MP1521
High Efficiency
White LED Driver
Monolithic Power Systems
General Description
The MP1521 is a constant-current Boost
Regulator.
This IC has three individual
current-sensing feedbacks, which are ideal for
driving multiple strings of series connected
LEDs. The OLS pin is designed for detecting
loose or open LED connections.
The IC uses a peak current, constant minimum
off-time architecture. The feedback pins
measure voltage across sense resistors in
series with the LED strings. When supplying a
bias current of 20mA, the voltage drop across
the sense resistor is only 0.4V to minimize
power loss. Either a DC voltage or a PWM
signal at the BRT/PWM input can control the
LED brightness.
The MP1521 has an on board power MOSFET
switch that is protected by current limit, open
load shutdown, thermal shutdown, and under
voltage lockout.
Features
On Board Power MOSFET
Drive 9 White LEDs from 2.7V Input
Drive 15 White LEDs from 5V Input
Up to 90% Efficiency
Over 80mA Output Current Capacity
Open Load Shutdown
Low Current Sensing Feedback Voltage
Up to 3 String Current Sensing Feedback
Analog and Digital PWM Brightness Control
UVLO, Thermal Shutdown
Soft Start
Internal Current Limit
Available in Compact MSOP10 and
Tiny
3 mm x 3 mm
QFN16 Package
Evaluation Boards Available
Applications
Handheld Computers, Cell phones, PDAs,
Digital and Video Cameras, Small LCD
Displays
Li Ion and NiMH Battery Applications
Ordering Information
Part Number
∗
MP1521EK
MP1521EQ
EV0005
EV0021
Package
MSOP10
QFN16 (3x3)
Temperature
-20° to +85°C
-20° to +85°C
MP1521EK Evaluation Board
MP1521EQ Evaluation Board
∗
For Tape & Reel, add suffix –Z (i.e., MP1521EK–Z)
For Lead Free, add suffix –LF (i.e., MP1521EK–LF–Z)
Typical Application Circuit (One String of Six (6) LEDs)
L1
D1
1
V
IN
C
IN
Efficiency vs LED current, 1 string of 6
Using Toko D62CB 47µH Inductor
100
Efficiency (%)
90
80
70
60
50
5
10
15
20
25
LED Current (mA)
V
IN
= 5V
V
IN
= 4V
V
IN
= 3V
IN
SW
10
ON
OFF
MP1521
EN
2
C
OUT
GND
9
GND
EN
3
REF
OLS
8
BRT/
PWM
4
BRT/PWM
FB1
7
White LEDs
5
FB3
FB2
6
MP1521 Rev 2.7 10/06/04
www.monolithicpower.com
1
MP1521
High Efficiency
White LED Driver
Monolithic Power Systems
Absolute Maximum Ratings
Supply Voltage (V
IN
)
Switch Voltage to GND
V
EN
, V
REF
, V
BRT
, V
FB1
, V
FB2
, V
FB3
Storage Temperature
28V
-0.5V to 28V
–0.3V to 6V
-55°C to +150°C
Recommended Operating Conditions
Supply Voltage (V
IN
)
Operating Temperature
Switch Voltage to GND
2.7V to 25V
-20°C to +85°C
-0.5V to 25V
150°C/W
60°C/W
Package Thermal Characteristics
Thermal Resistance,
Θ
JA
(MSOP10)
Thermal Resistance,
Θ
JA
(QFN16)
Electrical Characteristics
(V
IN
= 5.0V, T
A
= 25°C unless specified otherwise)
Parameters
Supply Current (shutdown)
Supply Current (quiescent)
Minimum Off Time, normal
operation
Minimum Off Time, start-up
(Note 1)
UVLO
Under Voltage Lockout
Enable
Threshold
Hysterisis
Leakage Current
Open Load Shutdown
Threshold
Leakage Current
Reference
Output Voltage
Load Regulation
Line Regulation
Feedback Comparator
Threshold
Hysterisis (Note 2)
FB Input Bias Current
PWM Mode
Output-Off Threshold
BRT Input Bias
BRT Full Scale Threshold
(Note 3)
Output Switch
Switch On Resistance (Note 2) R
ON
Switch Current Limit
Leakage Current
Thermal Shutdown (Note 2)
Symbol
I
IN (OFF)
I
IN (ON)
t
OFF (NORMAL)
t
OFF (START)
Condition
Min
V
EN
<0.4V
V
EN
>1.5V, V
BRT
=0V, V
FB
=0V
V
EN
>1.5V, V
BRT
=0.6V
400
V
FB
=0
V
EN
>1.5V, V
BRT
=0.6V,
1.5
V
FB
=0V
V
IN RISING
V
IN FALLING
2.1
1.9
0.8
V
EN
=3V
Measured at OLS Pin
V
OLS
=15V
V
OLS
=25V
I
REF
=0A
I
REF
=0 to 100µA
V
IN
=3 to 15V, I
REF
=0A
V
BRT
=1.2V
V
BRT
=1.2V
V
FB
=0.2V
V
BRT
=0.6V
26
45
1.20
1.235
2.5
2.5
400
30
-50
-30
1.0
60
Typ
1
550
500
Max
3
700
700
2.4
2.6
2.4
2.0
1
29
1
70
1.27
5
5
420
0
0.26
0
1.2
V
IN
=5.0V
300
V
SW
=25V
160
0.5
400
0.6
5
Units
µA
µA
nS
µS
V
V
V
mV
µA
V
µA
µA
V
mV
mV
mV
mV
nA
V
nA
V
Ω
mA
µA
°C
380
-150
0.18
-100
Notes:
1) First 64 pulses after EN switches high.
2) Guaranteed by design.
3) The output current is off when BRT is driven below the output-off threshold and full-scale when driven above the full-scale
threshold. To PWM the output drive BRT above and below these thresholds.
MP1521 Rev 2.7 10/06/04
www.monolithicpower.com
2
MP1521
High Efficiency
White LED Driver
Monolithic Power Systems
Pin Description
MSOP10
QFN16
SW
14
NC
16
15
IN
EN
REF
BRT
FB3
1
2
3
4
5
10
9
8
7
6
SW
GND
OLS
FB1
FB2
NC
EN
NC
REF
1
2
3
13
12
NC
GND
GND
OLS
FB1
11
10
8
9
IN
5
4
Top
View
6
7
FB3
BRT
Part Marking: MP1521EK
Part Marking: A1xx
Table 1: MP1521 MSOP10 and QFN16 Pin Assignments
MSOP
Pin #
1
QFN
Pin#
16
Pin
Name
IN
Pin Function
Input supply pin. It can be connected to the regulator’s input supply, or to the
output for boot-strapped operation (boot-strapped operation is required for V
IN
of
less than 3V).
Regulator On/Off control input. Logic high (V
EN
>1V) turns on IC, logic low puts it
into low current shutdown mode. Place a 100KΩ resistor in series with EN if it is
connected to a voltage greater than 10V.
1.23V reference used for setting the analog control voltage at BRT input. This
reference can supply up to 200µA to an external load.
Do not
connect any
bypass capacitors to this pin. It is internally compensated.
Analog or PWM input for controlling LED brightness. When used in analog
mode, apply a DC voltage of 0.3 to 1.2V. This input voltage is attenuated by 1/3
to set the voltage across sense resistors at FB1, FB2, and FB3. Maximum
brightness (20mA) occurs with 1.2V applied. To control brightness in PWM
mode, apply a PWM square wave with a minimum voltage less the 0.18V and a
maximum greater than 1.2V. When a voltage less than 0.18V is applied to this
pin, the output switch is turned off. The regulator operates at maximum
brightness when more than 1.2V is applied. The frequency of the PWM signal
must be between 100 to 400 Hz.
Feedback input for measuring voltage across current-sense resistor that is in
series with LED string. Unused feedback pins must be connected to any used
feedback pin.
Open Load Shutdown pin senses regulator output voltage to protect IC during
open load operation. When this pin’s voltage exceeds 28V, the output switch is
shut off. The output switch remains off until the IC is reset by cycling the enable
pin (EN) or input supply (IN) pin.
Signal and power ground. Connect directly to ground plane.
Output switch pin, power DMOS low-side switch.
No Connect
2
2
EN
3
4
REF
4
5
BRT
5
6
7
8
7
8
9
10
FB3
FB2
FB1
OLS
9
10
-
11, 12
14
1, 3, 6,
13, 15
GND
SW
N/C
MP1521 Rev 2.7 10/06/04
www.monolithicpower.com
FB2
NC
3
MP1521
High Efficiency
White LED Driver
Monolithic Power Systems
Figure 2: MP1521 Functional Block Diagram
IN
EN
Internal
Regulator
& Enable
Circuit
UVLO &
Thermal
Shutdown
SW
OLS
17V
160K
1.23V
20K
-
+
500nS
One-Shot
Timer
-
+
Control
Logic &
Output
Driv er
0.5 W
Power
FET
Open Load
Shutdown
Comparator
Current Limit
Comparator
-
+
20K
1.2V
-
+
-
+
-
+
10K
+
-
GND
0.25V
REF
Bandgap
Reference
1.23V
Feedback
Comparator
FB1
FB2
FB3
BRT
MP1521 Rev 2.7 10/06/04
www.monolithicpower.com
4
MP1521
High Efficiency
White LED Driver
Monolithic Power Systems
Functional Description
Operation
The MP1521 utilizes a constant off-time
control scheme (Refer to the Block
Diagram). For maximum brightness, the
BRT pin is biased at or above 1.2V. Its
voltage is routed through the unity gain
buffer whose the output of is clamped at
1.2V. A 20K/10K divider from the Buffer to
the non-inverting input of the Feedback
comparator divides the BRT voltage by
three to set the current sense threshold. If
any of the feedback pins FB1, FB2, or FB3
is lower than this threshold the Power FET
is turned on. The Power MOSFET and
inductor current increases until the current
limit comparator trips at 0.5A turning off the
MOSFET for 500ns. At the end of 500ns if
any of the Feedback pins are still lower
than the current sense threshold. The
Power FET is again turned on. This cycle
repeats itself until all three of the feedback
pins exceed the threshold.
Analog Brightness Adjust
Analog Brightness Adjust is accomplished
by applying a voltage between 0.26V to
1.2V on the BRT pin. The 1.23V REF
output can be used to bias a potentiometer
whose wiper drives the BRT pin.
PWM/ Brightness Adjust
PWM Brightness Adjust is accomplished
by driving the BRT pin with a square wave
whose low voltage is below 0.2V and
whose high voltage is greater than 1.2V. A
voltage below 0.2V forces the Power
MOSFET off. A voltage greater than 1.2V
will cause all three feedback pins to
regulate at or above 0.4V
Open Load Shut-Off
Open Load Shut-Off is implemented by
connecting the OLS pin to V
OUT
. In the
fault condition where the connection to the
LED’s is open, V
OUT
will rise up, as all the
feedback pins are absent of voltage. Once
V
OUT
exceeds 28V, the part will cease
operation until the EN pin is cycled off and
on. To disable the open load shut-off
feature, connect the OLS pin to GND.
MP1521 Rev 2.7 10/06/04
www.monolithicpower.com
5
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