LM27961
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SNVS326A – NOVEMBER 2004 – REVISED MAY 2013
LM27961 Dual-Display White LED Driver with 3/2x Switched Capacitor Boost
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FEATURES
DESCRIPTION
•
The LM27961 is a charge-pump-based white-LED
driver that is ideal for mobile phone display
backlighting. It is intended to drive 4 LEDs for a main
phone display backlight and 3 LEDs for a sub-display
backlight. Regulated internal current sources deliver
excellent current and brightness matching in all LEDs.
1
2
•
•
•
•
•
•
•
Drives 4 Individual Common-Anode LEDs with
up to 20mA each for a Main Display Backlight
Drives 3 Individual Common-Cathode LEDs
with up to 20mA each for a Sub-Display
Backlight
Independent Resistor-Programmable Current
Setting
Excellent Current and Brightness Matching
High-Efficiency 3/2x Charge Pump
Extended Li-Ion Input: 2.7V to 5.5V
PWM Brightness Control: 100Hz - 1kHz
18-bump Thin DSBGA Package: (2.1mm x
2.4mm x 0.6mm)
APPLICATIONS
•
•
•
•
Mobile Phone Display Lighting
Mobile Phone Keypad Lighting
PDAs
General LED Lighting
The LED driver current sources are split into two
independently controlled groups. The primary group
(Group A) can be used to backlight a main phone
display with up to 4 LEDs. The low-side current
drivers of Group A accommodate common-anodetype LEDs. The second group (Group B) can
backlight a secondary display with up to 3 LEDs. The
high-side current drivers of Group B accommodate
common-cathode-type LEDs. Both Group A and
Group B can also drive standard two-terminal LEDs,
and provide other general lighting functions (keypad
lighting, fun lighting, etc). The brightness of the two
LED groups can be adjusted independently with
external resistors.
The LM27961 works off an extended Li-Ion input
voltage range (2.7V to 5.5V). Voltage boost is
achieved with a high-efficiency 3/2×-gain charge
pump.
The LM27961 is available in TI’s chip-scale 18-bump
DSBGA package.
Typical Application Circuit
C1
1 PF
VIN
2.7V to 5.5V
C1+
VIN
C2
1 PF
C1-
C2+
C2-
(1x when VIN > 4.7V)
CIN
ENA
ENB
POUT
3/2x Charge Pump
CPOUT
LM27961
1 PF
D1A
D2A
D3A
1 PF
D4A
ISETA
D1B
D2B
D3B
ISETB
GND
RSETA
RSETB
Capacitors:
TDK C1608X5R1A105K,
or equivalent
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2004–2013, Texas Instruments Incorporated
LM27961
SNVS326A – NOVEMBER 2004 – REVISED MAY 2013
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Connection Diagram
7
6
5
4
3
2
1
7
6
5
4
3
2
1
A
B
C
D
E
E
D
Top View
C
B
A
Bottom View
Figure 1. 18-Bump Thin DSBGA Package, Large Bump
Package Number YZR0018
PIN DESCRIPTION
Pin #s
Pin Names
C1
VIN
Pin Descriptions
Input voltage. Input range: 2.7V to 5.5V.
D2
GND
Ground
A3
POUT
Charge pump output. Approximately 1.5×VIN
A1, B2, A5, E1
D6, E5, D4, E3
C1+, C1-, C2+, C2-
Flying capacitor connections.
D1A, D2A, D3A, D4A LED Outputs - Group A
C5, B4, C3
D1B, D2B, D3B
LED Outputs - Group B
B6
EN-A
Enable for Group-A LEDs (current outputs). Logic input.
High = Group-A LEDs ON. Low = Group A LEDs OFF.
Pulsing this pin with a PWM signal (100Hz-1kHz) can be used to dim LEDs.
A7
EN-B
Enable for Group-B LEDs (current outputs). Logic input.
High = Group-B LEDs ON. Low = Group B LEDs OFF.
Pulsing this pin with a PWM signal (100Hz-1kHz) can be used to dim LEDs.
E7
ISETA
Placing a resistor (RSETA) between this pin and GND sets the LED current for Group A LEDs.
LED Current = 100 × (1.25V ÷ RSETA).
C7
ISETB
Placing a resistor (RSETB) between this pin and GND sets the LED current for Group B LEDs.
LED Current = 100 × (1.25V ÷ RSETB).
Table 1. Operational States
ENA
ENB
Mode of Operation
L
L
Shutdown
H
L
Enabled. Group A LEDs ON. Group B LEDs OFF
L
H
Enabled. Group B LEDs ON. Group A LEDs OFF
H
H
Invalid for normal operation
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
2
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Absolute Maximum Ratings
(1) (2) (3)
VIN pin voltage
-0.3V to 7.1V
ENA, ENB pin voltages
-0.3V to (VIN+0.3V)w/ 6.0V max
IDxx Pin Voltages
-0.3V to (VPOUT+0.3V)w/ 6.0V max
Continuous Power Dissipation
Internally Limited
(4)
Junction Temperature (TJ-MAX)
150ºC
Storage Temperature Range
-65ºC to +150º C
Maximum Lead Temperature (Soldering, 10 sec.)
265ºC
ESD Rating (5)
Human Body Model - IDxx Pins:
Human Body Model - All other Pins:
Machine Model - IDxx Pins:
Machine Model - All Other Pins:
1.0kV
2.0kV
100V
200V
(1)
(2)
(3)
(4)
(5)
Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under
which operation of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed
performance limits and associated test conditions, see the Electrical Characteristics tables.
If Military/Aerospace specified devices are required, please contact the TI Sales Office/Distributors for availability and specifications.
All voltages are with respect to the potential at the GND pin.
Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ = 160°C (typ.) and
disengages at TJ = 120°C (typ.). The thermal shutdown function is guaranteed by design.
The Human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin. The machine model is a 200pF
capacitor discharged directly into each pin. MIL-STD-883 3015.7
Operating Rating
(1) (2)
Input Voltage Range
2.7V to 5.5V
Junction Temperature (TJ) Range
-30°C to +125°C
Ambient Temperature (TA) Range
-30°C to +85°C
(3)
(1)
(2)
(3)
Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under
which operation of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed
performance limits and associated test conditions, see the Electrical Characteristics tables.
All voltages are with respect to the potential at the GND pin.
In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may
have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP =
125°C), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance of the
part/package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX).
Thermal Properties
Juntion-to-Ambient Thermal
Resistance (θJA), (1)
(1)
100°C/W
Junction-to-ambient thermal resistance is highly dependent on application and board layout. In applications where high maximum power
dissipation exists, special care must be paid to thermal dissipation issues in board design.
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Electrical Characteristics (1) (2)
Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range.
Unless otherwise specified: VIN = 3.6V; VDxA = 0.6V; VDxB = 3.6V; ENA = 1.5V and ENB = GND, or ENA = GND and ENB =
1.5V; RSETA = RSETB = 8.35kΩ; CIN, C1, C2 , and CPOUT = 1µF. Specifications related to output current(s) and current setting
pins (IDxx and ISETx) apply to both Group A and Group B. (3)
Symbol
Parameter
Condition
3.0V ≤ VIN ≤ 4.2V, and VIN = 5.5V
0.45V ≤ VDxA ≤ 3.8V or
2.5V ≤ VDxB ≤ 3.8V;
RSET = 8.35kΩ
IDxx
Output Current Regulation
Min
Typ
Max
Units
13.5 (10%)
15
16.5
(+10%)
mA
(%)
3.0V ≤ VIN ≤ 5.5V;
0.6V ≤ VDxA ≤ 3.8V or
2.5V ≤ VDxB ≤ 3.8V;
RSET = 6.25kΩ
20
mA
3.0V ≤ VIN ≤ 5.5V;
0.3V ≤ VDxA ≤ 3.8V or
2.5V ≤ VDxB ≤ 3.8V;
RSET = 12.5kΩ
10
mA
2.7V ≤ VIN ≤ 3.0V;
0.45V ≤ VDxA ≤ 3.8V or
2.5V ≤ VDxB ≤ 3.8V;
RSET = 8.35kΩ
15
mA
0.6
%
IDxx-MATCH
Current Matching Between Any
Two Group A Outputs or Group B
Outputs
VIN = 3.0V
IQ
Quiescent Supply Current
2.7V ≤ VIN ≤ 4.2V;
No Load Current,
ENA or ENB = ON
4.4
6.75
mA
ISD
Shutdown Supply Current
2.7V ≤ VIN ≤ 5.5V,
ENA and ENB = OFF
2.3
5
µA
VSET
ISET Pin Voltage
2.7V ≤ VIN ≤ 5.5V
1.25
IDxx/ISET
Output Current to Current Set
Ratio
ROUT
Charge Pump Output Resistance
(5)
(4)
V
100
VIN = 3.0V
2.7
Ω
320
mV
VHR
I
= 95% X IDxx (nom)
Current Source Headroom Voltage Dxx
RSET = 8.35kΩ
Requirement (6)
(IDxx (nom) ≈ 15mA)
fSW
Switching Frequency
3.0V ≤ VIN ≤ 4.2V
tSTART
Start-up Time
IDx = 90% steady state
350
µs
1.5x to 1x Threshold
4.75
V
1.5x/1x
Charge pump gain cross-over:
Gain = 1.5 when VIN is below
threshold. Gain = 1 when VIN is
above threshold.
1x to 1.5x Threshold
4.55
V
375
500
625
kHz
Logic Pin Specifications: EN, ENA, ENB
VIL
Input Logic Low
2.7V ≤ VIN ≤ 5.5V
0
0.5
V
VIH
Input Logic High
2.7V ≤ VIN ≤ 5.5V
1.1
VIN
V
(1)
(2)
(3)
(4)
(5)
(6)
4
All voltages are with respect to the potential at the GND pin.
Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the
most likely norm.
CIN, CPOUT, C1, and C2 : Low-ESR Surface-Mount Ceramic Capacitors (MLCCs) used in setting electrical characteristics
For the two groups of outputs on a part (Group A and Group B), the following are determined: the maximum output current in the group
(MAX), the minimum output current in the group (MIN), and the average output current of the group (AVG). For each group, two
matching numbers are calculated: (MAX-AVG)/AVG and (AVG-MIN)/AVG. The largest number of the two (worst case) is considered the
matching figure for the group. The matching figure for a given part is considered to be the highest matching figure of the two groups.
The typical specification provided is the most likely norm of the matching figure for all parts.
Output resistance (ROUT) models all voltage losses in the charge pump. ROUT can be used to estimate the voltage at the charge pump
output (POUT): VPout = (1.5 × VIN) – (ROUT × IOUT). In the equation, IOUT is the total output current: the sum of all active Dxx output
currents and all current drawn from POUT. The equation applies when the charge pump is operating with a gain of 3/2 (VIN ≤ 4.75V typ.).
Headroom voltage: VHR = VPout – VLEDx . If headroom voltage requirement is not met, LED current regulation will be compromised.
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Electrical Characteristics(1)(2) (continued)
Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range.
Unless otherwise specified: VIN = 3.6V; VDxA = 0.6V; VDxB = 3.6V; ENA = 1.5V and ENB = GND, or ENA = GND and ENB =
1.5V; RSETA = RSETB = 8.35kΩ; CIN, C1, C2 , and CPOUT = 1µF. Specifications related to output current(s) and current setting
pins (IDxx and ISETx) apply to both Group A and Group B. (3)
Symbol
ILEAK
(7)
Parameter
Input Leakage Current
Condition
VENx = 0V
VENx = 3V
Min
Typ
Max
0.1
(7)
10
Units
µA
There is a 300kΩ(typ.) pull-down resistor connected internally between each enable pin (ENA, ENB) and GND.
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Typical Performance Characteristics
Unless otherwise specified: VIN = 3.6V; VLEDxA = 3.6V; VLEDxB = 3.6V; ENA = VIN and ENB = GND, or ENA = GND and ENB =
VIN; RSETA = RSETB = 8.35kΩ; CIN, C1, C2 , and CPOUT = 1µF.
LED Current (D1A, D2A,D3A, D4A)
vs. Input Voltage
15.5
LED Current (DxA) vs. Input Voltage
15.6
TA = 25oC
D1A - D4A Enabled
D1A - D4A Enabled
ID1A - DIODE CURRENT (mA)
IDXA - DIODE CURRENT (mA)
15.6
15.4
15.3
15.2
15.1
15.5
TA = 85oC
15.4
15.3
TA = 25oC
15.2
15.1
TA = -40oC
15
2.7
3.1
3.5
3.9
4.3
4.7
5.1
15
2.7
5.5
3.1
VIN - INPUT VOLTAGE (V)
3.5
Figure 2.
4.3
4.7
5.1
5.5
Figure 3.
Quiescent Current vs. Input Voltage,
Charge Pump Output Voltage
vs. Output Current
6.5
6
TA = 25oC
TA = 85oC
VPOUT - OUTPUT VOLTAGE (V)
IQ - QUIESCENT CURRENT (mA)
3.9
VIN - INPUT VOLTAGE (V)
5.5
TA = 25oC
5
4.5
4
TA = -40oC
3.5
6
No Diodes Connected
VIN = 5.5V
VIN = 4.2V
5.5
VIN = 3.6V
5
VIN = 3.3V
4.5
VIN = 3.0V
4
VIN = 2.7V
3.5
3
2.7
3.2
3.7
4.2
0
VIN - INPUT VOLTAGE (V)
60 70
80
90
Figure 5.
Charge Pump Output Voltage
vs. Output Current
Charge Pump Output Voltage
vs. Input Voltage (No Load Current)
7
No Diodes Connected
5.4
5.35
VPOUT - OUTPUT VOLTAGE (V)
VPOUT - OUTPUT VOLTAGE (V)
40 50
Figure 4.
VIN = 3.6V
TA = -40oC
5.3
5.25
TA = 85oC
5.2
5.15
TA = 25oC
5.1
10
20
30
40
50
60
70
80
90
IPOUT - OUTPUT CURRENT (mA)
TA = 25oC
6.5
D1A - D4A Enabled
6
5.5
5
4.5
4
3.5
3
2.7
3.1
3.5
3.9
4.3
4.7
5.1
5.5
VIN - INPUT VOLTAGE (V)
Figure 6.
6
30
IPOUT - OUTPUT CURRENT (mA)
5.45
0
10 20
Figure 7.
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Typical Performance Characteristics (continued)
Unless otherwise specified: VIN = 3.6V; VLEDxA = 3.6V; VLEDxB = 3.6V; ENA = VIN and ENB = GND, or ENA = GND and ENB =
VIN; RSETA = RSETB = 8.35kΩ; CIN, C1, C2 , and CPOUT = 1µF.
Charge Pump Output Resistance
vs Output Current
Input Current vs. Input Voltage
100
ROUT - OUTPUT RESISTANCE (:)
3.2
IIN - INPUT CURRENT (mA)
95
90
TA = 25oC
85
D1A - D4A Enabled
80
75
70
65
60
2.7
VIN = 3.6V
No Diodes Connected
3
TA = 85oC
2.8
TA = 25oC
2.6
2.4
2.2
TA = -40oC
2
3.1
3.5
3.9
4.3
4.7
5.1
5.5
10
VIN - INPUT VOLTAGE (V)
70
Figure 9.
Charge Pump Switching Frequency
vs. Input Voltage
Diode Current (DxA)
vs. Headroom Voltage (DxA)
520
D1A - D4A Enabled
510
500
TA = -40oC
490
TA = 25oC
480
470
460
90
18
VIN = 3.6V
TA = 85oC
TA = 25oC
IDXA - DIODE CURRENT (mA)
fSW - SWITCHING FREQUENCY (kHz)
50
Figure 8.
530
15
TA = -40oC
12
9
TA = 85oC
6
3
VIN = 3.0V
450
D1A - D4A Enabled
440
2.7
0
3.2
3.7
0
4.2
0.2
0.3
0.4
0.5
Figure 11.
Diode Current (DxB)
vs. Headroom Voltage (DxB)
Diode Current (DxA or DxB)
vs. PWM Duty Cycle (ENA or ENB)
OUTPUT CURRENT BANKA OR BANKB (mA)
Figure 10.
18
TA = -25oC
15
TA = -40oC
12
9
TA = 85oC
6
3
VIN =3.0V
D1B - D3B Enabled
0
0
0.1
VHRA - HEADROOM VOLTAGE BANKA (V)
VIN - INPUT VOLTAGE (V)
IDXB - DIODE CURRENT (mA)
30
IPOUT - OUTPUT CURRENT (mA)
0.1
0.2
0.3
0.4
0.5
VHRB - HEADROOM VOLTAGE BANKB (V)
Figure 12.
16
fPWM = 100 Hz.
14
12
10
8
6
4
2
0
0
20
40
60
80
100
ENA OR ENB PWM DUTY CYCLE (%)
Figure 13.
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Typical Performance Characteristics (continued)
Unless otherwise specified: VIN = 3.6V; VLEDxA = 3.6V; VLEDxB = 3.6V; ENA = VIN and ENB = GND, or ENA = GND and ENB =
VIN; RSETA = RSETB = 8.35kΩ; CIN, C1, C2 , and CPOUT = 1µF.
IDXX - DIODE CURRENT BANKA OR BANKB (mA)
Diode Current (DxA)
vs. RSETx
Input Voltage (Top)
and Output Voltage (Bottom) Waveforms
35
30
25
20
15
10
5
0
0
20
40
60
RSET (:)
80
100
Vertical Scale = (100mV/div),
Horizontal Scale = 1µs/div
Figure 14.
Figure 15.
ENx Signal (Top)
and Charge Pump Start-Up (Bottom) Waveforms
Vertical Scale = (2V/div),
Horizontal Scale = 100µs/div)
Figure 16.
8
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BLOCK DIAGRAM
POUT
CPOUT
1 PF
C1
1 PF
C2
1 PF
LM27961
BG
D1A
D2A
D3A
D4A
+
-
VIN
2.7V to 5.5V
3/2x Charge Pump +
Pass Mode
GND
CIN
100:1
DxA
Gain
Ctrl.
1 PF
ENA
500 kHz.
Switch
Frequency
ENB
SoftStart
100:1
DxB
Gain
Ctrl.
1.25V
Bandgap (BG)
BG
+
ISETB
GND
D1B
D2B
+
ISETA
D3B
RSETB
RSETA
CIRCUIT DESCRIPTION
Overview
The LM27961 is primarily intended for Lithium-Ion battery driven white-LED drive applications, and is well suited
to drive white LEDs that are used for backlighting small-format displays. The part has seven matched constantcurrent outputs, each capable of driving up to 20mA (or more) through white LEDs. The well-matched current
sources ensure the current through all the LEDs is virtually identical. This keeps brightness of all LEDs matched
to near perfection so that they can provide a consistent backlight over the entire display.
Charge Pump
The core of the LM27961 is a 1.5x/1x dual-mode charge pump. The input of the charge pump is connected to the
VIN pin. The recommended input voltage range of the LM27961 is 2.7V to 5.5V. The output of the charge pump is
the POUT pin (“Pump OUTput”). The output voltage of the charge pump is unregulated and varies with input
voltage and load current.
The charge pump operates in the 1.5x mode when the input voltage is below 4.75V (typ.). In this mode, the
input-to-output voltage gain of the charge pump is 1.5, and the voltage at the output of the charge pump will be
approximately 1.5x the input voltage (V(POUT) ≈ 1.5 * VIN ). When in the 1.5x mode, the charge pump provides
the voltage boost that is required to drive white LEDs from a Li-Ion battery. (White LEDs typically have a forward
voltage in the range of 3.3V to 4.0V. A Li-Ion battery typically is not considered to be fully discharged until the
battery voltage falls to 3.0V (approx.) )
The charge pump operates in the 1x mode when the input voltage is above 4.75V (typ.). In these conditions,
voltage boost is not required to drive the LEDs, so the charge pump merely passes the input voltage to POUT
(V(POUT) ≈ VIN). This reduces the input current and the power dissipation of the LM27961 when the input voltage
is high.
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Regulated Current Outputs
The matched current outputs are generated with a precision current mirror that is biased off the charge pump
output. Matched currents are ensured with the use of tightly matched internal devices and internal mismatch
cancellation circuitry.
There are seven regulated current outputs. These seven outputs are split into two groups, a group of 4 common
anode outputs and a group of 3 common cathode outputs. There is an ON/OFF control pin for each group (ENA
and ENB).
The DC current through the LEDs is programmed with an external resistor. Changing currents on-the-fly can be
achieved with the use of digital pulse (PWM) signals.
Enable Pins: ENA, ENB
The LM27961 has 2 enable pins. Both are active-high logic (HIGH = ON). There are internal pull-down resistors
(300kΩ typ.) that are connected internally between each of the enable pins and GND.
ENA and ENB can both enable and disable the part. When the voltage on both pins are low (1.1V), the part is active. The charge pump
is ON, and the corresponding output current drivers are active.
ENA and ENB are used to turn the output currents ON and OFF. ENA activates/deactivates the four GroupA
outputs (D1A-D4A). ENB activates/deactivates the three GroupB outputs (D1B-D3B).
Setting LED Currents
The output currents of the LM27961 can be set to a desired value simply by connecting an appropriately sized
resistor (RSETx) between the ISETx pins of the LM27961 and GND. RSETA sets the current for the GroupA outputs
and RSETB sets the current for the GroupB outputs. The output currents (LED currents) are proportional to the
current that flows out of the ISETx pins. The output currents are a factor of 100 greater than the ISETx current. The
feedback loop of an internal amplifier sets the voltage of the ISETx pin to 1.25V (typ.). Placing a resistor between
ISETx and GND programs the ISETx current, and thus the LED currents. The statements above are simplified in the
equations below:
IDxx = 100 × (VSETx / RSETx)
RSETx = 100 × (1.25V / IDxx)
(1)
(2)
Maximum Output Current, Maximum LED Voltage, Minimum Input Voltage
The LM27961 can drive 4 LEDs at 15mA each from an input voltage as low as 2.7V, so long as the LEDs have a
forward voltage of 3.5V or less (room temperature).
The statement above is a simple example of the LED drive capabilities of the LM27961. The statement contains
the key application parameters that are required to validate an LED-drive design using the LM27961: LED
current (ILEDx), number of active LEDs (N), LED forward voltage (VLED), and minimum input voltage (VIN-MIN).
The equation below can be used to estimate the total output current capability of the LM27961:
ILED_MAX = ((1.5 x VIN) - VLED) / ((N x ROUT) + kHR)
ILED_MAX = ((1.5 x VIN ) - VLED) / ((N x 2.7Ω) + 22mV/mA)
(3)
(4)
ROUT – Output resistance. This parameter models the internal losses of the charge pump that result in voltage
droop at the pump output POUT. Since the magnitude of the voltage droop is proportional to the total output
current of the charge pump, the loss parameter is modeled as a resistance. The output resistance of the
LM27961 is typically 2.7Ω (VIN = 3.0V, TA = 25°C). In equation form:
VPOUT = 1.5×VIN – N×ILED×ROUT
(5)
kHR – Headroom constant. This parameter models the minimum voltage required to be present across the current
sources for them to regulate properly. This minimum voltage is proportional to the programmed LED current, so
the constant has units of mV/mA. The typical kHR of the LM27961 is 22mV/mA. In equation form:
(VPOUT – VLED) > kHR×ILED
10
(6)
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The "ILED-MAX" equation (Equation 3) is obtained from combining the ROUT equation (Equation 5) with the kHR
equation (Equation 6) and solving for ILED. Maximum LED current is highly dependent on minimum input voltage
and LED forward voltage. Output current capability can be increased by raising the minimum input voltage of the
application, or by selecting an LED with a lower forward voltage. Excessive power dissipation may also limit
output current capability of an application.
Parallel Dx Outputs for Increased Current Capability
Outputs D1A through D4A, or D1B through D3B may be connected together in any combination to drive higher
currents through fewer LEDs. For example in Figure 17, outputs D1A and D2A are connected together to drive
one LED. D3A and D4A are connected to drive a second LED.
VPOUT
VIN
LM27961
LED2
D1B
D2B
D3B
LED1
D3A and D4A
D1A and D2A
ILED1 = ID1A + ID2A
ILED2 = ID3A + ID4A
Figure 17. Two Parallel Connected LEDs
With this configuration, two parallel current sources of equal value provide current to one of the LEDs. RSET
should therefore be chosen so that the current through each output is programmed to 50% of the desired current
through the parallel connected LED. For example, if 40mA is the desired drive current for the parallel connected
LED, RSETx should be selected so that the current through each of the outputs is 20mA. Other combinations of
parallel outputs may be implemented in similar fashions, such as in Figure 18.
LED3A
D3A
LED4A
D4A
VPOUT
VIN
LM27961
LED2A
D1B
D2B
D3B
LEDB
D1A D2A
LED1A
ILEDB = ID1B + ID2B + ID3B
Figure 18. One Parallel Connected LED
Connecting outputs in parallel does not affect internal operation of the LM27961 and has no impact on the
Electrical Characteristics and limits previously presented. The available diode output current, maximum diode
voltage, and all other specifications provided in the Electrical Characteristics table apply to parallel output
configurations, just as they do to the standard application circuit on pg1 of the datasheet.
Soft Start
The LM27961 contains internal soft-start circuitry to limit input inrush currents when the part is enabled. Soft start
is implemented with a controlled turn-on of the internal voltage reference. During soft start, the current through
the LED outputs rise at the rate of the reference voltage ramp. Due to the soft-start circuitry, turn-on time of the
LM27961 is approximately 350µs (typ.).
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Thermal Protection
Internal thermal protection circuitry disables the LM27961 when the junction temperature exceeds 160°C (typ.).
This feature protects the device from being damaged by high die temperatures that might otherwise result from
excessive power dissipation. The device will recover and operate normally when the junction temperature falls
below 120°C (typ.). It is important that the board layout provides good thermal conduction. This will help to keep
the junction temperature within specified operating ratings.
APPLICATIONS INFORMATION
Power Efficiency
Efficiency of LED drivers is commonly taken to be the ratio of power consumed by the LEDs (PLED) to the power
drawn at the input of the part (PIN). With a 1.5x charge pump, the input current is approximately 1.5x the output
current (total LED current). For a simple approximation, the current consumed by internal circuitry can be
neglected and the efficiency of the LM27961 can be predicted as follows:
PLED
E=
PIN
#
VLED
1.5 × VIN
(7)
Neglecting IQ will result in a slightly higher efficiency prediction, but this impact will be no more than a few
percentage points when several LEDs are driven at full power.
Adjusting LED Brightness (PWM control)
Perceived LED brightness can be adjusted using a PWM control signal to turn the LM27961 current sources ON
and OFF at a rate faster than perceptible by the eye. When this is done, the total brightness perceived is
proportional to the duty cycle (D) of the PWM signal (D = the percentage of time that the LED is on in every
PWM cycle). A simple example: if the LEDs are driven at 15mA each with a PWM signal that has a 50% duty
cycle, perceived LED brightness will be about half as bright as compared to when the LEDs are driven
continuously with 15mA. A PWM signal thus provides brightness (dimming) control for the solution.
The minimum recommended PWM frequency is 100Hz. Frequencies below this may be visibly noticeable as
flicker or blinking. The maximum recommended PWM frequency is 1kHz. Frequencies above this may cause
interference with internal current driver circuitry.
In cases where a PWM signal must be connected to the ENx pins, measures can be taken to reduce the
magnitude of the charge-pump turn-on voltage spikes. More input capacitance, series resistors and/or ferrite
beads may provide benefits.
If the current and voltage spikes can be tolerated, connecting the PWM signal to the EN pin does provide a
benefit: lower supply current when the PWM signal is active. When the PWM signal is low, the LM27961 will be
shutdown and input current will only be a few micro-amps. This results in a lower time-averaged input current.
Capacitor Selection
The LM27961 requires 4 external capacitors for proper operation. Surface-mount multi-layer ceramic capacitors
are recommended. These capacitors are small, inexpensive and have very low equivalent series resistance (ESR
