Micrel MIC2291 1.2a pwm boost regulator photo flash led driver Datasheet

MIC2291
1.2A PWM Boost Regulator Photo Flash
LED Driver
General Description
Features
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
appropriate voltage ratings for their applications.
The MIC2291 is available in low-profile, Thin SOT23 5-pin
and 8-pin 2mm × 2mm MLF® package options. The
MIC2291 has a junction temperature range of –40°C to
+125°C.
Data sheets and support documentation can be found on
Micrel’s web site at www.micrel.com.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
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
1µA shutdown current
Over temperature protection
UVLO
Low-profile Thin SOT23-5 package option
2mm × 2mm MLF® package option
–40°C to +125°C junction temperature range
Applications
•
•
•
•
•
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Photo Flash LED driver
Cell phones
PDAs
GPS systems
Digital cameras
IP phones
LED flashlights
Typical Application
10µH
10µH
100mA
MIC2291-15xML
MIC2291xD5
5
1-Cell
Li Ion
3V to 4.2V
VIN
SW
1
1µF
4
FB
EN
100mA
1-Cell
Li Ion
3V to 4.2V
0.22µF
ceramic
3
95mV
1µF
VIN
SW
EN
OVP
FB
GND
GND
0.22µF
95mV
2
Thin SOT23 Flash LED Driver
2mm x 2mm Flash LED Driver with Output OVP
PowerPAK is a trademark of Siliconix, Inc.
MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc.
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
May 2007
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M9999-051507
Micrel, Inc.
MIC2291
Ordering Information
Part Number
Marking*
Overvoltage
Protection
Junction
Temp. Range
Package
Lead Finish
MIC2291BD5
SSAA
―
–40° to +125°C
5-Pin Thin SOT23
Standard
MIC2291YD5
SSAA
―
–40° to +125°C
5-Pin Thin SOT23
Pb-Free
MIC2291-15BML
STA
15
–40° to +125°C
8-Pin 2mm x 2mm MLF®
Standard
MIC2291-15YML
STA**
15
–40° to +125°C
8-Pin 2mm x 2mm MLF®
Pb-Free
–40° to +125°C
®
Standard
®
Pb-Free
MIC2291-34BML
STC
MIC2291-34YML
STC**
34
34
–40° to +125°C
8-Pin 2mm x 2mm MLF
8-Pin 2mm x 2mm MLF
Notes:
*
Under bar / Over bar symbol may not be to scale.
**
Over bar symbol located after Pin 1 identifier.
Pin Configuration
FB GND SW
1
2
3
4
EN
5
VIN
OVP
1
8
PGND
VIN
2
7
SW
EN
3
6
FB
AGND
4
5
NC
EP
®
8-Pin 2mm x 2mm MLF (ML)
5-Pin TSOT23 (D5)
Pin Description
Pin Number
TSOT23-5
Pin Number
1
7
Pin Name
2x2 MLF-8
2
May 2007
Pin Name
SW
GND
Switch node (Output): Internal power BIPOLAR collector.
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
―
EP
GND
No connect (no internal connection to die).
Ground (Return): Exposed backside pad.
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MIC2291
Absolute Maximum Ratings(1)
Operating Ratings(2)
Supply Voltage (VIN) .......................................................12V
Switch Voltage (VSW)....................................... –0.3V to 34V
Enable Pin Voltage (VEN)................................... –0.3V to VIN
FB Voltage (VFB)...............................................................6V
Switch Current (ISW) .........................................................2A
Storage Temperature (Ts) .........................–65°C to +150°C
ESD Rating(3) .................................................................. 2kV
Supply Voltage (VIN).......................................... 2.5V to 10V
Junction Temperature (TJ) ........................ –40°C to +125°C
Package Thermal Resistance
2x2 MLF-8 (θJA) .................................................93°C/W
Thin SOT23-5 (θJA)..........................................256°C/W
Electrical Characteristics(4)
TA = 25°C, VIN = VEN = 3.6V; VOUT = 10V; IOUT = 40mA, bold values indicate –40°C< TJ < +125°C, unless noted.
Symbol
Parameter
Condition
Min
Typ
Max
Units
10
V
2.1
2.4
V
VIN
Supply Voltage Range
2.5
VUVLO
Under Voltage Lockout
1.8
IVIN
Quiescent Current
VFB > 200mV, (not switching)
2.8
5
mA
ISD
Shutdown Current
VEN = 0V(5)
0.1
1
µA
VFB
Feedback Voltage
(±5%)
95
100
mV
IFB
Feedback Input Current
VFB = 95mV
Line Regulation(7)
3V ≤ VIN ≤ 5V
Load Regulation(7)
5mA ≤ IOUT ≤ 40mA
DMAX
Maximum Duty Cycle
ISW
Switch Current Limit
VSW
Switch Saturation Voltage
ISW = 1.0A
ISW
Switch Leakage Current
VEN = 0V, VSW = 10V
VEN
Enable Threshold
TURN ON
TURN OFF
IEN
Enable Pin Current
fSW
Oscillator Frequency
VOVP
Overvoltage Protection
TJ
Overtemperature
Threshold Shutdown
90
–450
0.5
85
nA
1
0.5
%
90
%
1.2
A
550
mV
0.01
5
µA
0.4
V
V
1.5
VEN = 10V(6)
MIC2291BML- 15 only
MIC2291BML- 34 only
20
40
µA
1.05
1.2
1.35
MHz
13
30
14
32
16
34
V
V
150
10
Hysteresis
%
°C
°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, TJ(max), the
junction-to-ambient thermal resistance, θJA, and the ambient temperature, TA. The maximum allowable power dissipation will result in excessive die
temperature, and the regulator will go into thermal shutdown.
2. The device is not guaranteed to function outside its operating rating.
3. Devices are ESD sensitive. Handling precautions recommended. Human body model.
4. Specification for packaged product only.
5. ISD = IVIN.
6. See “Typical Characteristics” section for other VEN.
7. Guaranteed by design.
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MIC2291
Typical Characteristics
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Micrel, Inc.
MIC2291
Functional Characteristics
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MIC2291
Functional Diagram
VIN
FB
OVP*
EN
OVP*
SW
PWM
Generator
gm
VREF
95mV
1.2MHz
Oscillator
GND
Ramp
Generator
*OVP available on MLF® package option only
Figure 1. MIC2291 Block Diagram
The gm 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 gm 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, maintaining the constant
frequency current-mode PWM control. The LED is set by
the feedback resistor:
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, gm error
amplifier, PWM generator, and a 500mA bipolar output
transistor. The oscillator generates a 1.2MHz clock. The
clock’s two functions are to trigger the PWM generator
that turns on the output transistor and to reset the slope
compensation ramp generator. The current amplifier is
used to measure the switch cur-rent 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.
May 2007
ILED =
95mW
R FB
The Enable pin shuts down the output switching and
disables control circuitry to reduce input current-toleakage levels. Enable pin input current is zero at zero
volts.
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Micrel, Inc.
MIC2291
Over Voltage Protection
For MLF® package of MIC2291, there is an over voltage
protection function. If the feedback resistors are
disconnected 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.
Application 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
cur-rent 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 discharged 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.
VIN
D1
1A/40V
Schottky
10µH
Component Selection
VOUT
Inductor
Inductor selection is a balance between efficiency,
stability, cost, size and rated current. For most
applications a 10µH 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;
MIC2291-34xML
1-Cell
Li Ion
VIN
SW
EN
OVP
3xLED
C2
1µF
FB
GND
R2
GND
GND
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;
D =1
VIN
VOUT
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 volt-age, 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.
May 2007
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frhpz =
VOUT
VIN
× L × IOUT × 2π
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 increasing the output capacitor
value (decreasing loop gain).
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MIC2291
VIN
Output Capacitor
A 1µF 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.
PWM
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 cur-rent, the maximum output current and
the maximum reverse voltage is rated greater than the
output voltage.
EN
FB
Figure 3. PWM Dimming Method
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
dynamically 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.
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 desired current can be calculated as follows;
VIN
VREF
ILED
Where VREF is equal to 95mV.
VIN
SW
EN
FB
GND
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.
May 2007
SW
GND
Input Capacitor
A minimum 1µF 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 the inductor and the MIC2291,
with short traces for good noise performance.
R2 =
VIN
5.11k
49.9k
DC
Equivalent
Figure 4. Continuous Dimming
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M9999-051507
Micrel, Inc.
MIC2291
Package Information
5-Pin Thin SOT23 (D5)
8-Pin 2mm x 2mm MLF® (ML)
May 2007
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M9999-051507
Micrel, Inc.
MIC2291
MICREL, INC. 2180 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 a Purchaser’s own risk and Purchaser agrees to fully
indemnify Micrel for any damages resulting from such use or sale.
© 2004 Micrel, Incorporated.
May 2007
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