August, 2004
1
M9999-081104
MIC2291
Micrel
MIC2291
1.2A PWM Boost Regulator
Photo Flash LED Driver
General Description
The MIC2291 is a 1.2MHz Pulse Width Modulation (PWM),
boost-switching regulator that is optimized for high-current,
white LED photo flash applications. With a guaranteed switch
current of 1.2A, the MIC2291 easily drives a string of 3 white
LEDs in series at 100mA, ensuring a high level of brightness
and eliminating several ballast resistors.
The MIC2291 implements a constant frequency, 1.2MHz
PWM control scheme. The high frequency PWM operation
saves board space by reducing external component sizes.
The added benefit of the constant frequency PWM scheme,
in contrast to variable frequency topologies, is much lower
noise and input ripple injected back to the battery source.
To optimize efficiency, the feedback voltage is set to only
95mV. This reduces the power dissipation in the current set
resistor, and allows the lowest total output voltage, hence
minimal current draw from the battery.
The MIC2291 is available with 2 levels of over-voltage
protection, 15V, and 34V. This allows designers to choose
the smallest possible external components with the appropri-
ate voltage ratings for their applications.
The MIC2291 is available in low-profile, Thin SOT23 5-lead
and 8-lead 2mm
2mm MLFTM package options. The MIC2291
has a junction temperature range of 40
C to +125
C.
All support documentation can be found on Micrel's web
site at www.micrel.com.
Typical Application
10
H
0.22
F
ceramic
0.95
95mV
1-Cell
Li Ion
3V to 4.2V
1
F
2
3
1
4
5
MIC2291BD5
VIN
EN
SW
FB
GND
100mA
Thin SOT23 Flash LED Driver
Features
2.5V to 10V input voltage
Output voltage up to 34V
1.2A switch current
1.2MHz PWM operation
95mV feedback voltage
Overvoltage protection (OVP)
Options for 15V and 34V
Stable with ceramic capacitors
<1% line and load regulation
1
A shutdown current
Over temperature protection
UVLO
Low-profile Thin SOT23-5 package option
2mm
2mm MLFTM package option
40
C to +125
C junction temperature range
Applications
Photo Flash LED driver
Cell phones
PDAs
GPS systems
Digital cameras
IP phones
LED flashlights
Micrel, Inc. 1849 Fortune Drive San Jose, CA 95131 USA tel + 1 (408) 944-0800 fax + 1 (408) 474-1000 http://www.micrel.com
MLF and
Micro
LeadFrame are trademarks of Amkor Technology, Inc.
PowerPAK is a trademark of Siliconix, Inc.
10
H
0.22
F
1
F
1-Cell
Li Ion
3V to 4.2V
0.95
MIC2291-15BML
VIN
EN
SW
OVP
FB
GND
95mV
100mA
2mm
2mm Flash LED Driver with Output OVP
MIC2291
Micrel
M9999-081104
2
August, 2004
Pin Description
Pin Number
Pin Number
TSOT-23-5 2mm
2mm MLFTM
Pin Name
Pin Function
1
7
SW
Switch node (Output): Internal power BIPOLAR collector.
2
GND
Ground (Return): Ground.
3
6
FB
Feedback (Input): Output voltage sense node. Connect the cathode of the
LED to this pin. Connect current set resistor from this pin to ground.
4
3
EN
Enable (Input): Logic high (
1.5V) enables regulator. Logic low (
0.4V) shuts
down regulator.
5
2
VIN
Supply (Input): Input Voltage.
--
1
OVP
Overvoltage protection (Input): Connect to the output to clamp the maximum
output voltage.
--
4
AGND
Analog ground. Internally connected to ground.
--
8
PGND
Power ground.
--
5
NC
No connect (no internal connection to die).
--
EP
GND
Ground (Return): Exposed backside pad.
Pin Configuration
FB GND
EN
VIN
SW
3
1
5
2
4
TSOT-23-5 (BD5)
OVP
VIN
EN
AGND
PGND
SW
FB
NC
1
2
3
4
8
7
6
5
EP
8-Pin MLFTM (BML)
(Top View)
Fused Lead Frame
Ordering Information
Marking
Overvoltage
Junction
Part Number
Code
Protection
Temp. Range
Package
Lead Finish
MIC2291BD5 SSAA
--
40
C to 125
C
Thin SOT-23-5
Standard
MIC2291YD5 SSAA
--
40
C to 125
C
Thin SOT-23-5
Pb-Free
MIC2291-15BML STA
15V
40
C to 125
C 2mm
2mm MLFTM
Standard
MIC2291-15YML STA
15V
40
C to 125
C 2mm
2mm MLFTM
Pb-Free
MIC2291-34BML STC
34V
40
C to 125
C 2mm
2mm MLFTM
Standard
MIC2291-34YML STC
34V
40
C to 125
C 2mm
2mm MLFTM
Pb-Free
August, 2004
3
M9999-081104
MIC2291
Micrel
Absolute Maximum Ratings
(1)
Supply Voltage (V
IN
) ..................................................... 12V
Switch Voltage (V
SW
) ..................................... 0.3V to 34V
Enable Pin Voltage (V
EN
) ................................... 0.3 to V
IN
FB Voltage (V
FB
) ............................................................. 6V
Switch Current (I
SW
) ....................................................... 2A
Ambient Storage Temperature (T
S
) ......... 65
C to +150
C
ESD Rating
(3)
................................................................ 2kV
Operating Ratings
(2)
Supply Voltage (V
IN
) ........................................ 2.5V to 10V
Junction Temperature Range (T
J
) ........... 40
C to +125
C
Package Thermal Impedance
8-lead 2mm
2mm MLFTM (
JA
) ......................... 93
C/W
Thin SOT-23-5 (
JA
) .......................................... 256
C/W
Electrical Characteristics
(4)
T
A
= 25
C, V
IN
= V
EN
= 3.6V, V
OUT
= 10V, I
OUT
= 40mA, unless otherwise noted. Bold values indicate 40
C
T
J
125
C.
Symbol
Parameter
Condition
Min
Typ
Max
Units
V
IN
Supply Voltage Range
2.5
10
V
V
UVLO
Under Voltage Lockout
1.8
2.1
2.4
V
I
VIN
Quiescent Current
V
FB
> 200mV, (not switching)
2.8
5
mA
I
SD
Shutdown Current
V
EN
= 0V
(5)
0.1
1
A
V
FB
Feedback Voltage
(
5%)
90
95
100
mV
I
FB
Feedback Input Current
V
FB
= 95mV
450
nA
Line Regulation
3V
V
IN
5V
0.5
1
%
Load Regulation
5mA
I
OUT
40mA
0.5
2
%
D
MAX
Maximum Duty Cycle
85
90
%
I
SW
Switch Current Limit
1.2
A
V
SW
Switch Saturation Voltage
I
SW
= 1.0A
550
mV
I
SW
Switch Leakage Current
V
EN
= 0V, V
SW
= 10V
0.01
5
A
V
EN
Enable Threshold
TURN ON
1.5
V
TURN OFF
0.4
V
I
EN
Enable Pin Current
V
EN
= 10V
(6)
20
40
A
f
SW
Oscillator Frequency
1.05
1.2
1.35
MHz
V
OVP
Overvoltage Protection
MIC2291BML- 15 only
13
14
16
V
MIC2291BML- 34 only
30
32
34
V
T
J
Overtemperature
150
C
Threshold Shutdown
Hysteresis
10
C
Notes:
1. Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply when operating
the device outside of its operating ratings. The maximum allowable power dissipation is a function of the maximum junction temperature, T
J
(max),
the junction-to-ambient thermal resistance,
JA
, and the ambient temperature, T
A
. The maximum allowable power dissipation will result in excessive
die temperature, and the regulator will go into thermal shutdown.
2. This device is not guaranteed to operate beyond its specified operating ratings.
3. Devices are inherently ESD sensitive. Handling precautions required. Human body model.
4. Specification for packaged product only.
5. I
SD
= I
VIN
.
6. See "Typical Characteristics "section for other V
EN
.
MIC2291
Micrel
M9999-081104
4
August, 2004
Typical Characteristics
0
100
200
300
400
500
600
700
-40 -20 0
20 40 60 80 100 120
V
SW
SATURATION VOLTAGE (mV)
TEMPERATURE (C)
Switch Saturation
vs. Temperature
I
SW
= 1V
85
87
89
91
93
95
97
99
-40 -20 0
20 40 60 80 100 120
MAXIMUM DUTY CYCLE (%)
TEMPERATURE (C)
Maximum Duty Cycle
vs. Temperature
V
IN
= 3.6V
85
87
89
91
93
95
97
99
-40 -20 0
20 40 60 80 100 120
MAXIMUM DUTY CYCLE (%)
TEMPERATURE (C)
Maximum Duty Cycle
vs. Temperature
V
IN
= 3.6V
1.1
1.12
1.14
1.16
1.18
1.2
1.22
1.24
1.26
1.28
1.3
2.5
4
5.5
7
8.5
10
ENABLE THRESHOLD (V)
SUPPLY VOLTAGE (V)
Enable Threshold
vs. Supply Voltage
V
IN
= 3.6V
80
85
90
95
100
105
110
-40 -20 0
20 40 60 80 100 120
FEEDBACK VOLTAGE (mV)
TEMPERATURE (C)
Feedback Voltage vs.
Temperature
50
55
60
65
70
75
80
85
90
0
20 40 60 80 100 120 140 160
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency 12V
OUT
3V
4.2V
3.6V
0
0.2
0.4
0.6
0.8
1
1.2
1.4
-40 -20 0
20 40 60 80 100 120
CURRENT LIMIT (A)
TEMPERATURE (C)
Current Limit
vs. Temperature
V
IN
= 3.6V
0
50
100
150
200
250
300
SWITCH SATURATION VOLTAGE (mV)
SUPPLY VOLTAGE (V)
Switch Saturation
vs. Supply Voltage
I
SW
= 500mA
0
5
10
15
20
25
0.8
0.9
1
1.1
1.2
1.3
1.4
-40
0
40
80
120
FREQUENCY (MHz)
TEMPERATURE (C)
Frequency
vs. Temperature
V
IN
=
3.6V
80
82
84
86
88
90
92
94
96
98
100
2.5
4
5.5
7
8.5
10
MAXIMUM DUTY CYCLE (%)
SUPPLY VOLTAGE (V)
Maximum Duty Cycle
vs. Supply Voltage
V
IN
=3.6V
0
100
200
300
400
500
600
700
0
200
400
600
800
1000
SATURATION VOLTAGE (V)
SWITCH CURRENT (mA)
Saturation Voltage
vs. Current
V
IN
= 3.6V
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
2.5
4
5.5
7
8.5
10
CURRENT LIMIT (A)
SUPPLY VOLTAGE
Current Limit vs.
Supply Voltage
V
IN
=3.6V
August, 2004
5
M9999-081104
MIC2291
Micrel
Functional Description
The MIC2291 is a constant frequency, PWM current mode
boost regulator. The block diagram is shown above. The
MIC2291 is composed of an oscillator, slope compensation
ramp generator, current amplifier, g
m
error amplifier, PWM
generator, and a 500mA bipolar output transistor. The
oscillator generates a 1.2MHz clock. The clock's two func-
tions are to trigger the PWM generator that turns on the output
transistor and to reset the slope compensation ramp genera-
tor. The current amplifier is used to measure the switch
current by amplifying the voltage signal from the internal
sense resistor. The output of the current amplifier is summed
with the output of the slope compensation ramp generator.
This summed current-loop signal is fed to one of the inputs of
the PWM generator.
Functional Diagram
GND
V
REF
PWM
Generator
Ramp
Generator
1.2MHz
Oscillator
SW
EN
FB
OVP*
VIN
95mV
*
OVP available on MLF
TM
package option only
g
m
OVP*
Figure 1. MIC2291 Block Diagram
The g
m
error amplifier measures the LED current through the
external sense resistor and amplifies the error between the
detected signal and the 95mV reference voltage. The output
of the g
m
error amplifier provides the voltage-loop signal that
is fed to the other input of the PWM generator. When the
current-loop signal exceeds the voltage-loop signal, the
PWM generator turns off the bipolar output transistor. The
next clock period initiates the next switching cycle, maintain-
ing the constant frequency current-mode PWM control. The
LED is set by the feedback resistor:
I
LED
=
95mv
R
FB
The Enable pin shuts down the output switching and disables
control circuitry to reduce input current-to-leakage levels.
Enable pin input current is zero at zero volts.
MIC2291
Micrel
M9999-081104
6
August, 2004
Applications Information
DC to DC PWM Boost Conversion
The MIC2291 is a constant frequency boost converter. It
operates by taking a DC input voltage and regulating current
through series LED's by monitoring voltage across the sense
resistor (R2). LED current regulation is achieved by turning
on an internal switch, which draws current through the
inductor (L1). When the switch turns off, the inductor's
magnetic field collapses, causing the current to be dis-
charged into the output capacitor through an external schottkey
diode (D1). Regulation is then achieved by pulse width
modulation (PWM) to maintain a constant voltage on the FB
pin. This in turn provides constant LED current.
Figure 2. DC to DC PWM Boost Conversion
Duty Cycle Considerations
Duty cycle refers to the switch on-to-off time ratio and can be
calculated as follows for a boost regulator;
The duty cycle required for voltage conversion should be less
than the maximum duty cycle of 85%. Also, in light load
conditions where the input voltage is close to the output
voltage, the minimum duty cycle can cause pulse skipping.
This is due to the energy stored in the inductor causing the
output to overshoot slightly over the regulated output voltage.
During the next cycle, the error amplifier detects the output as
being high and skips the following pulse. This effect can be
reduced by increasing the minimum load or by increasing the
inductor value. Increasing the inductor value reduces peak
current, which in turn reduces energy transfer in each cycle.
Over Voltage Protection
For MLF package of MIC2291, there is an over voltage
protection function. If the feedback resistors are discon-
nected from the circuit or the feedback pin is shorted to
ground, the feedback pin will fall to ground potential. This will
cause the MIC2291 to switch at full duty-cycle in an attempt
to maintain the feedback voltage. As a result the output
voltage will climb out of control. This may cause the switch
node voltage to exceed its maximum voltage rating, possibly
damaging the IC and the external components. To ensure
the highest level of protection, the MIC2291 OVP pin will shut
the switch off when an over-voltage condition is detected
saving itself and other sensitive circuitry downstream.
Component Selection
Inductor
Inductor selection is a balance between efficiency, stability,
cost, size and rated current. For most applications a 10uH is
the recommended inductor value. It is usually a good balance
between these considerations.
Efficiency is affected by inductance value in that larger
inductance values reduce the peak to peak ripple current.
This has an effect of reducing both the DC losses and the
transition losses. There is also a secondary effect of an
inductors DC resistance (DCR). The DCR of an inductor will
be higher for more inductance in the same package size. This
is due to the longer windings required for an increase in
inductance. Since the majority of input current (minus the
MIC2291 operating current) is passed through the inductor,
higher DCR inductors will reduce efficiency.
Also, to maintain stability, increasing inductor size will have
to be met with an increase in output capacitance. This is due
to the unavoidable "right half plane zero" effect for the
continuous current boost converter topology. The frequency
at which the right half plane zero occurs can be calculated as
follows;
frhpz
V
V
L I
2
IN
2
OUT
OUT
=
The right half plane zero has the undesirable effect of
increasing gain, while decreasing phase. This requires that
the loop gain is rolled off before this has significant effect on
the total loop response. This can be accomplished by either
reducing inductance (increasing RHPZ frequency) or in-
creasing the output capacitor value (decreasing loop gain).
Output Capacitor
A 1
F or greater output capacitor is sufficient for most
designs. An X5R or X7R dielectric ceramic capacitors are
recommended for designs with the MIC2291. Y5V values
may be used, but to offset their tolerance over temperature,
more capacitance is required.
Diode Selection
The MIC2291 requires an external diode for operation. A
schottkey diode is recommended for most applications due to
their lower forward voltage drop and reverse recovery time.
Ensure the diode selected can deliver the peak inductor
current, the maximum output current and the maximum
reverse voltage is rated greater than the output voltage.
Input Capacitor
A minimum 1
F ceramic capacitor is recommended for
designing with the MIC2291. Increasing input capacitance
will improve performance and greater noise immunity on the
source. The input capacitor should be as close as possible to
D
1
V
V
IN
OUT
= -
10
H
C2
1
F
1-Cell
Li Ion
R2
MIC2291-34BML
VIN
EN
SW
OVP
FB
GND
D1
1A/40V
Schottky
3xLED
GND
GND
V
IN
V
OUT
August, 2004
7
M9999-081104
MIC2291
Micrel
the inductor and the MIC2291, with short traces for good
noise performance.
Feedback Resistors
The MIC2291 utilizes a feedback pin to compare the output
to an internal reference. The LED current is adjusted by
selecting the appropriate feedback resistor value. The de-
sired current can be calculated as follows;
Where V
REF
is equal to 95mV.
Dimming Control
There are two techniques for dimming control. One is PWM
dimming, and the other is continuous dimming.
1. PWM dimming control is implemented by applying
a PWM signal on EN pin as shown in Figure 1. The
MIC2291 is turned on and off by the PWM signal.
With this method, the LEDs operate with either
zero or full current. The average LED current is
increased proportionally to the duty-cycle of the
PWM signal. This technique has high-efficiency
because the IC and the LEDs consume no current
during the off cycle of the PWM signal. Typical
PWM frequency should be between 100Hz and
10kHz.
PWM
VIN
EN
SW
FB
GND
V
IN
Figure 3. PWM Dimming Method
VIN
EN
SW
FB
5.11k
49.9k
GND
DC
Equivalent
V
IN
Figure 4. Continuous Dimming
R2
V
REF
LED
I
=
2. Continuous dimming control is implemented by
applying a DC control voltage to the FB pin of the
MIC2291 through a series resistor as shown in
Figure 2. The LED intensity (current) can be dy-
namically varied applying a DC voltage to the FB
pin. The DC voltage can come from a DAC signal,
or a filtered PWM signal. The advantage of this
approach is that a high frequency PWM signal
(>10kHz) can be used to control LED intensity.
MIC2291
Micrel
M9999-081104
8
August, 2004
TIME (100
s/div)
INDUCT
OR CURRENT
(500mA/div)
SWITCH VOL
T
AGE
(10V/div)
Normal Operating Waveforms
OUTPUT
VOL
T
AGE
(500mA/div)
TIME (100
s/div)
LED CURRENT
(200mA/div)
CONTROL
(5A/div)
Load Step Response
TIME (100
s/div)
LED CURRENT
(200mA/div)
CONTROL
(5A/div)
Enable Response
TIME (100
s/div)
LED CURRENT
(2mA/div)
INPUT
VOL
T
A
GE
(2V/div)
Line Transient
August, 2004
9
M9999-081104
MIC2291
Micrel
Package Information
All Dimensions are in millimeters
5-Pin TSOT (BD5)
8-Pin MLFTM (BML)
MICREL, INC.
1849 FORTUNE DRIVE
SAN JOSE, CA 95131
USA
TEL
+ 1 (408) 944-0800
FAX
+ 1 (408) 474-1000
WEB
http://www.micrel.com
The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use.
Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can
reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into
the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser's
use or sale of Micrel Products for use in life support appliances, devices or systems is at Purchaser's own risk and Purchaser agrees to fully indemnify
Micrel for any damages resulting from such use or sale.
2004 Micrel, Incorporated.
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