charge pump modes — 1x, 1.5x and 2x
PWM dimming control with low pass filter provides
DC backlight current (not pulsed)
Two independently configurable backlight banks
PWM frequency range — 200Hz to 50kHz
Seven adjustable current sinks — 500µA to 25mA
Backlight current accuracy ±1.5% typical
Backlight current matching ±0.5% typical
LED float detection
Charge pump frequency — 250kHz
Low shutdown current — 0.1µA typical
Ultra-thin package — 3 x 3 x 0.6(mm)
Fully WEEE and RoHS compliant, and halogen free.
The SC656 is a high efficiency charge pump LED driver
using Semtech’s proprietary charge pump technology.
Performance is optimized for use in single-cell Li-ion
battery applications.
The device provides backlight current using up to seven
matched current sinks. The load and supply conditions
determine whether the charge pump operates in 1x, 1.5x,
or 2x mode. The seven backlights can be configured as a
single group or split into two independent banks by
setting the state of the BANK2 and BANK1 pins. If only one
bank is needed, the BANK2, BANK1, ENS and ISETS pins
must be grounded.
The maximum current per LED in each bank is set by a
resistor connected to ISETM or ISETS. LED current can be
set between 500µA and 25mA. Backlight current is varied
by applying a pulse-width modulated (PWM) signal to the
ENM pin for the main LED bank and the ENS pin for the
sub LED bank. The resulting DC current in each LED (IBL) is
equal to the maximum current setting multiplied by the
duty cycle of the PWM signal. During PWM operation, a
low-pass filter is used to develop a DC current through the
LED. The resulting power conversion is more efficient than
comparable pulsed current solutions. Backlight fading is
initiated when the duty cycle is changed.
Applications
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Cellular phones, smart phones, and PDAs
LCD display modules
Portable media players
Digital cameras
Personal navigation devices
Display/keypad backlighting and LED indicators
The 3 x 3 (mm) package and minimal number of small
external components make the SC656 an ideal backlight
driver solution for space-limited designs.
Typical Application Circuit
VBAT = 2.9V to 5.5V
SC656
IN
CIN
2.2µF
COUT
2.2µF
GND
RISETM
RISETS
ISETM
BL1
ISETS
BL2
BANK2
BANK1
September 24, 2009
OUT
BL3
BL4
BL5
PWM Signal
ENM
BL6
PWM Signal
ENS
BL7
C1+ C1-
C2+ C2-
C1
2.2µF
C2
2.2µF
© 2009 Semtech Corporation
US Patents: 6,504,422; 6,794,926
SC656
ENS
1
IN
OUT
C2+
C1+
C1-
Ordering Information
ENM
Pin Configuration
20
19
18
17
16
15
C2-
2
14
GND
ISETM
3
13
BANK2
ISETS
4
12
BANK1
11
BL7
T
6
7
8
9
10
BL3
BL4
BL5
BL6
5
BL2
BL1
TOP VIEW
Device
Package
SC656ULTRT(1)(2)
MLPQ-UT-20 3×3
SC656EVB
Evaluation Board
Notes:
(1) Available in tape and reel only. A reel contains 3,000 devices.
(2) Lead-free package only. Device is WEEE and RoHS compliant, and
halogen free.
MLPQ-UT-20; 3x3, 20 LEAD
θJA = 35°C/W
Marking Information
656
yyww
xxxx
yyww = Date Code
xxxx = Semtech Lot Number
SC656
Absolute Maximum Ratings
Recommended Operating Conditions
IN, OUT (V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +6.0
Ambient Temperature Range (°C). . . . . . . . . . -40 ≤ TA ≤ +85
C1+, C2+ (V). . . . . . . . . . . . . . . . . . . . . . . . -0.3 to (VOUT + 0.3)
Input Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9 to 5.5
Pin Voltage — All Other Pins (V). . . . . . . . . -0.3 to (VIN + 0.3)
Output Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . 2.5 to 5.25
OUT Short Circuit Duration. . . . . . . . . . . . . . . . . . Continuous
Voltage Difference between any two LEDs (V) . . . ∆VF ≤ 1.0 (2)
ESD Protection Level(1) (kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Thermal Information
Thermal Resistance, Junction to Ambient(3) (°C/W) . . . . 40
Maximum Junction Temperature (°C). . . . . . . . . . . . . . . +150
Storage Temperature Range (°C) . . . . . . . . . . . . -65 to +150
Peak IR Reflow Temperature (10s to 30s) (°C) . . . . . . . +260
Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters
specified in the Electrical Characteristics section is not recommended.
NOTES:
(1) Tested according to JEDEC standard JESD22-A114-B.
(2) ∆VF(max) = 1.0V when VIN = 2.9V, higher VIN supports higher ∆VF(max)
(3) Calculated from package in still air, mounted to 3 x 4.5(in), 4 layer FR4 PCB per JESD51 standards.
Electrical Characteristics
Unless otherwise noted, TA = +25°C for Typ, -40°C to +85°C for Min and Max, TJ(MAX) = 125°C, VIN = 3.7V, CIN= COUT = C1= C2= 2.2µF, (ESR = 0.03Ω),
500µA < IFS_BL < 25mA, Duty Cycle of PWM = 100%, All 7 LEDs connected and enabled as a single bank.
Parameter
Shutdown Current
Quiescent Current
Symbol
Conditions
IQ(OFF)
IQ
Min
Typ
Max
Units
TA = 25°C
0.1
2
µA
1x mode, VIN = 3.7V, 7 LEDs at 1mA on main bank,
PWM duty cycle = 5%, RISETM = 4.99kΩ
2.5
1.5x mode, VIN = 3.2V, 7 LEDs at 1mA on main bank,
PWM duty cycle = 5%, RISETM = 4.99kΩ
2.8
2x mode, VIN = 2.9V, 7 LEDs at 1mA on main bank,
PWM duty cycle = 5%, RISETM = 4.99kΩ
3.0
mA
IOUT(MAX)
VIN > 3.0V, sum of all active LED currents,
VOUT(MAX) = 4.2V
175
Backlight Current Setting
IFS_BL
PWM duty cycle = 100%, 200kΩ ≥ RISETX ≥ 4kΩ
0.5
Current Gain(1)
IGAIN
Gain from IISETX to IFS_BL
100
A/A
VIN - ISET
Voltage across RISETX
1
V
Backlight Current Matching(2)
IBL-BL
IFS_BL = 12mA, Duty = 100%
Backlight Current Accuracy
IBL_ACC
IFS_BL = 12mA, Duty = 100%
EN/PWM Input Frequency
fEN/PWM
Duty-cycle percentage changes linearly with IFS_BL
ENM, ENS Minimum High
tHIGH_MIN
Maximum Total Output Current
Current Set Voltage
-3.5
mA
25
±0.5
+3.5
±1.5
0.2
%
%
50
1
mA
kHz
µs
SC656
Electrical Characteristics (continued)
Parameter
Symbol
Conditions
Current Transition Settling Time
ts
Duty cycle change from 100% to 50%(1)(4)
ENM/ENS Low Time
tLT
Time that voltage on ENM or ENS can be low without disabling the device
1x Mode to 1.5x Mode
Falling Transition Voltage
V TRANS1x
IOUT = 70mA, IBLn = 10mA, VOUT = 3.2V
3.26
V
1.5x Mode to 2x Mode
Falling Transition Voltage
V TRANS1.5x
IOUT = 70mA, IBLn = 10mA, VOUT = 4.0V(3)
2.90
V
Current Sink Off-State
Leakage Current
IBLn(off )
VIN = VBLn = 4.2V
0.1
Charge Pump Frequency
fPUMP
VIN = 3.2V
250
OUT pin shorted to GND
50
Output Short Circuit Current Limit
IOUT(SC)
VOUT > 2.5V
400
Increasing VIN — lockout released
2.4
V
500
mV
Under Voltage Lockout Threshold
UVLO Hysteresis
Min
Typ
Max
Units
0.5
s
5
ms
1
µA
kHz
mA
VUVLO
VUVLO-HYS
Over-Voltage Protection
VOVP
OUT pin open circuit, VOUT = VOVP, VIN rising threshold
5.7
Over-Temperature
TOT
Rising Temperature
165
°C
25
°C
OT Hysteresis
TOT-HYS
6.0
V
Digital Logic Pins — ENM, ENS, BANK2, AND BANK1
Input High Threshold
VIH
VIN = 5.5V
1.4
V
Input Low Threshold
VIL
VIN = 2.9V
0.4
V
Input High Current
IIH
VIN = 5.5V
1
µA
Input Low Current
IIL
VIN = 5.5V
1
µA
Notes:
(1) Guaranteed by design
(2) Current matching equals ± [IBL(MAX) - IBL(MIN] / [IBL(MAX) + IBL(MIN)].
(3) Test voltage is VOUT = 4.0V — a relatively extreme LED voltage used to force a transition during test. Typically VOUT = 3.2V for white LEDs.
(4) The settling time is affected by the magnitude of change in the PWM duty cycle.
SC656
Typical Characteristics
CIN = COUT= C1 = C2 = 2.2µF — 0603 size (1608 metric)
Backlight Accuracy (7 LEDs) — 25mA Each
VOUT = 3.56V, IOUT = 175mA, 25°C
8
6
6
4
4
2
MAX LED
0
MIN LED
-2
VOUT = 3.56V, IOUT = 175mA, 25°C
8
Backlight Matching (%�
Backlight Accuracy (%�
Backlight Matching (7 LEDs) — 25mA Each
-4
2
0
-2
-4
-6
-6
-8
4.2
3.9
3.6
VIN (V�
3.3
3
-8
2.7
4.2
Backlight Accuracy (7 LEDs) — 12mA Each
8
4
MAX LED
2
MIN LED
0
-2
-4
3.3
3
2.7
VOUT = 3.42V, IOUT = 84mA, 25°C
4
2
0
-2
-4
-6
-6
4.2
3.9
3.6
VIN (V�
3.3
3
-8
2.7
4.2
Backlight Accuracy (7 LEDs) — 5mA Each
8
VOUT = 3.28V, IOUT = 35mA, 25°C
8
3.6
VIN (V�
3.3
3
2.7
VOUT = 3.28V, IOUT = 35mA, 25°C
6
Backlight Matching (%�
4
MAX LED
2
0
MIN LED
-2
-4
-6
-8
3.9
Backlight Matching (7 LEDs) — 5mA Each
6
Backlight Accuracy (%�
VIN (V�
6
Backlight Matching (%�
Backlight Accuracy (%�
6
-8
3.6
Backlight Matching (7 LEDs) — 12mA Each
VOUT = 3.42V, IOUT = 84mA, 25°C
8
3.9
4
2
0
-2
-4
-6
4.2
3.9
3.6
VIN (V�
3.3
3
2.7
-8
4.2
3.9
3.6
VIN (V�
3.3
3
2.7
SC656
Typical Characteristics (continued)
Backlight Current (7 LEDs) — 25mA Each
Charge Pump Efficiency (7 LEDs) — 25mA Each
100
VOUT = 3.56V, IOUT = 175mA, 25°C
350
Battery Current (mA�
90
Efficiency (%�
VOUT = 3.56V, IOUT = 175mA, 25°C
400
80
70
60
300
250
200
150
50
4.2
3.9
3.6
VIN (V�
3.3
3
2.7
100
4.2
VOUT = 3.42V, IOUT = 84mA, 25°C
3.6
VIN (V�
3.3
3
2.7
Backlight Current (7 LEDs) — 12mA Each
Charge Pump Efficiency (7 LEDs) — 12mA Each
100
3.9
VOUT = 3.42V, IOUT = 84mA, 25°C
135
125
Battery Current (mA�
90
Efficiency (%�
115
80
105
70
60
50
95
85
4.2
3.9
3.6
VIN (V�
3.3
3
75
2.7
4.2
Charge Pump Efficiency (7 LEDs) — 5mA Each
100
VOUT = 3.28V, IOUT = 35mA, 25°C
80
VIN (V�
3.3
3
2.7
VOUT = 3.28V, IOUT = 35mA, 25°C
70
Battery Current (mA�
Efficiency (%�
3.6
Backlight Current (7 LEDs) — 5mA Each
90
80
70
60
50
3.9
60
50
40
30
4.2
3.9
3.6
VIN (V�
3.3
3
2.7
20
4.2
3.9
3.6
VIN (V�
3.3
3
2.7
SC656
PWM Accuracy — 4.2V
PWM Accuracy — 4.2V
Typical Characteristics (continued)
Percentage of Maximum IBL — 4.2V
PWM Accuracy — 4.2V
20
VIN = 4.2V, RISET = 4.99kΩ, Calculated IBL = (100/RISET) x Duty Cycle
100
Percentage of Maximum IBL (%�
Calculate� IBL (mA�
16
12
50kHz
8
32kHz
4
VIN = 4.2V, RISET = 4.99kΩ
80
60
50kHz
40
32kHz
20
200Hz
200Hz
0
4
0
8
12
Mea�ure� IBL (mA�
PWM Accuracy — 4.2V
16
0
20
0
PWM Accuracy — 3.7V
20
100
Percentage of Maximum IBL (%�
Calculate� IBL (mA�
16
12
50kHz
32kHz
4
0
0
200Hz
4
8
12
Mea�ure� IBL (mA�
60
50kHz
40
32kHz
20
0
100
Percentage of Maximum IBL (%�
Calculate� IBL (mA�
12
0
32kHz
50kHz
200Hz
0
4
8
12
Mea�ure� IBL (mA�
200Hz
40
60
PWM Duty Cycle (%�
20
PWM80 Accuracy
100 — 4.2V
Percentage of Maximum IBL — 2.9V
16
4
100
80
0
16 Accuracy
20
PWM
— 4.2V
VIN = 2.9V, RISET = 4.99kΩ, Calculated IBL = (100/RISET) x Duty Cycle
8
80
VIN = 3.7V, RISET = 4.99kΩ
PWM Accuracy — 2.9V
20
PWM Accuracy — 4.2V
Percentage of Maximum IBL — 3.7V
VIN = 3.7V, RISET = 4.99kΩ, Calculated IBL = (100/RISET) x Duty Cycle
8
40
60
PWM Duty Cycle (%�
20
16
20
VIN = 2.9V, RISET = 4.99kΩ
80
50kHz
60
40
32kHz
20
0
0
200Hz
20
40
60
80
100
PWM Duty Cycle (%�
SC656
Typical Characteristics (continued)
Ripple — 1X Mode
Ripple — 1X Mode
VIN=4.2V, RISET = 4kΩ, 7 Backlights — 25 mA each, 25°C
VIN=4.2V, RISET = 5.56kΩ, 7 Backlights — 18 mA each, 25°C
VIN (100mV/div)
VIN (100mV/div)
VOUT (100mV/div)
VOUT (100mV/div)
Time (10µ������
s�����
/div)
Time (10µ������
s�����
/div)
Ripple — 1.5X Mode
Ripple — 1.5X Mode
VIN=3.2V, RISET = 4kΩ, 7 Backlights — 25 mA each, 25°C
VIN=3.2V, RISET = 5.56kΩ, 7 Backlights — 18 mA each, 25°C
VIN (100mV/div)
VIN (100mV/div)
VOUT (100mV/div)
VOUT (100mV/div)
Time (10µ������
s�����
/div)
Time (10µ������
s�����
/div)
Ripple — 2X Mode
Ripple — 2X Mode
VIN=2.9V, RISET = 4kΩ, 7 Backlights — 25 mA each, 25°C
VIN=2.9V, RISET = 5.56kΩ, 7 Backlights — 18 mA each, 25°C
VIN (100mV/div)
VIN (100mV/div)
VOUT (100mV/div)
VOUT (100mV/div)
Time (10µ������
s�����
/div)
Time (10µ������
s�����
/div)
SC656
Typical Characteristics (continued)
Start-up — 0% to 100%
Start-up — 0% to 50%
VIN = 3.7V, 0 to 100% duty cycle, RISET = 4.99kΩ, no PWM
VIN = 3.7V, 0 to 50% duty cycle, RISET = 4.99kΩ, fPWM = 32kHz
20mA
10mA
IBL (10.0mA/div)
IBL (10.0mA/div)
0mA—
0mA—
VPWM (2V/div)
VPWM (2V/div)
0V—
50%
0V—
100%
Time (200m������
s�����
/div)
Time (200m������
s�����
/div)
IBL Settling Time — 100% to 50%
IBL Settling Time — 50% to 100%
VIN = 3.7V, RISET = 4.99kΩ, fPWM = 32kHz
VIN = 3.7V, RISET = 4.99kΩ, fPWM = 32kHz
20mA
IBL (10.0mA/div)
10mA
20mA
0mA—
0mA—
VPWM (2V/div)
VPWM (2V/div)
0V—
50%
10mA
IBL (10.0mA/div)
100%
0V—
100%
50%
Time (200m������
s�����
/div)
Time (200ms/div)
DC Backlight Current — 32kHz PWM
DC Backlight Current — 200Hz PWM
VIN = 3.7V, 50% duty cycle, RISET = 4.99kΩ, IBL = 10mA
VIN = 3.7V, 50% duty cycle, RISET = 4.99kΩ, IBL = 10mA
IBL (10.0mA/div)
IBL (10.0mA/div)
0mA—
0mA—
VPWM (2V/div)
VPWM (2V/div)
0V—
0V—
Time (20�����
s�����
/div)
Time (1m������
s�����
/div)
SC656
Pin Descriptions
Pin #
Pin Name
Pin Function
1
ENS
2
IN
3
ISETM
Current setting pin — connect a resistor between ISETM and IN to set the main bank LED
current.
4
ISETS
Current setting pin — connect a resistor between ISETS and IN to set the sub bank LED
current.
5
BL1
Current sink output for backlight LED 1 — leave this pin open if unused
6
BL2
Current sink output for backlight LED 2 — leave this pin open if unused
7
BL3
Current sink output for main backlight LED 3 — leave this pin open if unused
8
BL4
Current sink output for main backlight LED 4 — leave this pin open if unused
9
BL5
Current sink output for main backlight LED 5 — leave this pin open if unused
10
BL6
Current sink output for main backlight LED 6 — leave this pin open if unused
11
BL7
Current sink output for main backlight LED 7 — leave this pin open if unused
12
BANK 1
Logic input that, along with BANK 2, controls the configuration of the main and sub bank functions. See
application information section for details.
13
BANK 2
Logic input that, along with BANK 1, controls the configuration of the main and sub bank functions. See
application information section for details.
14
GND
Ground pin
15
C2-
Negative connection to bucket capacitor 2
16
C1-
Negative connection to bucket capacitor 1
17
C1+
Positive connection to bucket capacitor 1
18
C2+
Positive connection to bucket capacitor 2
19
OUT
Charge pump output — all LED anode pins are connected to this pin.
20
ENM
Enable pin for the main display LED bank — also used as the PWM dimming control input for this bank
Enable pin for sub display LED bank — also used as the PWM dimming control input for this bank
Battery voltage input
10
SC656
Block Diagram
C1+ C1- C2+ C217
IN
2
GND
14
VIN
16
18
15
Fractional Charge Pump
(1x, 1.5x, 2x)
VOUT
19
OUT
5
BL1
6
BL2
7
BL3
8
BL4
9
BL5
10
BL6
11
BL7
Oscillator
ENM
ISETM
20
3
Control
Interface,
Level
Converter,
Digital LPF
Current
Setting
Block
4
Current
Setting
Block
ENS
1
Control
Interface,
Level
Converter,
Digital LPF
BANK2
13
BANK1
12
ISETS
LED Bank
Configuration
Logic
11
SC656
Applications Information
General Description
This design is optimized for handheld applications supplied from a single Li-Ion cell and includes the following
key features:
•
•
•
A high efficiency fractional charge pump that
supplies power to all LEDs
Seven matched current sinks that control LED
backlighting current, providing 500µA to 25mA
per LED
Two LED bank options with independent current
settings and enable pins with PWM control of
LED brightness.
High Current Fractional Charge Pump
The backlight outputs are supported by a high efficiency,
high current fractional charge pump output at the OUT
pin. The charge pump multiplies the input voltage by 1,
1.5, or 2 times. The charge pump switches at a fixed frequency of 250kHz in 1.5x and 2x modes and is disabled in
1x mode to save power and improve efficiency.
The mode selection circuit automatically selects the 1x,
1.5x, or 2x mode based on circuit conditions such as LED
voltage, input voltage, and load current. The 1x mode is
the most efficient mode, followed by 1.5x and 2x modes.
Circuit conditions such as low input voltage, high output
current, or high LED voltage place a higher demand on
the charge pump output. A higher numerical mode (1.5x
or 2x) may be needed momentarily to maintain regulation
at the OUT pin during intervals of high demand. The
charge pump responds to momentary high demands,
setting the charge pump to the optimum mode to deliver
the output voltage and load current while optimizing efficiency. Hysteresis is provided to prevent mode toggling.
The charge pump requires two bucket capacitors for
proper operation. One capacitor must be connected
between the C1+ and C1- pins and the other must be connected between the C2+ and C2- pins as shown in the
Typical Application Circuit diagram. These capacitors
should be equal in value, with a minimum capacitance of
1µF to support the charge pump current requirements.
The device also requires at least 1µF capacitance on the IN
pin and at least 1µF capacitance on the OUT pin to minimize noise and support the output drive requirements of
IOUT up to 90mA. For output currents higher than 90mA, a
nominal value of 2.2µF is recommended for COUT and CIN.
Capacitors with X7R or X5R ceramic dielectric are strongly
recommended for their low ESR and superior temperature and voltage characteristics. Y5V capacitors should
not be used as their temperature coefficients make them
unsuitable for this application.
Capacitor Recommendations
The full rated output of 175mA is achieved using 2.2µF
0603 size capacitors for input, output, and bucket
capacitors.
For applications which do not require the full 175mA
output capability of the SC656 , a lower cost and smaller
size capacitor option may be used. The 1µF capacitor in
Table 1 may be used with no loss in accuracy, for up to
90mA of output current.
Table 1 — Capacitor Recommendations
Capacitance
Value of
CIN = COUT
= C1 = C2
Size Code
EIA (JIS)
Application IOUT Limit
2.2µF
0603 (1608)
up to IOUT = 175mA(1)
1.0µF
0402 (1005)
up to IOUT = 90mA(2)(3)
Notes:
(1) Note that 2.2µF in the 0402 size is not equivalent to 2.2µF in the
0603 size, so 0402 may not be substituted for this application.
(2) Larger size capacitors may be substituted.
(3) Exceeding 90mA, or using less than 1.0µF, may cause excessive
peak-to-peak output ripple, (>120mV), and some loss of accuracy
in 1.5x mode.
Bank Control Options
The backlight drivers can be configured as a single bank
or as two independently controlled banks. The configuration of the banks is determined by the BANK2 and
BANK1 pins as described in Table 2. The ENM and ISETM
pins control the brightness of LEDs assigned to the main
bank, and the ENS and ISETS pins allow the sub bank
current to be set independently as described in the following section. Note that when both BANK2 and BANK1
are set to 0, the sub bank feature is disabled. In this case,
both ENS and ISETS should be tied to GND.
12
SC656
Applications Information (continued)
Bank 2
Bank 1
Main Bank
Sub Bank
0
0
BL1 — BL7
none
0
1
BL2 — BL7
BL1
1
0
BL3 — BL7
BL1 — BL2
1
1
BL4 — BL7
BL1 — BL3
LED Backlight Current Sinks
The full scale backlight current (IFS_BL) is set via the current
through the ISET pin controlling the corresponding LED
bank (ISETM or ISETS) . The IFS_BL is regulated to the value
of the ISETM or ISETS pin current multiplied by an internal
gain of 100A/A. RISETM and RISETS are used to control the
current into the ISETM and ISETS pins. The relationship
between each resistance RISETx and the full scale backlight
current is:
RISETx = 100/IFS_BL
All backlight current sinks have matched currents, even
when there is a variation in the forward voltages (∆VF ) of
the LEDs. A ∆VF of 1.0V is supported when the input
voltage is at 2.9V. Higher ∆VF LED mis-match is supported
when VIN is higher than 2.9V. All current sink outputs are
compared and the lowest output is used for setting the
voltage regulation at the OUT pin. This is done to ensure
that sufficient bias exists for all LEDs.
Any unused LED driver outputs must be left open for
normal operation.
PWM Operation
A PWM signal on the ENM or ENS pin can be used to
adjust the DC current through the LEDs. When the duty
cycle is 100%, the backlight current through each LED (IBL)
equals the full scale current set by the corresponding ISET
pin. As the duty cycle decreases, the ENM or ENS input
samples the control signal and converts the duty cycle to
a DC current level. In conventional PWM controlled
systems, the output current pulses on and off with the
PWM input to achieve an average output current.
Providing a DC current through the LEDs instead of a
pulsed current provides an efficiency advantage over
other PWM controlled systems by allowing the charge
pump to remain in 1x mode longer since the maximum
current is equal to the average current.
PWM Sampling
The sampling system that translates the PWM signal to
a DC current requires the ENM and ENS pins to have a
minimum high time tHIGH_MIN to set the DC level. High
time less than tHIGH_MIN impacts the accuracy of the target
I BL. The minimum duty cycle needed to support the
minimum high time specification varies with the applied
PWM frequency (see figure 1). Note that use of a lower
PWM frequency, from 200Hz to 10kHz, will support
lower minimum duty cycle and an extended backlight
dimming range.
5
tHIGH_MIN = 1µs
4
Minimum Duty Cycle (%�
Table 2 — Backlight Bank Configuration Settings
3
2
1
0
0.2
10
20
30
40
50
PWM Frequency (kHz�
Figure 1 — Minimum Duty Cycle
Shutdown Mode
Shutdown occurs after ENM and ENS are both held low
for an interval of 15ms or more. When the ENM and ENS
pins are both held low for 5ms or less, the device will not
shutdown.
Protection Features
The SC656 provides several protection features to safeguard the device from catastrophic failures. These features
include:
•
•
•
•
Output Open Circuit Protection
Over-Temperature Protection
Charge Pump Output Current Limit
LED Float Detection
13
SC656
Applications Information (continued)
Output Open Circuit Protection
Over-Voltage Protection (OVP) at the OUT pin prevents
the charge pump from producing an excessively high
output voltage. In the event of an open circuit between
the OUT pin and all current sinks (no loads connected),
the charge pump runs in open loop and the voltage rises
up to the OVP limit. OVP operation is hysteretic, meaning
the charge pump will momentarily turn off until VOUT is
sufficiently reduced. The maximum OVP threshold is 6.0V,
allowing the use of a ceramic output capacitor rated
at 6.3V.
Charge Pump Output Current Limit
The device limits the charge pump current at the OUT pin.
If the OUT pin is shorted to ground, or VOUT is lower than
2.5V, the typical output current limit is 70mA. The output
current is limited to 225mA when over loaded resistively
with VOUT greater than 2.5.
LED Float Detection
Float detect is a fault detection feature of the LED backlight outputs. If an output is programmed to be enabled
and an open circuit fault occurs at any backlight output,
that output will be disabled to prevent a sustained output
OVP condition from occurring due to the resulting open
loop. Float detect ensures device protection but does not
ensure optimum performance.
Over-Temperature Protection
The Over-Temperature (OT) protection circuit prevents the
device from overheating and experiencing a catastrophic
failure. When the junction temperature exceeds 165­°C, the
device goes into thermal shutdown with all outputs disabled until the junction temperature is reduced. All register information is retained during thermal shutdown.
Hysteresis of 20°C is provided to ensure that the device
cools sufficiently before re-enabling.
PCB Layout Considerations
The layout diagram in Figure 2 illustrates a proper two
layer PCB layout for the SC656 and supporting components. Following fundamental layout rules is critical for
achieving the performance specified in the Electrical
Characteristics table. The following guidelines are recommended when developing a PCB layout:
OUT
Ground Plane
COUT
GND
C1
C1-
C1+
C2+
OUT
ENM
C2
CIN
ENS
IN
C2-
GND
IN
RSETM
GND
SC656
ISETM
BANK2
ISETS
BANK1
BL1
BL6
BL5
BL4
BL3
BL7
BL2
RSETS
GND
Figure 2 — Recommended Layout
14
SC656
Applications Information (continued)
•
•
•
•
•
Place all bucket and decoupling capacitors —
C1, C2, CIN, and COUT — as close to the device as
possible.
All charge pump current passes through pins
IN, OUT, C1+, C2+, C1-, and C2-. Therefore,
ensure that all connections to these pins make
use of wide traces so that the voltage drop on
each connection is minimized.
The GND pin should be connected to a ground
plane using multiple vias to ensure proper
thermal connection for optimal heat transfer.
Make solid ground connections between the
grounds of the COUT, CIN, and the GND pin on the
device.
Resistors RSETM and RSETS should be connected as
shown in Figure 2, close to pins IN and ISET.
The placement and routing shown minimizes
parasitic capacitance at the ISET pin.
Figure 3 — Layer 1
•
•
•
Figure 3 shows the pads on layer 1 that should
be connected with vias to layer 2. CIN, COUT and
the GND pin all use vias to connect to the
ground plane.
Figure 4 shows layer 2, which functions as the
ground plane. Layer 2 is also used for routing
signals to pins ENM, ENS, BANK1, and BANK2. A
void in the copper beneath the ISETM and ISETS
pins serves to reduce capacitance coupled from
these pins to ground.
Avoid coupling noise to the ENM and ENS pins.
This will help prevent unintended clocking of
the PWM. The layout should be routed to
achieve the least possible trace to trace capacitance between ENM and ENS. Also, minimize
trace capacitance between ENM or ENS and any
high speed signals.
Figure 4 — Layer 2
15
SC656
Outline Drawing — MLPQ-UT-20 3x3
A
D
B
DIM
PIN 1
INDICATOR
(LASER MARK)
E
A2
A
aaa C
C
A1
SEATING
PLANE
A
A1
A2
b
D
D1
E
E1
e
L
N
aaa
bbb
DIMENSIONS
INCHES
MILLIMETERS
MIN NOM MAX MIN NOM MAX
.020
.024 0.50
0.60
.000
.002 0.00 0.05
(.006)
(0.152)
.006 .008 .010 0.15 0.20 0.25
.114 .118 .122 2.90 3.00 3.10
.061 .067 .071 1.55 1.70 1.80
.114 .118 .122 2.90 3.00 3.10
.061 .067 .071 1.55 1.70 1.80
.016 BSC
0.40 BSC
.012 .016 .020 0.30 0.40 0.50
20
20
.003
0.08
.004
0.10
D1
e
LxN
E/2
E1
2
1
N
D/2
bxN
bbb
C A B
NOTES:
1.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2.
COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
3.
DAP IS 1.90 x 1.90mm.
16
SC656
Land Pattern — MLPQ-UT-20 3x3
K
DIMENSIONS
R
(C)
H
G
Y
X
P
Z
DIM
INCHES
MILLIMETERS
C
G
H
K
P
R
X
Y
Z
(.114)
(2.90)
.083
.067
.067
.016
.004
.008
.031
.146
2.10
1.70
1.70
0.40
0.10
0.20
0.80
3.70
NOTES:
1. CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
2.
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
3.
THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD
SHALL BE CONNECTED TO A SYSTEM GROUND PLANE.
FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR
FUNCTIONAL PERFORMANCE OF THE DEVICE.
Contact Information
Semtech Corporation
Power Management Products Division
200 Flynn Road, Camarillo, CA 93012
Phone: (805) 498-2111 Fax: (805) 498-3804
www.semtech.com
17