PAM PAM2841

PAM2841
1.5A SW Current, 40V Precision WLED Driver
Features
n
n
n
n
n
n
n
n
n
n
n
Description
Capable of Driving 10 or more WLEDs
Chip Enable with Soft-start
Analog and PWM Dimming
Peak Efficiency up to 90%
Low Quiescent Current
Fixed Frequency of 1MHz
Over Current Protection
Over Voltage Protection
Thermal Protection
UVLO
Tiny Pb-Free Packages (RoHS
Compliant): MSOP-8 and DFN 2X2
The PAM2841 is a white LED driver, capable of
driving 10 or more WLEDs in series (depending on
forward voltage of the LEDs) with a range of input
voltages from 2.7V to 5.5V.
The PAM2841 features over current protection , over
voltage protection , under voltage lockout and over
temperature protection, which prevent the device
from damage.
LED dimming can be done by four methods as
described in the Application Information hereinafter.
Applications
n WLED Driver System
Typical Application
D1(SS14)
L1
V IN
22 μ H
C1
10 μ F
PGND
SW
R1
910kΩ
C3
1μF
VIN
C2
ENA
1μF
OVP
PAM2841
Comp
R2
27kΩ
FB
10 LEDs
GND
C4
10nF
R3
10Ω
D1(SS14)
V IN
L1
C1
22 μ H
10 μ F
PGND
SW
R1
910kΩ
C3
1μF
VIN
C2
PAM2841
ENA
1μF
3x9 LEDs
OVP
Comp
R2
27kΩ
FB
GND
R3
1.1Ω
C4
10nF
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
1
PAM2841
1.5A SW Current, 40V Precision WLED Driver
Block Diagram
Comp
200mV
Reference
FB
OVP
SW
VIN
Comparator
+
GM
-
PWM
+
PWM Logic
And Driver
+
Σ
CS
Ramp
Generator
ENA
Shutdown And
Soft-start
1.0MHz
Oscillator
GND
PGND
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
2
PAM2841
1.5A SW Current, 40V Precision WLED Driver
Pin Configuration & Marking Information
TOP View
MSOP-8
8
1
P2841
XXXYW
2
3
7
XXX: Internal Code
Y: Year
W: Weekly
6
4
5
DFN2X2
1
3
EMX
YW
2
8
7
6
4
EM: Product Code
X: Internal Code
Y: Year
W: Weekly
5
Pin number
Name
Description
1
PGND
Power Ground
2
VIN
Input Voltage
3
ENA
Chip Enable, Active High
4
Comp
Compensation Node
5
GND
Chip Ground
6
FB
7
OVP
Over Voltage
8
SW
Drain of Main Switch.
Feedback
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
3
PAM2841
1.5A SW Current, 40V Precision WLED Driver
Absolute Maximum Ratings
These are stress ratings only and functional operation is not implied . Exposure to absolute
maximum ratings for prolonged time periods may affect device reliability . All voltages are with
respect to ground .
O
Supply Voltage.............................................6.0V
Output Current............................................50mA
I/O Pin Voltage Range.........GND-0.3V to V DD+0.3V
Maximum SW Pin Voltage...............................40V
O
Storage Temperature.....................-40 C to 150 C
O
Maximum Junction Temperature..................150 C
O
Soldering Temperature.......................300 C,5sec
Recommended Operating Conditions
Junction Temperature Range.............0 OC to 125 OC
Supply Voltage Range........................2.8V to 5.5V
O
O
Operation Temperature Range..........-40 C to 85 C
Thermal Information
Parameter
Symbol
Thermal Resistance
θJA
(Junction to Ambient)
Thermal Resistance
θJC
(Junction to Case)
Package
Maximum
Unit
MSOP
180
°C/W
DFN
102
°C/W
MSOP
40
°C/W
DFN
20
°C/W
Electrical Characteristic
V EN=V DD=5.0V, 10 LEDs, T A=25 C , unless otherwise noted .
O
Parameters
Supply Voltage Range
Symbol
Test Conditions
VDD
IQ
No Switching
Shutdown Current
ISD
VENA=low
IO
200
R3=5.1Ω
40
R3=6.8Ω
30
R3=10Ω
20
R3=20Ω
10
Output Voltage Range
VO
Feedback Voltage
VFB
SW On Resistance
RDS(ON)
SW Current Limit
ILIM
SW Frequency
fSW
Maximum Duty Cycle
DC
Over Voltage Protection Threshold
OVP
Open Load
VEL
Chip Shutdown
VEH
Chip On
1.2
UVLO
VIN falling
2.0
Enable Threshold Voltage
Under Voltage Lockout
TYP.
2.7
Quiescent Current
Output Current
MIN.
VINx1.1
MAX.
UNITS
5.5
V
300
μA
1
μA
mA
40
V
200
206
mV
ISW=100mA
0.35
0.5
Ω
Switch On
1.5
VENA=high
194
0.7
1.0
A
1.3
MHz
95
%
1.2
V
0.4
2.2
2.4
V
V
UVLO Hysterisis
VTH
0.2
V
Over Temperature Shutdown
OTS
150
°C
Over Temperature Hysterisis
OTH
30
°C
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
4
PAM2841
1.5A SW Current, 40V Precision WLED Driver
Typical Operating Characteristics
T A=25 ° C,V DD=5V, unless otherwise noted .
2. Efficiency VS Load Current
1. Efficiency VS Input Voltage
6 LED, L=22 µ H
90%
95 %
85%
90 %
80%
85 %
75%
Efficiency
Efficiency
I LED=20mA, L=22 µ H
70%
65%
60%
8LED
12LED
2
3
4
5
V DD =5V
VDD=3.6V
65%
10LED
50%
75%
70%
6LED
55%
80 %
60%
6
0
10
20
Input Voltage(V)
30
40
50
60
Output Current(mA)
3. Efficiency VS Input Voltage
4. LED Current V S Output Voltage
L= 47µ H
I LED=20mA, 10 LED
0.5
0.9
VDD=5V
VDD=3V
0.85
LED Current (A)
0.4
Efficiency
0.8
0.75
0.7
L=4.7uH
L=10uH
L=22uH
L=33uH
L=47uH
0.65
0.3
0.2
0.1
0
0.6
2
3
4
5
6
8
Input Voltage(V)
12
16
20
24
28
32
Output Voltage (V)
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
5
36
PAM2841
1.5A SW Current, 40V Precision WLED Driver
Typical Operating Characteristics
T A=25 ° C,V DD=5V, unless otherwise noted .
5. Feedback Voltage VS Input voltage
6. Feeback Voltage VS Temperature
10 LED, ILED=20mA
0.3
0.25
0.25
0.2
0.2
Vfb(V)
0.3
0.15
0.15
0.1
0.1
0.05
0.05
0
0
2
3
4
5
6
0
20
40
Input Voltage (V)
60
80
100
120
140
Temperature
8. Frequency VS Input Voltage
7. LED current VS Input voltage
10 LED
L=22 μ H, 10 LED
25
1.2
20
1
Frequency(Mhz)
ILED(mA)
Vfb (V)
10 LED, ILED=20mA
15
10
0.8
0.6
0.4
R=10
5
0.2
R=15
R=20
0
0
2
3
4
5
2
6
3
4
5
6
Input Voltage(V)
Vin(V)
Power Analog Microelectronics , Inc
www.poweranalog.com
6
08/2008 Rev 1.3
PAM2841
1.5A SW Current, 40V Precision WLED Driver
Typical Operating Characteristics
T A=25 ° C,V DD=5V, unless otherwise noted .
10. Frequency VS Temperature
( 10 LED, I LED=20mA)
200
1.13
180
1.12
160
Frequency(Mhz)
Quiescent Current (uA)
9. Quiescent Current VS Input Voltage
140
120
100
80
60
1.11
1.1
1.09
1.08
1.07
40
1.06
20
0
1.05
2
3
4
5
0
6
20
Input voltage (V)
40
60
80
100
120
140
Temperature
11. LED Current VS Duty Cycle of PWM
([email protected] Pin)
12. LED Current VS Duty Cycle of PWM
([email protected] Pin)
25
20
18
20
Theoretic
LED Current(mA)
LED Current (mA)
16
14
12
f=100Hz
10
8
6
f=100HZ
4
f=200Hz
2
0
0%
40%
60%
80%
10
5
f=200Hz
theoretic
20%
15
0
0%
100%
20%
40%
60%
80%
100%
Duty Cycle of PWM
Duty Cycle of PWM
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
7
PAM2841
1.5A SW Current, 40V Precision WLED Driver
Typical Operating Characteristics
T A=25 ° C,V DD=5V, unless otherwise noted .
13. Start-up Waveform
14. Steady State Waveform
Vin=5V,10LED,Iled=20mA
Vin=5V,10LED,Iled=20mA
ENA
DC coupling
SW
DC coupling
IL
DC coutpling
IL
DC coupling
VOUT
DC coupling
Vout
AC coupling
15.Open load
16. Open load
Vout
AC coupling
Vout
DC coutpling
OVP
AC coupling
SW
DC coupling
SW
DC coutpling
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
8
PAM2841
1.5A SW Current, 40V Precision WLED Driver
Application Information
Inductor Selection
to the device.
The selection of the inductor affects steady state
operation as well as transient behavior and loop
stability. These factors make it the most important
component in power regulator design. There are
three important inductor specifications, inductor
value, DC resistance and saturation current.
Considering inductor value alone is not enough.
Output Capacitor
The output capacitor is mainly selected to meet
the requirement for the output ripple and loop
stability. This ripple voltage is related to the
capacitor's capacitance and its equivalent series
resistance (ESR). A output capacitor of 1μF
minimum is recommended and maybe need a
larger capacitor. The total output voltage ripple
has two components: the capacitive ripple
caused by the charging and discharging on the
output capacitor, and the ohmic ripple due to the
capacitor's equivalent series resistance (ESR):
The inductor value determines the inductor ripple
current. Choose an inductor that can handle the
necessary peak current without saturation, the
inductor DC current given by:
Iin_dc=Vout*Iout/(Vin* η )
η =efficiency.
V RIPPLE=V RIPPLE(C)+V RIPPLE(ESR)
Inductor values can have ±20% tolerance with no
current bias. When the inductor current
approaches saturation level, its inductance can
decrease 20% to 35% from the 0A value
depending on how the inductor vendor defines
saturation current. Using an inductor with a
smaller inductance value causes discontinuous
PWM when the inductor current ramps down to
zero before the end of each switching cycle. This
reduces the boost converter's maximum output
current, causes large input voltage ripple and
reduces efficiency. Large inductance value
provides much more output current and higher
conversion efficiency. For these reasons, an
inductor within 4.7μH to 22μH value range is
recommended.
V RIPPLE(C)≈
2
I OUT)
1/2*(L/(C OUT *((V OUT(MAX) -V IN(MIN) ))))*(I
2
PEAK
-
V RIPPLE(ESR)=I PEAK*R ESR(COUT)
Where I PEAK is the peak inductor current.
Multilayer ceramic capacitors are an excellent
choice as they have extremely low ESR and are
available in small footprints. Capacitance and
ESR variation with temperature should be
considered for best performance in applications
with wide operating temperature ranges.
Dimming Control
Schottky Diode Selection
There are 4 different types of dimming control
methods:
The high switching frequency of the PAM2841
demands a high-speed rectification for optimum
efficiency. Ensure that the diode average and
peak current rating exceeds the average output
current and peak inductor current. In addition, the
diode's reverse breakdown voltage must exceed
the open protection voltage.
1). Using an External PWM Signal to EN Pin
With the PWM signal applied to the EN pin, the
PAM2841 is alternately turned on or off by the
PWM signal. The LEDs operate at either zero or
full current. The average LED current changes
proportionally with the duty cycle of the PWM
signal. A 0% duty cycle turns off the PAM2841 and
leads to zero LED current. A 100% duty cycle
generates full current.Also the recommend
dimming frequency is between 100Hz and 200Hz.
Input and Output Capacitor Selection
Input Capacitor
At least a 1 μ F input capacitor is recommended to
reduce the input ripple and switching noise for
normal operating conditions. Larger value and
lower ESR (Equivalent Series Resistance) may
be needed if the application require very low input
ripple. It follows that ceramic capacitors are a
good choice for applications. Note that the input
capacitor should be located as close as possible
I AVE=I STATE* (T ON –T STARTUP)/(T ON+T OFF)
where T ON: on time of a period
T STARTUP: 0.85ms
T OFF: off time of a period
I STATE: on state current (full current)
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
9
PAM2841
1.5A SW Current, 40V Precision WLED Driver
L1
1
VIN
2
1
3). Using a DC Voltage
For some applications, a simple and direct way
to control brightness is use an external variable
DC voltage to vary the voltage drop on feedback
resistor. This will make the PAM2841 adjust the
output current to follow the change of feedback
voltage. The circuit is shown in Figure 3. As the
DC voltage increases, the voltage drop on R4
increases and the voltage drop on R3
decreases. Thus, the LED current decreases.
The selection of R4 and R5 will make the current
from the variable DC source much smaller than
the LED current and much larger than the FB pin
current. For VCC range from 0V to 2V, the
selection of resistors in Figure 3 gives dimming
control of LED current from 0mA to 20mA.
D1
2
22uH
C1
10uF
C3
1uF
R1
910k
U12
1
0
2
3
1uF
C2
4
8
PGND
SW
VIN
OV
OVP
ENA
FB
COMP
7
0
6
R2
27k
5
GND
PAM2841
PWM
C4
10n
Figure. 1
2). Using an External PWM Signal to NMOS
Gate.
When PWM signal is at high level, N MOSFET
turned on, then pull comp pin down, then the
LED current should be zero. When PWM signal
is at low level, N MOSFET turned off. The circuit
uses resistor R1 to set the on state current. The
average LED current changes proportionally
with the duty cycle of the PWM signal. A 100%
duty cycle turns off the PAM2841 and leads to
zero LED current. A 0% duty cycle generates full
current.
L1
1
VIN
2
1
D1
2
22uH
C1
10uF
C3
1uF
R1
910k
U33
1
0
PGND
2
3
1uF
SW
VIN
OV
OVP
ENA
C2
4
FB
COMP
GND
8
7
0
R2
27k
6
5
PAM2841
ENA
C4
10n
R4
R5
90k
5k
R3
10
I AVE=I STATE* (T ON –T STARTUP)/(T ON+T OFF)
Figure. 3
where T ON: on time of a period
T STARTUP: 0.08ms
T OFF: off time of a period
I STATE: on state current (full current)
Also the recommend frequency is between 100
and 500Hz. Frequency<100Hz can naturally
causes LEDs to blink visibly.
L1
1
VIN
2
1
4). Using a Filtered PWM Signal
The filtered PWM signal can be considered as an
adjustable DC voltage. Such regulated signal is
often with some grade of ripple because of some
simple configuration of circuit. With appropriated
arrangement of PWM frequency and level, and
filter parameters, it can be used to replace the
variable DC voltage source in dimming control.
The circuit is shown in Figure 4.
D1
2
22uH
C1
10uF
C3
1uF
DC
R1
910k
U12
1
0
2
3
1uF
C2
4
PGND
SW
VIN
OV
OVP
ENA
FB
COMP
GND
8
7
6
0
L1
R2
27k
1
VIN
2
1
D1
2
22uH
5
C1
10uF
C3
1uF
PAM2841
ENA
Q1
PWM
R1
910k
U33
1
0
2
C4
10n
3
1uF
R3
10
C2
4
PGND
SW
OVP
OV
VIN
ENA
FB
COMP
GND
8
7
0
R2
27k
6
5
PAM2841
ENA
C4
10n
R4
R5
90k
Figure. 2
5V
5k
R3
10
R6
PWM
10k
0V
C5
100nF
0
Figure. 4
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
10
PAM2841
1.5A SW Current, 40V Precision WLED Driver
Layout Considerations
Layout Example
Top Layer
As for all switching power supplies, especially
those in high frequency and high current ones,
layout is an important design step. If layout is not
carefully done, the regulator could suffer from
instability as well as noise problems.
(1) Use separate traces for power ground and
signal ground. Power ground and signal ground
are connected together to a quite ground (input
connector)
(2) To prevent radiation of high frequency
resonance, proper layout of the high frequency
switching path is essential. Minimize the length
and area of all traces connecting to the SW pin.
The loop including the PWM switch, schottky
diode and output capacitor, contains high current
rising and falling in nanosecond and thus it
should be kept as short as possible.
Bottom Layer
(3) The input capacitor should be close to both
the VIN pin and the GND pin in order to reduce
the IC supply ripple.
(4) Keep the signal ground traces short and as
close to the IC as possible. Small signal
components should be placed as close as
possible to the IC, thus minimizing control signal
noise interference.
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
11
PAM2841
1.5A SW Current, 40V Precision WLED Driver
Ordering Information
PAM2841 X X
Shipping
Package Type
Part Number
Marking
Package
Shipping
PAM2841SR
P2841
MSOP-8
3,000 units/Tape & Reel
PAM2841GR
EMX
DFN 2x2-8
3,000 units/Tape & Reel
YW
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
12
PAM2841
1.5A SW Current, 40V Precision WLED Driver
Outline Dimensions
MSOP-8
E
HE
e
DETAIL A
b
A1
C
A
A2
D
L
L1
DETAIL A(S=3:1)
REF.
Millimeter
REF.
Millimeter
Min
Max
Min
Max
A
--
1.10
L
0.40
0.70
A1
0.05
0.15
L1
A2
0.78
0.94
B
0.22
0.38
D
2.90
3.10
C
0.08
0.23
E
2.90
3.10
e
HE
4.75
5.05
0.95 BSC
0.65 BSC
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
13
PAM2841
1.5A SW Current, 40V Precision WLED Driver
Outline Dimensions
DFN 2x2
D
A
B
INDEX AREA
(D/2 xE/2)
E
4
TOP VIEW
C
A
A1
9
8
SEATING
PLANE
SIDE VIEW
0.08 C
e
Nxb
Pin#1 ID
INDEX AREA
(D/2 xE/2)
NxL
NxK
E2
4
D2
BOTTOM VIEW
SYMBOL
A
A1
Lead
Pitch (e)
0.50
MIN
0.70
0.00
COMMON DIMENSION
NOM
MAX
0.75
0.80
0.02
0.05
Summary Table
Lead
Body
Count
Size
8
2X2
Pin #1 ID
R0.20
D BSC
E BSC
MIN
b NOM
MAX
MIN
D2 NOM
MAX
MIN
E2 NOM
MAX
MIN
L NOM
MAX
N
2.00
2.00
0.18
0.25
0.30
1.05
1.20
1.30
0.45
0.60
0.70
0.20
0.30
0.40
8
Unit: Millimeters
Power Analog Microelectronics , Inc
www.poweranalog.com
08/2008 Rev 1.3
14