Intersil ISL97634IRT18Z-TK White led driver with wide pwm dimming range Datasheet

ISL97634
®
Data Sheet
March 7, 2008
White LED Driver with Wide PWM
Dimming Range
Features
• Drives Up to 7 LEDs in Series (3.5V/20mA type)
The ISL97634 represents an efficient and highly integrated
PWM boost LED driver that is suitable for 1.8” to 3.5” LCDs
that employ 2 to 7 white LEDs for backlighting. With
integrated Schottky diode, OVP, and wide range of PWM
dimming capability, the ISL97634 provides a simple, reliable,
and flexible solution to the backlight designers.
The ISL97634 features a wide range of PWM dimming
control capability. It allows dimming frequency as low as DC
to 32kHz beyond audible spectrum. The ISL97634 also
features a feedback disconnect switch to prevent the output
from being modulated by the PWM dimming signal that
minimizes system disturbance.
The ISL97634 is available in the 8 Ld TDFN (2mmx3mm)
package. There are 14V, 18V, and 26V OVP options that are
suitable for 3 LEDS, 4 LEDs, and 7 LEDs (3.5V/20mA type)
backlight applications respectively. The ISL97634 is
specified for operation over the -40°C to +85°C ambient
temperature at input voltage from 2.4V to 5.5V.
Pinout
ISL97634
(8 LD TDFN)
TOP VIEW
GND 1
8 LX
VIN 2
7 VOUT
PWM/EN 3
6 FBSW
NC 4
5 FB
Typical Application Circuit
10µH or 22µH
VIN
VIN
PWM/EN
LX
• OVP (14V, 18V, and 26V for 3, 4 and 7 LEDs Applications)
• PWM Dimming Control From DC to 32kHz
• Output Disconnect Switch
• Integrated Schottky Diode
• 2.4V to 5.5V Input
• 85% Efficiency
• 1.4MHz Switching Frequency Allows Small LC
• 1µA Shutdown Current
• Internally Compensated
• 8 Ld TDFN (2mmx3mm)
• Pb-Free (RoHS Compliant)
Applications
• LED Backlighting for:
- Cell phones
- Smartphones
- MP3
- PMP
- Automotive Navigation Panel
- Portable GPS
Ordering Information
PART
NUMBER
(Note)
PART
MARKING
PACKAGE
(Pb-free)
Tape & Reel
PKG.
DWG.
#
ISL97634IRT14Z-T
ELE
8 Ld 2x3 TDFN
L8.2x3A
ISL97634IRT14Z-TK
ELE
8 Ld 2x3 TDFN
L8.2x3A
ISL97634IRT18Z-T
ELF
8 Ld 2x3 TDFN
L8.2x3A
ISL97634IRT18Z-TK
ELF
8 Ld 2x3 TDFN
L8.2x3A
ISL97634IRT26Z-T
ELG
8 Ld 2x3 TDFN
L8.2x3A
ISL97634IRT26Z-TK
ELG
8 Ld 2x3 TDFN
L8.2x3A
VOUT
NC
FBSW
GND
FN6264.3
FB
1
*Please refer to TB347 for details on reel specifications
NOTE: These Intersil Pb-free plastic packaged products employ
special Pb-free material sets; molding compounds/die attach
materials and 100% matte tin plate PLUS ANNEAL - e3 termination
finish, which is RoHS compliant and compatible with both SnPb and
Pb-free soldering operations. Intersil Pb-free products are MSL
classified at Pb-free peak reflow temperatures that meet or exceed
the Pb-free requirements of IPC/JEDEC J STD-020..
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright Intersil Americas Inc. 2006-2008. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
ISL97634
Absolute Maximum Ratings (TA = +25°C)
Thermal Information
Input Voltage (VIN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 6V
LX Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 28V
FBSW Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 28V
All Other Pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 6V
Thermal Resistance
Operating Conditions
θJA (°C/W)
θJC (°C/W)
8 Ld TDFN Package (Notes 1, 2). . . . .
77
12
Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . . . . +125°C
Storage Temperature . . . . . . . . . . . . . . . . . . . . . . .-65°C to +150°C
Pb-free reflow profile . . . . . . . . . . . . . . . . . . . . . . . . . .see link below
http://www.intersil.com/pbfree/Pb-FreeReflow.asp
Temperature Range . . . . . . . . . . . . . . . . . . . . . . . . . .-40°C to +85°C
CAUTION: Do not operate at or near the maximum ratings listed for extended periods of time. Exposure to such conditions may adversely impact product reliability and
result in failures not covered by warranty.
IMPORTANT NOTE: All parameters having Min/Max specifications are guaranteed over temperature of -40°C to +85°C unless otherwise stated. Typ values are for
information purposes only at TJ = TC = TA = +25°C.
NOTE:
1. θJA is measured in free air with the component mounted on a high effective thermal conductivity test board with “direct attach” features. See
Tech Brief TB379.
2. For θJC, the “case temp” location is the center of the exposed metal pad on the package underside.
Electrical Specifications
PARAMETER
VIN = VPWM/EN = 3V
DESCRIPTION
VIN
Supply Voltage
IIN
Supply Current
CONDITION
MIN
TYP
2.4
PWM/EN = 3V, enabled, not switching
0.8
PWM/EN = 0V, disabled
fSW
DMAX
ILIM
RSW(LX)
ILEAK
Switching Frequency
UNIT
5.5
V
1.5
mA
1
µA
1,600
kHz
1,300
1,450
Maximum Switching Duty
Cycle
90
95
%
LX Current
400
470
mA
mΩ
LX Switch ON-Resistance
ILX = 100mA
900
LX Switch Leakage Current
VLX = 28V
0.01
1
µA
95
100
mV
1
µA
VFB
Feedback Voltage
IFB
FB Pin Bias Current
RSW(FBSW)
MAX
90
VFB = 95mV
FBSW Switch ON-Resistance
Ω
10
VDIODE
Schottky Diode Forward
Voltage
IDIODE = 100mA, TA = +25°C
600
OVP
Overvoltage Protection
ISL97634IRT14Z
14
V
ISL97634IRT18Z
18
V
ISL97634IRT26Z
26
VIL
Logic Low Voltage of
PWM/EN
VIH
Logic High Voltage of
PWM/EN
850
1.5
mV
28
V
0.6
V
V
PWM_on
Minimum PWM On-Time
1.5
µs
EN_delay
EN to Vout Delay
200
µs
2
FN6264.3
March 7, 2008
ISL97634
Block Diagram
VIN (2.4V TO 5.5V)
L
CIN
VIN
PWM/EN
1.4MHZ OSCILLATOR AND RAMP
GENERATOR
LX
ISL97634
VOUT
COUT
PWM
COMPARATOR
PWM LOGIC
CONTROLLER
FET
DRIVER
2 LEDs to 7 LEDS
CURRENT
SENSE
GND
FBSW
PWM/EN
GM AMP
COMPENSATION
GM
AMPLIFIER
FB
95mV
BANDGAP
REFERENCE
GENERATOR
RSET
Pin Description
PIN NUMBER
PIN NAME
DESCRIPTION
1
GND
Ground Pin. Connect to local ground.
2
VIN
Input Supply Pin. Connect to the input supply voltage, the inductor and the input supply decoupling capacitor.
3
PWM/EN
PWM or Enable Pin. Connect external PWM signal allows pulse width modulation current operation. Enable
signal allows peak current operation or disable signal shuts down the device.
4
NC
No Connect
5
FB
Feedback Pin. Connect the sense resistor between FB and ground. The cathode of bottom LED can also
be connected at this pin if the output current is not to be PWMed.
6
FBSW
FB Disconnect Switch. Connect to the cathode of the bottom LED if the output current to be PWMed.
7
VOUT
Output Pin. Connect to the anode of the top LED and the output filter capacitor.
8
LX
Switching Pin. Connect to inductor.
3
FN6264.3
March 7, 2008
ISL97634
Typical Performance Curves
1.0
90
85
0.8
0.6
4 LEDs, 10µH
75
3 LEDs, 10µH
70
3 LEDs, 22µH
65
7 LEDs, 10µH
7 LEDs, 22µH
ILED_peak = 25mA
VIN = 4V
55
RSET = 4Ω
fPWM = 1kHz
0
20
0.4
0.2
60
50
Iq (mA)
EFFICIENCY (%)
80
40
60
80
PWM DUTY CYCLE (%)
0.0
-0.2
100
FIGURE 1. EFFICIENCY vs PWM DUTY CYCLE
0
1
2
3
VIN (V)
4
6
5
FIGURE 2. QUIESCENT CURRENT vs VIN (PWM/EN = HI)
20.08
19.76
19.74
20.04
IO (mA)
IO (mA)
19.72
20.00
19.70
19.68
19.66
19.96
19.64
19.92
0
5
10
15
VOUT (V)
20
25
30
19.62
2.5
FIGURE 3. LOAD REGULATION (VIN = 4V)
3.0
3.5
4.0
VIN (V)
4.5
5.0
5.5
FIGURE 4. LINE REGULATION
100
7 LEDs
VIN = 4V
RSET = 4Ω
L = 10µH
90
FB VOLTAGE (mV)
80
70
VOUT
VIN = 4V
1kHz
R1 = 4Ω
L1 = 22µH
LX
60
50
40
PWM/EN
30
20kHz
20
10
32kHz
ILED
0
0
20
40
60
80
100
120
DUTY CYCLE (%)
FIGURE 5. DIMMNG LINEARITY (FB VOLTAGE) vs DUTY
CYCLE
4
FIGURE 6. PWM DIMMING AT 1kHz, D = 1%
FN6264.3
March 7, 2008
ISL97634
VOUT
VOUT
LX
LX
PWM/EN
PWM/EN
VIN = 4V
ILED
R1 = 4Ω
L1 = 22µH
ILED
VIN = 4V
R1 = 4Ω
L1 = 22µH
FIGURE 7. PWM DIMMING AT 1kHz, D = 1% ZOOM IN
FIGURE 8. PWM DIMMING AT 1kHz, D = 99%
VOUT
LX
R1 = 4Ω
VIN = 4V
L1 = 22µH
PWM/EN
ILED
FIGURE 9. PWM DIMMING AT 20kHz, D = 50%
Detailed Description
The ISL97634 uses a constant frequency, current mode
control scheme to provide excellent line and load
regulation. There are three OVP models for driving 3, 4
and 7 LEDs (3.5V/20mA type) and their OVP thresholds
are set at 14V, 18V and 26V respectively. The ISL97634
operates from an input voltage of 2.4V to 5.5V and ambient
temperature from -40°C to +85°C. The switching frequency
is around 1.45MHz and allows the driver circuit to employ
small LC components. The peak forward current of the LED
is set using the RSET resistor. In the steady state mode, the
LED peak current is given by Equation1:
V FB
I LED = --------------R SET
(EQ. 1)
PWM Dimming
The ISL97634’s PWM/EN pin can be tied permanently to
high for a fixed current operation. On the other hand, the
ISL97634 can be applied with an external PWM signal to
5
pulse width modulated output current. It is well understood
that the LED brightness is a linear function of the LED
current. In addition, the average LED current corresponds to
the duty cycle “D” of the PWM signal as shown in
Equation 2:
V FB
I LED-AVG = --------------- ⋅ D
R SET
(EQ. 2)
As a result, PWM signal provides a means to dim the LED
brightness. PWM dimming offers the best LEDs matching
over DC dimming. It is because the LED peak current
operating point is far away from the knee of the diode I-V
curve where part to part variations are high. The PWM
dimming test results are shown in Figure 6 with two PWM
frequencies, 1kHz and 20kHz. The vertical scale parameter
FB is proportional to the current and therefore the
brightness.
For the ISL97634, PWM dimming provides linear dimming
adjustment with low frequency signal, such as 1kHz and
FN6264.3
March 7, 2008
ISL97634
below. The applied PWM dimming signal can be up to
32kHz; however, the dimming linearity is compromised at
low duty cycles as their durations are too short for the
ISL97634’s control loop to respond properly. This
non-ideality behavior does not cause any functional
problem. The PWM dimming linear responses in Figure 5
are expanded in Figure 10. At 1kHz PWM dimming, the duty
cycle can virtually vary from below 1% to DC. On the other
hand, at 20kHz PWM dimming, the linearity range is from
5% to DC only.
.
VOUT
LX
PWM/EN
VIN = 4V
10
9
FB VOLTAGE (mV)
8
7 LEDs
VIN = 4V
RSET = 4Ω
L = 10µH
ILED
R1 = 4Ω
L1 = 22µH
1kHz
7
FIGURE 12. PWM DIMMING AT 1kHz WITHOUT USING FBSW
6
The FBSW should be used for PWM dimming as illustrated
in “Typical Application Circuit” on page 1. During the PWM
off time, the FBSW is opened. The LEDs are floating and
therefore the output capacitor has no path to discharge. The
LED current responds accurately with the PWM signal (see
Figure 13). The output switches very quickly to the target
current with minimal settling ringing and without being
modulated by the PWM signal, and therefore minimizes any
system disturbance.
5
4
3
20kHz
2
1
32kHz
0
0
2
4
6
8
DUTY CYCLE (%)
10
12
FIGURE 10. DIMMING LINEARITY vs DUTY CYCLES ZOOM IN
The low level non-linearity effects at high frequency PWM
dimming is also reflected in the efficiency measurements in
Figure 11.
VOUT
LX
90
85
EFFICIENCY (%)
80
PWM/EN
75
R1 = 4Ω
L1 = 22µH
70
ILED
65
3 LEDs
VIN = 4V
RSET = 4Ω
L = 22µH
60
55
50
VIN = 4V
0
5
10
15
ILED (mA)
20
25
FIGURE 13. PWM DIMMING AT 1kHz USING FBSW
30
FIGURE 11. EFFICIENCY vs PWM DIMMING FREQUENCIES
Feedback Disconnect Switch
The ISL97634 functions properly without using the FBSW.
However, the output capacitor will discharge during the PWM
off time resulting in poor dimming linearity at low duty cycles.
The output discharge effect can be seen in Figure 12.
Moreover, the output is modulated by the PWM signal that
may create interference to other systems.
6
Overvoltage Protection
The ISL97634 comes with overvoltage protection. The OVP
trip points are at 14V, 18V and 26V for ISL97634IRT14Z,
ISL97634IRT18Z and ISL97634IRT26Z respectively. The
maximum numbers of LEDs and OVP threshold are shown in
Table 1. When the device reaches the OVP, the LX stops
switching, disabling the boost circuit until VOUT falls about 7%
below the OVP threshold. At this point, LX will be allowed to
switch again. The OVP event will not cause the device to
shutdown.
There are three OVP options so that the 3 LEDs application
should use the 14V OVP device and the 7 LEDs application
should use the 26V OVP device. An output capacitor that is
FN6264.3
March 7, 2008
ISL97634
only rated for the required voltage range can therefore be
used, which will optimize the component costs in some cases.
TABLE 1.
OVP
MAX NO. OF
LEDS
MAX ILED
ISL97634IRT14Z
14V
3
70mA
ISL97634IRT18Z
18V
4
50mA
ISL97634IRT26Z
26V
7
30mA
PART NO.
Compensation
The product of the output capacitor and the load create a
pole while the inductor creates a right half plane zero. Both
of these attributes degrade the phase margin but the
ISL97634 has internal compensation network that ensures
the device operates reliably under the specified conditions.
The internal compensation and the highly integrated
functions of the ISL97634 make it a design friendly device to
be used in high volume, high reliability applications.
Shutdown
Applications
When PWM/EN is taken low the ISL97634 enters into the
power-down mode where the supply current is reduced to
less than 1µA. The device resumes normal when the
PWM/EN goes high.
Analog Dimming
Components Selection
The input capacitance is typically 0.22µF. The output
capacitor should be in the range of 0.22µF to 1µF. X5R or
X7R type of ceramic capacitors of the appropriate voltage
rating are recommended.
When choosing an inductor, make sure the average and
peak current ratings are adequate by using Equations 3, 4 and
5 (80% efficiency assumed):
I LED ⋅ V OUT
I LAVG = --------------------------------0.8 ⋅ V IN
(EQ. 3)
Analog dimming is usually not recommended because of the
brightness non-linearity at low levels dimming. However,
some systems are EMI or noise sensitive that analog
dimming may be more suitable than PWM dimming under
those situations. The ISL97632 is part of the same family as
the ISL97634 and has been designed with a serial interface
to give access to 32 separate dimming levels. Alternatively
analog dimming can be achieved by applying a variable DC
voltage (VDim) at FB pin (see Figure 14) to adjust the LED
current. As the DC dimming signal voltage increases above
VFB, the voltages drop on R1 and R2 increase and the
voltage drop on RSET decreases. Thus, the LED current
decreases as shown in Equation 6:
V FB ⋅ ( R 1 + R 2 ) – V Dim ⋅ R 1
I LED = -------------------------------------------------------------------------R2 ⋅ R
(EQ. 6)
SET
1
I LPK = I LAVG + --- ⋅ ΔI L
2
(EQ. 4)
V IN ⋅ ( V OUT – V IN )
ΔI L = --------------------------------------------------L ⋅ V OUT ⋅ f OSC
(EQ. 5)
Where:
If VDIM is taken below FB, the inverse will happen and the
brightness will increase.
The DC dimming signal voltage can be a variable DC voltage
from a POT, a DCP (Digitally Controlled Potentiometer), or a
DC voltage generated by filtering a high frequency PWM
control signal.
L1
• ΔIL is the peak-to-peak inductor current ripple in Amps
22µH
• L is the inductance in H
VIN
• fOSC is the switching frequency, typically 1.45MHz
The ISL97634 supports a wide range of inductance values
(10µH to ~82µH). For lower inductor values or lighter loads,
the boost inductor current may become discontinuous. For
high boost inductor values, the boost inductor current will be
in continuous mode.
In addition to the inductor value and switching frequency, the
input voltage, number of LEDs and the LED current also
affects whether the converter operates in continuous
conduction or discontinuous conduction mode.
Both operating modes are allowed and normal. The
discontinuous conduction mode yields lower efficiency due
to higher peak current.
LEDs
VIN
3.3V
LX
VOUT
C1
1µF
C2
0.22µF
ISL97634
R1
PWM
FB
GND
R2
3.3k
RSET
4.75Ω
VDim
FIGURE 14. ANALOG DIMMING CONTROL APPLICATION
CIRCUIT
As brightness is directly proportional to LED currents, VDim
may be calculated for any desired “relative brightness” (F)
using Equation 7:
R1
R2
⎛
⎞
V Dim = ------- ⋅ V FB ⋅ ⎜ 1 + ------- – F⎟
R1
R2
⎝
⎠
(EQ. 7)
Where F = ILED (dimmed)/ILED (undimmed).
7
FN6264.3
March 7, 2008
ISL97634
The analog dimming circuit can be tailored to a desired
relative brightness for different VDim ranges using
Equation 8.
[ ( V Dim_max – V FB ) • R 1 ]
R 2 = ------------------------------------------------------------------[ V FB • ( 1 – F min ) ]
(EQ. 8)
Where VDim_max is the maximum VDim voltage and Fmin is
the minimum relative brightness (i.e., the brightness with
VDim_max applied).
i.e., VDim_max = 5V, Fmin = 10% (i.e., 0.1), R2 = 189k
i.e., VDim_max = 1V, Fmin = 10% (i.e., 0.1), R2 = 35k
Efficiency Improvement
Figure 1 shows the efficiency measurements during PWM
operation. The choice of the inductor has a significant impact
on the power efficiency. As shown in Equation 4, the higher
the inductance, the lower the peak current, therefore, the
lower the conduction and switching losses. On the other hand,
it has also a higher series resistance. Nevertheless, the
efficiency improvement effect by lowering the peak current is
greater than the resistance increases with larger value of
inductor. Efficiency can also be improved for systems that
have high supply voltages. Since the ISL97634 can only
supply from 2.4V to 5.5V, VIN must be separated from the
high supply voltage for the boost circuit as shown in Figure 15
and the efficiency improvement is shown in Figure 16.
C3 0.22µF
D1
L1
Vs = 12V
C1
1
C2
22µH
1µF
VIN = 2.7V TO 5.5V
0.1µF
D2
2
D3
D4
VIN
LX
VOUT
ISL97634
FBSW
D5
D6
FB
PWM/EN
GND
R1
4Ω
FIGURE 15. SEPARATE HIGH INPUT VOLTAGE FOR HIGHER
EFFICIENCY OPERATION
8
.
90
EFFICIENCY (%)
These equations are valid for values of R1 and R2 such that
both R1>>RSET and R2>>RSET.
VS = 12V
VS = 9V
85
80
VIN = 4V
7 LEDs
L1 = 22µH
R1 = 4Ω
fPWM
75
70
0
5
10
15
20
25
30
ILED (mA)
FIGURE 16. EFFICIENCY IMPROVEMENT WITH 9 AND 12V
INPUTS
8 LEDs Operation
For medium size LCDs that need more than 7 low power LEDs
for backlighting, such as a portable media player or automotive
navigation panel displays, the voltage range of the ISL97634 is
not sufficient. However, the ISL97634 can be used as an LED
controller with an external protection MOSFET connected in
cascode fashion to achieve higher output voltage. A conceptual
8 LEDs driver circuit is shown in Figure 17. A 60V logic level NChannel MOSFET is configured such that its drain ties between
the inductor and the anode of Schottky diode, its gate ties to the
input, and its source ties to the ISL97634 LX node connecting
to the drain of the internal switch. When the internal switch
turns on, it pulls the source of M1 down to ground and LX
conducts as normal. When the internal switch turns off, the
source of M1 will be pulled up by the follower action of M1,
limiting the maximum voltage on the ISL97634 LX pin to below
VIN, but allowing the output voltage to go much higher than the
breakdown limit on the LX pin. The switch current limit and
maximum duty cycle will not be changed by this setup, so input
voltage will need to be carefully considered to make sure that
the required output voltage and current levels are achievable.
Because the source of M1 is effectively floating when the
internal LX switch is off, the drain-to-source capacitance of M1
may be sufficient to capacitively pull the node high enough to
break down the gate oxide of M1. To prevent this, VOUT should
be connected to VIN, allowing the internal Schottky diode to
limit the peak voltage. This will also hold the VOUT pin at a
known low voltage, preventing the built in OVP function from
causing problems. This OVP function is effectively useless in
this mode as the real output voltage is outside its intended
range. If the user wants to implement their own OVP protection
(to prevent damage to the output capacitor), they should insert
a zener diode from VOUT to the FB pin. In this setup, it would
be wise not to use the FBSW to FB switch, as otherwise, the
zener diode will have to be a high power one capable of
dissipating the entire LED load power. Then the LED stack can
then be connected directly to the sense resistor via a 10kΩ
resistor to FB. A zener can be placed from VOUT to the FB pin
allowing an overvoltage event to pull-up on FB with a low
breakdown current (and thus low power zener diode) as a
result of the 10kΩ resistor.
FN6264.3
March 7, 2008
ISL97634
1
C1
1µF
L1
D0
2
2.2µ
10BQ100
M1
FQT13N06L
SK011C226KAR
VIN
C2
0.1µF
C3 4.7µF
VOUT
LX
ISL97634
FBSW
D1
D2
D3
D4
FB
PWM/EN
GND
D5
R1
6.3Ω
D6
D7
D8
FIGURE 17. CONCEPTUAL 8 LEDs HIGH VOLTAGE DRIVER
SEPIC Operation
For applications where the output voltage is not always
above the input voltage, a buck or boost regulation is
needed. A SEPIC (Single Ended Primary Inductance
Converter) topology, shown in Figure 18, can be considered
for such an application. A single cell Li-ion battery operating
a cellular phone backlight or flashlight is one example. The
battery voltage is between 2.5V and 4.2V, depending on the
state of charge. On the other hand, the output may require
only one 3V to 4V medium power LED for illumination
because the light guard of the backlight assembly is
optimized for cost efficiency trade-off reason.
In fact, a SEPIC configured LED driver is flexible enough to
allow the output to be well above or below the input voltage,
unlike the previous example. Another example is when the
number of LEDs and input requirements are different from
platform to platform, a common circuit and PCB that fit all the
platforms in some cases may be beneficial enough that it
outweighs the disadvantage of adding additional component
cost. L1 and L2 can be a coupled inductor in one package.
VIN = 2.7V TO 5.5V 1 L1 2
22µH
C1
1µF
VA
C3
VB D0
1µF
C4
L2
22µH
(EQ. 9)
where D is the on-time of the PWM duty cycle.
The convenience of SEPIC comes with some trade-off in
addition to the additional L and C costs. The efficiency is
usually lowered because of the relatively large efficiency loss
through the Schottky diode if the output voltage is low. The L2
series resistance also contributes additional loss. Figure 19
shows the efficiency measurement of a single LED application
as the input varies between 2.7V and 4.2V.
Note VB is considered the level-shifted LX node of a standard
boost regulator. The higher the input voltage, the lower the VB
voltage will be during PWM on period. The result is that the
efficiency will be lower at higher input voltages because the
SEPIC has to work harder to boost up to the required level.
This behavior is the opposite to the standard boost regulator’s
and the comparison is shown in Figure 19.
76
VIN = 2.7V
72
VIN = 4.2V
68
1 LED
L1 = L2 = 22µH
64
C3 = 1µF
R1 = 4.7Ω
60
D1
LX
VOUT
ISL97634
FBSW
0
5
10
ILED (mA)
15
20
FIGURE 19. EFFICIENCY MEASUREMENT OF A SINGLE LED
SEPIC DRIVER
FB
SDIN
D
V OUT = V IN -----------------(1 – D)
0.22µF
VIN
C2
0.1µF
The SEPIC works as follows; assume the circuit in Figure 18
operates normally when the ISL97634 internal switch opens
and it is in the PWM off state. After a short duration where few
LC time constants elapsed, the circuit is considered in the
steady-state within the PWM off period that L1 and L2 are
shorted. VB is therefore shorted to the ground and C3 is
charged to VIN with VA = VIN. When the ISL97634 internal
switch closes and the circuit is in the PWM on-state, VA is now
pulled to ground. Since the voltage in C3 cannot be changed
instantaneously, VB is shifted downward and becomes -VIN.
The next cycle when the ISL97634 switch opens, VB boosts
up to the targeted output like the standard boost regulator
operation, except the lowest reference point is at -VIN. The
output is approximated in Equation 9:
EFFICIENCY (%)
VIN = 2.7V TO 5.5V
PCB Layout Considerations
GND
R1
1Ω
FIGURE 18. SEPIC LED DRIVER
The simplest way to understand SEPIC topology is to think
about it as a boost regulator where the input voltage is level
shifted downward at the same magnitude and the lowest
reference level starts at -VIN rather than 0V.
9
The layout is very important for the converter to function
properly. RSET must be located as close as possible to the FB
and GND pins. Longer traces to the LEDs are acceptable.
Similarly, the supply decoupling cap and the output filter cap
should be as close as possible to the VIN and VOUT pins.
The heat of the IC is mainly dissipated through the thermal pad
of the package. Maximizing the copper area connected to this
pad if possible. In addition, a solid ground plane is always
helpful for the EMI performance.
FN6264.3
March 7, 2008
ISL97634
Thin Dual Flat No-Lead Plastic Package (TDFN)
L8.2x3A
2X
0.15 C A
A
D
8 LEAD THIN DUAL FLAT NO-LEAD PLASTIC PACKAGE
2X
MILLIMETERS
0.15 C B
SYMBOL
E
MIN
A
0.70
A1
-
6
A3
INDEX
AREA
b
TOP VIEW
D2
0.20
0.10
SIDE VIEW
C
SEATING
PLANE
D2
(DATUM B)
0.08 C
A3
7
0.75
0.80
-
-
0.05
-
0.25
0.32
1.50
1.65
1.75
1
7,8
3.00 BSC
-
8
1.65
e
1.80
1.90
7,8
0.50 BSC
-
k
0.20
-
-
-
L
0.30
0.40
0.50
8
N
8
Nd
4
D2/2
6
INDEX
AREA
5,8
C
E2
A
NOTES
2.00 BSC
E
//
MAX
0.20 REF
D
B
NOMINAL
2
3
Rev. 0 6/04
2
NX k
NOTES:
1. Dimensioning and tolerancing conform to ASME Y14.5-1994.
2. N is the number of terminals.
3. Nd refers to the number of terminals on D.
(DATUM A)
E2
4. All dimensions are in millimeters. Angles are in degrees.
E2/2
5. Dimension b applies to the metallized terminal and is measured
between 0.25mm and 0.30mm from the terminal tip.
NX L
6. The configuration of the pin #1 identifier is optional, but must be
located within the zone indicated. The pin #1 identifier may be
either a mold or mark feature.
N N-1
NX b
e
8
5
0.10
(Nd-1)Xe
REF.
M C A B
7. Dimensions D2 and E2 are for the exposed pads which provide
improved electrical and thermal performance.
8. Nominal dimensions are provided to assist with PCB Land
Pattern Design efforts, see Intersil Technical Brief TB389.
BOTTOM VIEW
CL
(A1)
NX (b)
L
5
SECTION "C-C"
C C
TERMINAL TIP
e
FOR EVEN TERMINAL/SIDE
All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems.
Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without
notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and
reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result
from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see www.intersil.com
10
FN6264.3
March 7, 2008
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