EL7801
®
Data Sheet
PRELIMINARY
April 5, 2006
FN7354.1
High Power LED Driver
Features
The EL7801 is a high-power LED backlight driver with an
integrated 36V FET designed to drive up to 8 high-power
LEDs in series while running from a 12V input supply. The
PWM converter runs from an internally generated 1MHz
clock. With efficiencies over 90% the regulator provides tight
control of LED current and may be configured in either boost
or buck topologies, allowing from 1 to 8 series diodes to be
driven from a 12V input.
• Drives 1-8 high-power LEDs in series, up to 32V
LED light level may be controlled either by:
• LED over-temperature protection
• 2.7V to 16V input voltage range
• Boost or buck configurable switch
• 3A integrated FET
• Automotive load dump protection
• Light output temperature compensation
1. LED DC bias current set via the LEVEL pin, or
• LED disconnect
2. External low frequency PWM control via the
ENABLE/PWM pin.
• PWM/analog light level control
In both control modes optional over temperature thermal
protection of the LED reduces the LED DC bias current
above a customer set temperature, protecting the LED from
thermal damage. An optional fault monitor drives an external
FET between the input supply and inductor, providing short
circuit current protection for the LED and inductor as well as
load dump protection for automotive applications. For low
cost applications the pass transistor may be omitted and the
fault pin bypassed.
The EL7801 is packaged in a 20 Ld 4mm x 4mm QFN
package and is specified for operation over the -40°C to
+105°C temperature range.
• Small, 20 Ld 4mm x 4mm QFN package
• Pb-free plus anneal available (RoHS compliant)
Applications
• Display backlighting
- Automotive
- LCD monitor
- Notebook displays
• LED accent lighting
• Automotive lighting
Pinout
Ordering Information
20 Ld 4x4 QFN
MDP0046
EL7801ALZ-T13
13”
20 Ld 4x4 QFN
MDP0046
NOTE: Intersil Pb-free plus anneal products employ special Pb-free
material sets; molding compounds/die attach materials and 100%
matte tin plate termination finish, which are 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.
1
16 VBAT
7”
VDC 1
15 ENL
14 MODE
VHI 2
THERMAL
PAD
OVP 3
13 EN/PWM
SWD1 4
12 SWS1
SWD2 5
11 SWS2
TMAX 10
EL7801ALZ-T7
17 NC
MDP0046
FB 9
20 Ld 4x4 QFN
18 GND
-
TEMP 8
EL7801ALZ
19 FAULT
PKG. DWG. #
LEVEL 7
PACKAGE
(Pb-free)
20 VIN
TAPE &
REEL
BOOST/BUCKN 6
PART
NUMBER
(Note)
EL7801
(20 LD 4X4 QFN)
TOP VIEW
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. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
EL7801
Absolute Maximum Ratings (TA = 25°C)
Supply Voltage, VIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18V
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . GND -0.3V to VSP +0.3V
Continuous Output Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1A
Battery Input, VBAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24V
Storage Temperature Range . . . . . . . . . . . . . . . . . .-65°C to +150°C
Ambient Operating Temperature . . . . . . . . . . . . . . .-40°C to +105°C
Operating Junction Temperature . . . . . . . . . . . . . . . . . . . . . . +125°C
Power Dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . See Curves
CAUTION: Stresses above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the
device at these or any other conditions above those indicated in the operational sections of this specification is not implied.
IMPORTANT NOTE: All parameters having Min/Max specifications are guaranteed. Typical values are for information purposes only. Unless otherwise noted, all tests
are at the specified temperature and are pulsed tests, therefore: TJ = TC = TA
Electrical Specifications
PARAMETER
VBAT = VIN = 12V, VDC = 5V, IOUT = 350mA, TA = -40°C to +105°C unless otherwise specified.
DESCRIPTION
CONDITIONS
MIN
TYP
MAX
UNIT
VIN
Input Supply Voltage
Normal operating range
2.7
16
V
VBAT
Input Supply Monitor
Normal operating range
2.7
16
V
Supply Fault Threshold
If VBAT > VBATFAULT, FAULT pin is
switched to ground
17.6
20
24.4
V
ISEN
Supply Current in VIN
No switching, EN/PWM = 1
TBD
2
mA
ISDIS
Supply Current in VIN
No switching, EN/PWM = 0
TBD
10
µA
RSWITCH
Power FET On Resistance
ISWITCH = 1.2A
0.15
Ω
VDC
Regulated Auxiliary Supply
VBATFAULT
4.75
5
5.25
V
ROUTOL
Auxiliary Supply Open Loop Output
Resistance
VIN < VDC
50
Ω
ROUTCL
Auxiliary Supply Closed Loop Output
Resistance
VIN > 6V, F < 100Hz
5
Ω
CMIN
VDC Filter (Compensation) Capacitor
IOUT
Output Drive Current
4 LED output string
ILIMBOOST
Power Switch Current Limit
BOOST/BUCKN = VDC
3.6
A
ILIMBOOST
Power Switch Current Limit
BOOST/BUCKN = GND
2.4
A
20
ns
34
V
TDELOV
0.1
Over Voltage Positive Going Voltage
Mode Threshold
Upper threshold to enter overvoltage fault
mode
OVPL
Over Voltage Negative Going Voltage
Mode Threshold
Lower threshold to exit overvoltage fault
mode
Switch Driver Supply
In buck mode (VHI - VIN),
In boost mode (VHI - GND)
Feedback Voltage
System in regulation, VLEVEL = 1V
Light Control Voltage Input Range
Mode = 1, analog control of LED current
VHIGATE
VFB
VLEVEL
FSW
FMOD
TSWITCH
RLSDRIVER
1000
Transition Delay from Current to Voltage LX fault protection activation
Mode
OVPH
Switching Frequency
TBD
17
GND
Mode = 1, modulation signal applied to
EN/PWM
Load Switch Transition Time
CGATE = 2nF
1
VDC
V
V
3
V
TBD
MHz
10
kHz
ns
25
TBD
Ω
TBD
50
TBD
ms
Load Switch Driver Impedance
Fault Timer Period
TDELAY
Start-up Delay
Timed LX switching delay
TBD
1
VGATE
External FET Gate Clamp
|VFAULT - VIN|
TBD
10
Fault Pin Charge Pump
VBAT = VIN = 3V
2
V
100
TFAULT
VFAULTPUMP
mA
TBD
0.2
TBD
External Light Modulation Frequency
µF
6
ms
TBD
V
V
FN7354.1
April 5, 2006
EL7801
Electrical Specifications
PARAMETER
VBAT = VIN = 12V, VDC = 5V, IOUT = 350mA, TA = -40°C to +105°C unless otherwise specified. (Continued)
DESCRIPTION
CONDITIONS
MIN
TYP
MAX
UNIT
VDC/3
V
VMODEL
Mode Pin Input Low Threshold
VDC = 5V
VMODEH
Mode Pin Input Low Threshold
VDC = 5V
2x
VDC/3
V
enFAULT
Input Level Applied to TMAX Pin to
Enable Fault Protection
VDC = 5V
0.94VDC
V
disFAULT
Input Level Applied to TMAX Pin to
Disable Fault Protection
VDC = 5V
0.96VDC
V
enTEMP
Input Level Applied to TEMP Pin to
Enable Temperature Compensation
VDC = 5V
0.06VDC
V
disTEMP
Input Level Applied to TEMP Pin to
Disable Temperature Compensation
VDC = 5V
0.04VDC
V
TTRIP
Internal Temperature Protection
Threshold
135
°C
THYS
Internal Temperature Protection
Hysteresis
25
°C
VEN/PWML
EN/PWM Pin Input Low Threshold
VEN/PWMH
EN/PWM Pin Input High Threshold
1.2
V
2.5
V
.
TABLE 1. LIGHT OUTPUT CONTROL, VDC = 5.0V
MODE
TEMP
1
(VDC - 0.25V) > V > 0.25V
Don’t Care
V < 0.25V
0
V < (VDC - 0.25V)
3
OPERATING MODE
Standard Mode light level to PWM modulation of EN/PWM input; LED bias current
determined by LEVEL voltage, nominal 1V
Disable temperature compensation
Fixed Bias Mode VFB level internally set to 0.4V, independent of VLEVEL
FN7354.1
April 5, 2006
EL7801
Typical Application Diagram
VBAT
VIN
VDC
0.1µF
VHI
FAULT
SWD1
VBAT
SWD2
VDC
TEMP
SENSOR
OVP
BOOST/BUCKN
PWM
TEMP
SWS1
TMAX
SWS2
EN/PWM
FB
MODE
ENL
LEVEL
GND
1V
BOOST MODE
FIGURE 1. TYPICAL APPLICATION CIRCUIT
Pin Descriptions
PIN
NAME
1
VDC
Internally regulated 5V supply, tracks VIN for input voltages less than 5V
2
VHI
Power FET gate drive supply
3
OVP
Overvoltage monitor input; tie to VOUT for normal operation
4
SWD1
NMOS power FET drain
5
SWD2
NMOS power FET drain
6
DESCRIPTION
BOOST/BUCKN Digital input, configures controller to operate in BOOST or BUCK mode, low for BOOST, high for BUCK
7
LEVEL
Sets LED bias current level; VFB(nominal) = VLEVEL/5
8
TEMP
Temperature reference, tie to GND to disable temperature compensation
9
FB
10
TMAX
Maximum LED temperature set point; If TEMP voltage exceeds TMAX, FB set point is reduced
11
SWS2
NMOS power FET source
12
SWS1
NMOS power FET source
13
EN/PWM
14
MODE
15
ENL
16
VBAT
17
N/C
Leave floating (internally connected)
18
GND
Ground return and FB ground reference
19
FAULT
20
VIN
LED current feedback
Chip enable, light modulation PWM input
Digital Input; tie to GND to set FB reference to 400mV, tie to VDC to control FB reference with LEVEL input
LED load isolation switch gate driver
Input supply monitor
Gate drive of fault FET. Driven low under fault conditions
Input supply and FB pin supply reference
4
FN7354.1
April 5, 2006
Functional Block Diagram
2.7V-16V
L
VBAT
FAULT
VIN
VDC
VHI
5
GND
START-UP
CHARGE PUMP
VSTART
FAULT CONTROL
AND TIMER
CLOCK AND RAMP
GENERATOR
HALT
LDO AND REF
REF
VSTART
CLK
RAMP
OVP
SWD2
VDC
POR
LEVEL (T)
INNER LOOP
PWM CONTROL
AND CURRENT
LIMIT
EN O/P
LEVEL
SWS1
FET
CURRENT
SENSE
LIGHT CONTROL
SWS2
VDC
EN O/P
ENL
MODE
EN/PWM
MODE CONTROL
BOOST/
BUCKN
TEMPERATURE
COMPENSATION
LOAD
CURRENT
SENSE
FB
EL7801
HALT
TEMP
TMAX
FIGURE 2. EL7801 BLOCK DIAGRAM
EL7801
REF
SWD1
CLK
RAMP
HALT
FN7354.1
April 5, 2006
EL7801
Theory of Operation
General Description
Switching Regulator
The EL7801 employs a current mode PWM control scheme
with a nominal switching frequency of 1MHz. This provides
fast transient response and enables the use of low profile
inductors and compact multilayer ceramic capacitors.
Settling time is optimized by the use of a simple control loop
without an error amplifier, relying instead on intrinsic gain
within the direct summing path. Due to the lower loop gain,
offset must be accounted for when setting up initial LED bias
current. Refer to the applications section of the datasheet for
further information. Figure 2 shows a block diagram of the
system.
Application Configurations
Operating Modes
VIN
FB
Voltage
Feedback
GND
0.5
LEVEL
SHIFT
RSENSE
The EL7801 is a flexible, highly integrated high-power LED
driver consisting of a PWM switching controller and
integrated 36V NDMOS power FET. The device can drive up
to 8 series high-power LED's at currents up to 1A. The
control loop can be configured as either as a boost or buck
regulator, providing an output voltage above or below the
input supply voltage, depending on the number of stacked
LED's. The controller operates from 2.7V to 16V and can be
powered by a single lithium ion battery, 5V or 12V regulated
supplies or automotive electrical systems. LED current is
sensed through a low value resistor in series with the LED.
The resistor may be referenced to ground or the input rail,
allowing operation with supplies that span the output
voltage, for example a lithium ion battery driving one LED.
Load current can be adjusted using a thermistor to correct
for the reduction in optical efficiency of white LED's with
increasing temperature. The thermistor is also used to
implement a thermal protection scheme to limit the
maximum LED temperature to a preset customer level.
+
EL7801
VDC /2
FIGURE 3. FB REFERENCE AUTO SWITCH
Start-up
To maximize external PWM switching speed, the EL7801
doesn't include an internal soft-start circuit. When VDC
exceeds the power on reset threshold, switching is delayed
for 1ms (TDELAY) allowing the output capacitor to charge
through the inductor. If soft-start control is required, a
suitable application circuit is shown in Figure 4.
VBAT
VBAT FAULT
EL7801
10µH
L1
VOUT
VIN
COUT
C1
4.7nF
SWD1
SWD2
20µF
R1
FB
SWS1 SWS2
R2
2k
100
0.5
RSENSE
FIGURE 4. EXTERNAL SOFT-START CIRCUIT
The EL7801 can operate as either a buck or boost regulator.
Hardwire BUCK/BOOSTN to GND for boost mode or to VDC
for buck mode. In buck mode the power NDMOS drive circuit
is "floated" (boot-strapped) allowing the NDMOS gate to be
driven above VIN to fully enhance the power NDMOS. An
internal Schottky diode between VDC (5V) and VHI reduces
external component count. Use a ceramic capacitor of at
least 50nF between VHI and SWS1/2 to bootstrap VHI.
Light Level Control
LED Load Connection
LED color temperature varies with bias current. In
backlighting applications PWM dimming offers better control
of color temperature because current through the LED's is
kept constant. A 5V gate driver (ENL) synchronized to
EN/PWM can be used to control an external FET and
disconnect the LED stack during the PWM off period. The
switch prevents discharge of the output capacitor by the LED
load, maintaining a constant bias independent of PWM duty
cycle. Operation at 1kHz PWM rate is shown in Figure 5 and
EL7801 includes an auto-sensing FB level shift circuit that
enables the LED load to be connected to either GND or VIN.
An internal sense circuit monitors the FB pin voltage. When
the level exceeds VDC/2, the feedback reference voltage is
switched from GND to VIN. Refer to the application section
of the datasheet for typical application schematics.
6
Two light control schemes are provided:
1. An external PWM signal via the EN/PWM pin, providing
low frequency PWM dimming.
2. Bias current level adjustment via the LEVEL input or fixed
internal bias.
PWM Dimming
FN7354.1
April 5, 2006
EL7801
Figure 6. The load disconnect switch improves PWM
dynamic range, linearity and color temperature control. To
further improve the linearity of PWM dimming, an internal
timer delays system shutdown via EN/PWM for 50ms.
The value of VFB should be limited to between 50mV and
450mV for linear operation and is internally limited to 500mV.
LEVEL voltages above 2.5V will have no effect on LED
current. With MODE tied to GND, voltage across the
feedback resistor is set at ~400mV via an internal reference.
In either operating mode, if LED temperature control is
enabled the value of VFB will be reduced when maximum
LED temperature is exceeded.
Input Overvoltage
For automotive applications, an external high voltage NFET
driven by the FAULT pin disconnects the device from the
input supply in response to voltage spikes on the input
supply. During start-up an internal charge pump drives the
FAULT pin above the input voltage, ensuring the NFET is
fully enhanced and powering up the device. In normal
operation the switching node of the boost regulator or the
floating supply of the buck regulator is used to pump FAULT
above VIN. On detection of an overvoltage, the FAULT pin is
discharged to GND. The gate to source voltage of the
NDMOS is internally limited to ±10V to prevent voltage
stress.
FIGURE 5. OPERATION WITH ENL
Fault Protection
The external NFET is also used as a fault protection switch,
disconnecting the input supply if a fault occurs for more than
50ms. The system monitors feedback voltage regulation,
output overvoltage and input overvoltage. For applications
not requiring input voltage or fault protection, connect VBAT
and VIN directly together. All faults except input supply
overvoltage latch the EL7801 into an off state that can be
cleared by either power cycling the input supply or the
EN/PWM pin. Connecting the TMAX pin to VDC disables the
fault latch function (LED over temperature control is also
disabled).
Output Overvoltage Protection (OVP)
FIGURE 6. OPERATION WITH NO ENL
Bias Current Dimming
Current in the LED load is determined by the value of the
feedback resistor and the target feedback regulation voltage:
V FB
I LED = ----------------------R SENSE
With MODE tied to VDC, voltage across the feedback
resistor is set by VLEVEL:
If the FB pin is shorted to ground or an LED fails open
circuit, output voltage in BOOST mode can increase to
potentially damaging voltages. An optional overvoltage
protection circuit can be enabled by connection of the OVP
pin to the output voltage. The device will stop switching if the
output voltage exceeds OVPH and re-start when the output
voltage falls below OVPL. During sustained OVP fault
conditions, VOUT will saw-tooth between the upper and
lower threshold voltages at a frequency determined by the
magnitude of current available to discharge the output
capacitor and the value of output capacitor used.
The OVP threshold can be set to a lower value by using an
external zener diode and resistor, as shown in Figure 7. R1
should be adjusted to minimize offset in the FB voltage due
to FB pin input current. A value of 100Ω is recommended.
V LEVEL
V FB = --------------------5
7
FN7354.1
April 5, 2006
EL7801
10µH
L1
0.47uF
VOUT
VIN
EL7801
VIN
COUT
SWD1
SWD2
VDC
EL7801
LDO
20µF
ZOVP
FB
SWS1 SWS2
Thermistor
Close to
LED's
CREG
RT
R1
100K
0.5
VBAT FAULT
RSENSE
VBAT
+
100
0.5
RSENSE
+
-
-
+
RT
100K
TEMP
-
RT
10K
GND
Temp Compensation
FIGURE 7. EXTERNAL OVP CIRCUIT
Over Temperature Shutdown
FIGURE 9. TEMPERATURE COMPENSATION CIRCUIT
An internal sense circuit disables PWM switching if the die
temperature exceeds 135°C. Switching is re-enabled when
the temperature falls below 100°C.
Internal 5V LDO
An internal LDO between VIN and VDC regulates VDC to
5V, to power control and gate drive circuits when VIN
exceeds 5.1V. In normal operation decouple VDC with at
least 0.47µF. In applications where the input supply is less
than 5.5V, VDC should be tied directly to VIN.
Temperature Compensation
140
120
110
100
40
90
70
60
50
-20
30%
30
80
0
20
40
60
80
100
120
JUNCTION TEMPERATURE, TJ (°C)
FIGURE 8. HIGH POWER WHITE LED LIGHT OUTPUT
VARIATION WITH JUNCTION TEMPERATURE
EL7801 incorporates a supply referenced temperature
interface to increase LED load current with temperature.
Disable the function by connecting the TEMP pin to GND.
8
% BIAS VARIATION
RELATIVE LIGHT OUTPUT (%)
At a constant current, high power white LED light intensity
reduces as junction temperature increases.
In use, connect a potential divider comprised of an NTC
thermistor and low temperature coefficient resistor between
VDC and GND. Locate the thermistor physically close to the
LED load for accurate temperature sensing. Connect the tap
point of the divider to the TEMP pin. Temperature changes
vary the VDC divider ratio and adjust the voltage present at
VTEMP, providing up to ±30% adjustment in FB bias level. A
10K resistor and Murata NCP18XH103f03RB thermistor will
set VTEMP at VDC/2 at room temperature. Different
thermistor and resistor values may be used to tailor the
system temperature coefficient for specific LED families.
Alternatively, temperature coefficient can be fine tuned by
inserting limit resistors in series and parallel with the
thermistor to bound impedance variation with temperature.
For the LED temperature variation shown in Figure 8, a
suitable arrangement is shown in Figure 11.
20
10
0
-10
-20
-30
-30%
-40
0
0.2
0.4
0.6
0.8
1.0
VTEMP/VDC
FIGURE 10. FB VOLTAGE VARIATION WITH VTEMP/VDC RATIO
FN7354.1
April 5, 2006
EL7801
input supply, improving system stability. The high switching
frequency of the loop causes almost all ripple current to flow
in the input capacitor, which must be rated accordingly.
VDC
Considerably more input current ripple is generated in buck
mode than boost mode. In buck mode input current is
alternately switched between IOUT and zero. The rms
current flow in the input capacitor is given by:
MURATA
NCP18XH103F03RB
R1
R2
23k
300
2
I CAPRMS = I OUT • ( D – D )
TEMP
Where: D = Duty Cycle
R3
4k
The input current is maximum for D = 0.5 and when IOUT
approaches current limit (2.4A) giving a value of around
1.2A.
A capacitor with low internal series resistance should be
chosen to minimize heating effects and improve system
efficiency, such as X5R or X7R ceramic capacitors, which
offer small size and a lower value of temperature and voltage
coefficient compared to other ceramic caps.
FIGURE 11. THERMISTOR VOLTAGE COEFFICIENT
ADJUSTMENT
LED Temperature Control
LED lifetime reduces dramatically with elevated
temperature. An over temperature control circuit utilizing the
thermistor voltage at TEMP reduces the LED bias current
when VTEMP exceeds the threshold voltage on TMAX. To
minimize noise injection use a potential divider between
VDC and GND to set the voltage on TMAX, as shown in
Figure 12. The value of TMAX for a specific threshold
temperature is determined by the choice of thermistor
temperature coefficient. Disable the function by connecting
the TMAX pin to GND.
VIN
VDC
RM1
RSENSE
20k
LDO
TMAX
RM2
80k
+
FB Level Adjust
Current
TEMP
Temp
Compensation
RT
10K
GND
EL7801
FIGURE 12. OVER-TEMPERATURE CIRCUIT
Component Selection
Input Capacitor
Switching regulators require input capacitors to deliver peak
charging current and to reduce the impedance of the input
supply. This reduces interaction between the regulator and
9
In automotive applications the input capacitor can be
protected from exposure to high voltages present during
fault conditions (load dump) by connecting it downstream of
the fault protection switch, as shown in Figures 19 and 20.
Inductor
Thermistor
Close to
LED's
CREG
0.5
0.47uF
In boost mode input current flows continuously into the
inductor, with an AC ripple component proportional to the
rate of inductor charging only and smaller value input
capacitors may be used. It is recommended that an input
capacitor of at least 10µF be used. Ensure the voltage rating
of the input capacitor is suitable to handle the full supply
range.
Careful selection of inductor value will optimise circuit
operation. Inductor type and value influence many key
parameters, including ripple current, current limit, efficiency,
transient performance and stability. Internal slope
compensation has been optimised for inductor values
between 4.7µH and 10µH. Ensure the inductor current
rating is capable of handling the current limit value in the
configuration used (2.4A for buck, 3.5A for boost). If an
inductor core is chosen with too low a current rating,
saturation in the core will cause the effective inductor value
to fall, leading to an increase in peak to average current
level, poor efficiency and overheating in the core.
Rectifier Diode
A high speed rectifier diode is necessary to prevent
excessive voltage overshoot, especially in the boost
configuration. Low forward voltage and reverse leakage
current will minimize losses, making Schottky diodes the
preferred choice. Similarly to the inductor, a diode with a
suitable current rating to handle current limit in the
configuration must be used.
FN7354.1
April 5, 2006
EL7801
Output Capacitor
where
The output capacitor acts to smooth the output voltage and
in the boost configuration supplies load current directly
during the conduction phase of the power switch. Ripple
voltage consists of two components, the first due to charging
and discharging of the capacitor; the second due to IR drop
across the ESR of the capacitor by inductor ripple current.
V OUT
D = --------------V IN
For a low ESR ceramic capacitor, output ripple is dominated
by the charging and discharging of the output capacitor.
Care should be taken to ensure the voltage rating of the
capacitor exceeds the maximum output voltage.
In boost mode:
Compensation
IO
D
V RIPPLE = ---------------- × ------- + I LPK × ESR
C OUT F S
The EL7801 employs a direct summing control loop with
current feedback. No error amplifier is used in the system.
The arrangement provides fast transient response and
makes use of the output capacitor to compensate the loop.
The effect of the pole associated with the inductor is
minimized by the current feedback. The number of LEDs,
their DC bias current and the value of feedback resistor alter
loop stability due to their effect on feedback factor which is
heavily influenced by the small signal impedance of the
LEDs. Generally, higher numbers of LEDs, lower bias levels
and smaller values of feedback resistor will require smaller
output capacitors to achieve loop stability. A combination of
low ESR electrolytic and ceramic capacitors may be used to
reduce implementation costs.
where:
V OUT – V IN
D = ------------------------------V OUT
and
IO
( V OUT – V IN ) ( 1 – D )
× ------------------ + -----------------------------------I LPK = -----------fs
2×L
1–D
In buck mode:
( V IN – V OUT ) × D
D
V RIPPLE = ----------------------------------------------- × -------------------------- + ESR
f × C
2 × fs × L
s
OUT
TABLE 2. BOOST MODE COMPENSATION. 2.7V OPERATION
VOUT (V)
7
10.5
14
17.5
21
24.5
28
4
5
6
7
8
DMAX
DMAX
VFB
IOUT
LED’s
2
3
50mV
50mA
Electrolytic
94µF
47µF
Ceramic
40µF
20µF
40µF
20µF
20µF
Electrolytic
94µF
Ceramic
60µF
60µF
40µF
40µF
40µF
Electrolytic
94µF
47µF
47µF
47µF
ILIM
ILIM
ILIM
Ceramic
60µF
40µF
40µF
40µF
Electrolytic
ILIM
ILIM
ILIM
ILIM
ILIM
ILIM
ILIM
100mV
200mV
200mV
100mA
350mA
1A
Ceramic
TABLE 3. BOOST MODE COMPENSATION. 5V OPERATION
VOUT (V)
7
10.5
14
17.5
21
24.5
28
4
5
6
7
8
40µF
20µF
20µF
20µF
20µF
40µF
40µF
40µF
40µF
VFB
IOUT
LED’s
2
3
50mV
50mA
Electrolytic
94µF
47µF
Ceramic
40µF
20µF
Electrolytic
141µF
47µF
Ceramic
60µF
60µF
60µF
Electrolytic
141µF
47µF
47µF
Ceramic
60µF
60µF
40µF
60µF
40µF
40µF
40µF
Electrolytic
94µF
47µF
ILIM
ILIM
ILIM
ILIM
ILIM
Ceramic
40µF
40µF
100mV
200mV
200mV
100mA
350mA
1A
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EL7801
TABLE 4. BOOST MODE COMPENSATION. 12V OPERATION.
VOUT (V)
7
10.5
14
17.5
21
24.5
28
2
3
4
5
6
7
8
DMIN
DMIN
DMIN
60µF
40µF
40µF
40µF
47µF
47µF
40µF
20µF
40µF
40µF
47µF
47µF
40µF
20µF
40µF
40µF
47µF
47µF
20µF
20µF
40µF
40µF
VFB
IOUT
LED’s
50mV
50mA
Electrolytic
Ceramic
100mV
100mA
Electrolytic
Ceramic
200mV
350mA
1A
DMIN
DMIN
DMIN
DMIN
Electrolytic
Ceramic
DMIN
A Note about Ceramic Capacitors:
Many ceramic capacitors have strong voltage and
temperature coefficients which reduces effective
capacitance as the applied voltage or operating temperature
is increased. Pay careful attention when selecting ceramic
capacitor type. X5R and X7R families provide much better
stability than Y5V, which should generally be avoided unless
additional capacitance is added to compensate for the
significant changes in value which occurs over voltage and
temperature.
TABLE 5. CERAMIC CAPACITOR VARIABILITY
CAPACITOR TYPE
DMIN
Electrolytic
Ceramic
200mV
DMIN
TYPICAL VOLTAGE
VARIATION
TEMPERATURE
VARIATION
DMIN
DMIN
• Place several via holes (thermal vias) under the chip to a
backside ground plane to improve heat dissipation
• Maximize the copper area around the thermal vias to
spread heat away from the chip.
The demo board is a good example of layout based on this
outline. Please refer to the EL7801 Application Brief for more
detailed information.
Cost-Sensitive Applications
For cost-sensitive applications, the BOM can be reduced
considerably by:
1. Removing temperature compensation
2. Removing the fault-protection switch
X7R, 10V
-30% at 10V
-15% at 125°C
3. Removing the load isolation switch
X5R, 25V
-50% at 25V
-9% at 85°C
Y5V, 6.3V
-90% at 6.3V
-65% at 85°C
4. Switching the FB into internal fixed bias mode (400mV
across VFB)
Layout Considerations
PCB layout is very important for the converter to function
properly. The following general guidelines should be
followed:
In this configuration, light level may be controlled using the
EN/PWM input to chop the output current.
In the absence of the load isolation switch, LED bias current
will vary with PWM duty cycle, due to the discharge of the
output capacitor by the LED’s during the PWM off time.
• Separate the Power Ground and Signal Ground; connect
them only at one point close to the GND pin
• Place the input capacitor close to VIN and SWS1,2 pins in
boost mode
• Make the following PC traces as short as possible:
- from SWD1,2 to the inductor in boost mode
- from SWS1,2 to the inductor in buck mode
- from Cout to PGND
• Feedback signals levels are small to improve efficiency.
Ensure the reference connection (GND or VIN) between
the sense resistor and IC pin doesn't carry switching
current.
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EL7801
Typical Boost Application Diagram
VBAT
VIN
FAULT
SWD1
VBAT
SWD2
VDC
TEMP
SENSOR
EN
VLEVEL (0V TO 2.5V)
VHI
OVP
TEMP
SWS1
TMAX
SWS2
EN/PWM
ENL
MODE
FB
LEVEL
GND
BUCK/BOOSTN
Minimum BOM Boost Application Diagram
VBAT
VIN
FAULT
SWD1
VBAT
SWD2
VDC
EN
VHI
OVP
TEMP
SWS1
TMAX
SWS2
EN/PWM
ENL
MODE
FB
LEVEL
GND
BUCK/BOOSTN
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EL7801
Typical Boost Application Diagram - Supply-Return Load
VBAT
VHI
VIN
FAULT
SWD1
VBAT
SWD2
VDC
TEMP
SENSOR
EN
VLEVEL (0V TO 2.5V)
OVP
TEMP
SWS1
TMAX
SWS2
EN/PWM
ENL
MODE
FB
LEVEL
GND
BUCK/BOOSTN
Minimum BOM Boost Application Diagram - Supply-Return Load
VBAT
VIN
FAULT
SWD1
VBAT
SWD2
VDC
EN
VHI
OVP
TEMP
SWS1
TMAX
SWS2
EN/PWM
ENL
MODE
FB
LEVEL
GND
BUCK/BOOSTN
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EL7801
Typical Buck Application Diagram
VBAT
VIN
FAULT
SWD1
VBAT
SWD2
VDC
TEMP
SENSOR
EN
VLEVEL (0V TO 2.5V)
VHI
OVP
TEMP
SWS1
TMAX
SWS2
EN/PWM
ENL
MODE
FB
LEVEL
GND
BUCK/BOOSTN
Minimum BOM Buck Application Diagram
VBAT
VIN
FAULT
SWD1
VBAT
SWD2
VDC
EN
VHI
OVP
TEMP
SWS1
TMAX
SWS2
EN/PWM
ENL
MODE
FB
LEVEL
GND
BUCK/BOOSTN
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EL7801
Automotive Applications
The protection circuit is applicable to buck, boost, and
supply-return load applications.
The LED load and EL7801 may be protected against load
dumps and other electrical faults in automotive supplies with
a minor addition to the standard application schematic:
A small reduction in efficiency is caused by the drop in the
power schottky.
• A reverse transient automotive-rated protection power
schottky must be added in series with the input supply
Unless alternative transient protection is provided,
minimum BOM automotive applications must include the
circuit changes noted above.
• A 500Ω current limit resistor must be inserted in series
with the VBAT pin
• The fault protection NFET must be specified to handle
100V VDS conditions.
Automotive Boost Application Diagram
VBAT
RLIM
VHI
VIN
500
FAULT
SWD1
VBAT
SWD2
VDC
TEMP
SENSOR
OVP
TEMP
SWS1
TMAX
SWS2
EN/PWM
EN
VLEVEL (0V TO 2.5V)
ENL
MODE
FB
LEVEL
GND
BUCK/BOOSTN
Automotive Minimum BOM Boost Application Diagram
VBAT
VIN
FAULT
SWD1
VBAT
SWD2
VDC
EN
VLEVEL (0V TO 2.5V)
VHI
OVP
TEMP
SWS1
TMAX
SWS2
EN/PWM
ENL
MODE
FB
LEVEL
GND
BUCK/BOOSTN
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EL7801
QFN Package Outline Drawing
NOTE: The package drawing shown here may not be the latest version. To check the latest revision, please refer to the Intersil website at
http://www.intersil.com/design/packages/index.asp
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
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