MIC2297 Datasheet by Microchip Technology

EBIIIEIEU z; .5); .5; .5 2); 4c; 52L a
MIC2297
40V PWM Boost Regulator
White LED Driver
MLF and MicroLeadFrame is a trademark 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
March 2008
M9999-032708
General Description
The MIC2297 is a 600KHz PWM boost-switching regulator
that is optimized for driving 6-10 series white LEDs. With
its internal 40V switch and a guaranteed switch current of
1.2A, the MIC2297 easily drives a string of 10 white LEDs
in series at 20mA, ensuring a high level of brightness and
eliminating several ballast resistors.
The MIC2297 implements constant frequency 600KHz
PWM control. The high frequency PWM operation saves
board space by reducing external component sizes. The
added benefit of the constant frequency PWM operation is
much lower noise and input ripple injected back to the
battery source than with variable frequency topologies.
To optimize efficiency, the feedback voltage is set to
200mV. This reduced voltage reduces the power
dissipation in the current set resistor, and allows the lowest
total output voltage, hence minimal current draw from the
battery.
The MIC2297 is available with output over-voltage
protection that protects the IC and external components in
case of open LED conditions.
The MIC2297 is available in low profile small size 10-pin
2.5mm x 2.5mm MLF
®
package. The MIC2297 has a
junction temperature range of –40°C to +125°C.
Features
• 2.5V to 10V input voltage range
• Output voltage up to 40V
• 1.2A switch current
• 600KHz PWM operation
• Trimmed 200mV feedback voltage
• Output over voltage protection (fixed or adjustable)
• PWM Brightness Control
• DAC Brightness Control
• <1% line regulation
• 1µA shutdown current
• Over temperature protection
• UVLO
• 10-pin 2.5mm x 2.5mm MLF
®
package
• –40
o
C to +125
o
C junction temperature range
Applications
• PDAs
• GPS systems
• Smart phones
• Mini PCs
• Digital cameras
• IP phones
• LED flashlights
___________________________________________________________________________________________________________
Typical Applications
VIN
EN
SW
FB
MIC2297
-42BML
0.47µF
/50V
6.8µH-22µH
10
OVP
PGND
1µF
1-Cell
Li Ion
3V to 4.2V
BRT
REF
AGND COMP
1µF
PWM
VIN
EN
SW
FB
MIC2297
-42BML
0.47µF
/50V
6.8µH-22µH
10
OVP
PGND
1µF
1-Ce ll
Li Ion
3V to 4.2V
BRT
REF
AGND COMP
0.1µF
1µF
0.1µF
10 Series LED Driver with Output OVP 10 Series LED Driver with PWM Brightness Control
Micrel, Inc. MIC2297
Mach 2008 2
M9999-032708
Ordering Information
Part Number Mark Code* Output Over
Voltage Protection Junction
Temp. Range Package Lead Finish
MIC2297-15YML S115 15V –40°C to 125°C 10-Pin 2.5 x2.5 MLF
®
Pb-Free
MIC2297-42YML S142 42V –40°C to 125°C 10-Pin 2.5 x2.5 MLF
®
Pb-Free
* ( ) Top bar symbol after PI indicator identifying Pb-Free may not be to scale.
Pin Configuration
COMP AGND5
1PGND
OVP
VIN
EN
10 SW
FB
REF
BRT
9
8
7
2
3
4
6
Pin Description
Pin Number Pin Name Pin Function
1 PGND Ground (Return).
2 OVP
Over Voltage Protection (Input): Connect to the output to
clamp the maximum output voltage. A resistor divider from
this pin to ground could be used to raise the OVP level of
the 15V OVP option.
3 VIN Supply (Input): Input voltage.
4 EN
Enable (Input): Logic high enables regulator. Logic low shuts
down regulator.
5 COMP Compensation Pin.
6 AGND
Analog Ground.
7 BRT
Brightness Control (Input): Either an analog (DAC) or filtered
PWM signal can be used. The gain equation is: VFB =
VBRT / 5. This pin should be left open if the brightness
function is not used. In that case, the FB will be set to its
default value of 200mV.
8 REF
Reference Voltage (Output): This node is equal to the
voltage on the FB pin. A capacitor from REF to ground
should be used to filter the BRT voltage if PWM dimming is
implemented. A capacitor from REF to ground can also be
used to implement a soft-start function. This pin can be left
open if not used.
9 FB
Feedback (Input): Output voltage sense node. Default value
is 200mV. Connect the cathode of the LED chain to this pin.
Connect current set resistor from this pin to ground.
10 SW Switch Node (Output): Internal power BIPOLAR collector.
EPad GND Ground (Return): Backside pad.
Micrel, Inc. MIC2297
Mach 2008 3
M9999-032708
Absolute Maximum Rating
(1)
Supply voltage (V
IN
)........................................................12V
Switch voltage (V
SW
) ........................................-0.3V to 50V
Enable pin voltage (V
EN
)....................................... -0.3 to V
IN
FB Voltage (V
FB
)...............................................................6V
V
BRT
..................................................................................6V
Switch Current (I
SW
) .........................................................3A
Ambient Storage Temperature (T
S
)............-65°C to +150°C
ESD Rating
(3)
................................................................ 2KV
Operating Range
(2)
Supply Voltage (V
IN
).......................................... 2.5V to 10V
Maximum Output Voltage (V
OUT
)....................................40V
Junction Temperature Range (T
J
)..............-40°C to +125°C
Package Thermal Impedance
MLF
®
-10 (θ
JA
).....................................................65°C/W
Electrical Characteristics
T
A
=25
o
C, V
IN
=V
EN
= 3.6V, V
OUT
= 30V, I
OUT
= 20mA, 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) 4 7 mA
I
SD
Shutdown Current V
EN
= 0V
(4)
0.1
1 µA
(+/-5%) 190 200 210
V
FB
Feedback Voltage
(+/±6.5%) (Over Temp) 187 213 mV
I
FB
Feedback Input Current V
FB
= 200mV -450 nA
Line Regulation
(5)
2.5V ≤ V
IN
≤ 4.5V 0.5
1 %
Load Regulation
(5)
5mA ≤ I
OUT
≤ 20mA 0.5 %
D
MAX
Maximum Duty Cycle 93 %
I
SW
Switch Current Limit V
IN
2.5V 1.2 1.7 2.5 A
V
SW
Switch Saturation Voltage V
IN
= 2.5V, I
SW
= 0.5A 220 mV
I
SW
Switch Leakage Current V
EN
= 0V, V
IN
= 10V 0.01 1 µA
V
EN
Enable Threshold TURN ON
TURN OFF
1.5
0.4 V
I
EN
Enable Pin Current V
EN
= 10V 20 40 µA
V
REF
Brightness Control Accuracy
V
BRT
= 0V
V
BRT
= 1V
V
BRT
= 5V
V
BRT
= OPEN
0.185
0.93
0.19
0.2
1.0
0.2
0.015
0.215
1.05
0.21
V
f
SW
Oscillator Frequency 525 600 675 KHz
V
OVP
Over Voltage protection MIC2297-42BML (nominal voltage)
MIC2297-15BML (nominal voltage)
40.5
15
42
16
47
18
V
150
°C
T
J
Over-Temperature 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, θ
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 rating.
3. IC devices are inherently ESD sensitive. Handling precautions required. Human body model.
4. I
SD
= I
VIN
.
5. Guaranteed by design.
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Micrel MIC2297
Mach 2008 4
M9999-032708
Typical Characteristics
3
3.5
4
4.5
5
5.5
6
2345678910
INPUT VOLTAGE (V)
Supply Current
vs. Input Voltage
Not Switching
V
FB
= 1V
95
96
97
98
99
100
2345678910
INPUT VOLTAGE (V)
Max Duty Cycle
vs. Input Voltage
400
500
600
700
800
2345678910
INPUT VOLTAGE (V)
Frequency
vs. Input Voltage
0
200
400
600
800
1000
1200
0 500 1000 1500
SWITCH CURRENT (mA)
Switch Current
vs. Switch Voltage
V
IN
= 3.6V
190
195
200
205
210
215
220
2345678910
INPUT VOLTAGE (V)
Switch Voltage
vs. Input Voltage
I
FSWITCH
= 0.5A
10
12
14
16
18
20
2345678910
INPUT VOLTAGE (V)
LED Current
vs. Input Voltage
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
2345678910
INPUT VOLTAGE (V)
BRT Voltage
vs. Input Voltage
100
120
140
160
180
200
220
240
260
280
300
2345678910
INPUT VOLTAGE (V)
FB Voltage
vs. Input Voltage
0
5
10
15
20
25
30
10001200
BRT VOLTAGE (mV)
LED Current
vs. BRT Voltage
R
SENSE
0
50
100
150
200
250
300
FB Voltage
vs. BRT Voltage
10001200
BRT VOLTAGE (mV)
70
72
74
76
78
80
2345678910
INPUT VOLTAGE (V)
Efficiency for 10 LEDs @ 20mA
vs. Input Voltage
L = 6.8µH
40
45
50
55
60
65
70
75
80
0 5 10 15 20 25 30 35
LED CURRENT (mA)
Efficiency for 10 LEDs
vs. LED Current
L = 6.8µH
V
IN
= 3.2V
V
IN
= 3.6V
V
IN
= 4.2V
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Micrel MIC2297
Mach 2008 5
M9999-032708
Typical Characteristics (continued)
35
40
45
50
55
60
65
70
75
80
85
0 10203040
LED CURRENT (mA)
Efficiency for 10 LEDs
vs. LED Current
V
IN
= 3.6V
V
IN
= 3.2V
V
IN
= 4.2V
L = 15µH
40
45
50
55
60
65
70
75
80
0 5 10 15 20 25 30 35
LED CURRENT (mA)
Efficiency for 9 LEDs
vs. LED Current
L = 6.8µH
V
IN
= 3.6V
V
IN
= 3.2V
V
IN
= 4.2V
35
40
45
50
55
60
65
70
75
80
85
0 5 10 15 20 25 30 35 40
LED CURRENT (mA)
Efficiency for 9 LEDs
vs. LED Current
V
IN
= 3.6V
V
IN
= 3.2V
V
IN
= 4.2V
L = 15µH
0
5
10
15
20
25
30
35
0 20406080100
DUTY CYCLE (%)
LED Current
vs. Duty Cycle
R
SENSE
5V
4V
3V 2V
PWM = 20kHz
2V peak PWM
VIN
EN
REF
BRT
AGND PGND
SW
OVP
COMP
FB
MIC2297BML
C3
0.22µF, 10V PWM
C4
0.1µF, 10V
C2
0.47µF, 50
V
C1
1µF, 16V
L1 = Murata LQH32CN100K11
L1
Micrel MIC2297
Mach 2008 6
M9999-032708
Functional Characteristics
Enable Characteristics
Output Volta
g
e
(10V/div)
Input Current
(100mA/div)
TIME (2ms/div)
V
IN
= 3.6V
10 LEDs @ 20mA
L = 15µH
C
OUT
= 0.47µV
Enable Volta
g
e
(2V/div)
Wavefor
m
1
SW Volta
g
e
(20V/div)
Inductor Current
(100mA/div)
TIME (1µs/div)
V
IN
= 3.6V
10 LEDs @ 5mA
L = 15µH
Output Volta
g
e
(100mV/div)
0A
Wavefor
m
2
SW Volta
g
e
(20V/div)
Inductor Current
(200mA/div)
TIME (1µs/div)
V
IN
= 3.6V
10 LEDs @ 20mA
L = 15µH
Outpu
t
Volta
g
e
(100mV/div)
0A
35m 124m 31ml
Micrel MIC2297
Mach 2008 7
M9999-032708
Block Diagram
+
+
1.245V
1.245V
PWM
Generator
Ramp
Generator
600kHz
Oscillator
FB COMP OVP EN
SW
GND
REF
BRT
MIC2297 Block Diagram
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Micrel MIC2297
Mach 2008 8
M9999-032708
Functional Description
The MIC2297 is a constant frequency, PWM current mode
boost regulator. The MIC2297 uses peak current mode
control. The block diagram is shown above. The MIC2297
is composed of an oscillator, slope compensation ramp
generator, current amplifier, gm error amplifier, PWM
generator, and a 1.2A bipolar output transistor. The
oscillator generates a 600kHz 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 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.
The gm error amplifier measures the LED current through
the external sense resistor and amplifies the error between
the detected signal and the 200mV 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 current is set by the feedback
resistor:
FB
LED
R
mV
I200
=
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.
DC-to-DC PWM Boost Conversion
The MIC2297 is a constant-frequency boost converter. It
operates by taking a DC input voltage and regulating a
higher DC output voltage. Figure 2 shows a typical circuit.
Boost 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. This causes the current to be discharged into
the output capacitor through an external Schottky diode
(D1). Waveforms 1 and 2 show Output Voltage ripple, SW
Voltage, and Indicator Current for 5mA and 20mA LED
current respectively. Voltage regulation is achieved by
modulating the pulse width or pulse-width modulation
(PWM).
VIN
EN
SW
FB
MIC2297
-42BML
0.47µF
/50V
6.8µH-22µH
10
OVP
PGND
1µF
1-Cell
Li Ion
3V to 4.2V
BRT
REF
AGND COMP
1µF
0.1µF
Figure 2. Typical Application Circuit
Duty Cycle Considerations
Duty cycle refers to the switch on-to-off time ratio and can
be calculated as follows for a boost regulator:
out
in
V
V
D−= 1
However, at light loads the inductor will completely
discharge before the end of a switching cycle. The current
in the inductor reaches 0A before the end of the switching
cycle. This is known as discontinuous conduction mode
(DCM). DCM occurs when:
2
peak
out
in
out
I
V
V
I⋅<
Where
)
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⋅
⋅
−
=
out
ininout
peak
V
V
fL
VV
I
In DCM, the duty cycle is smaller than in continuous
conduction mode. In DCM the duty cycle is given by:
in
inoutout
V
VVILf
D)(2 −⋅⋅⋅⋅
=
The duty cycle required for voltage conversion should be
less than the maximum duty cycle of 95%. 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.
Micrel MIC2297
Mach 2008 9
M9999-032708
Over-Voltage Protection
The MIC2297 has an over-voltage protection function. If an
LED is disconnected from the circuit or the feedback pin is
shorted to ground, the feedback pin will fall to ground
potential. This will cause the MIC2297 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
MIC2297 OVP pin will shut the switch off when an over-
voltage condition is detected, saving itself and the output
capacitor.
Brightness Control
In the MIC2297, the reference to the voltage error amplifier
is pinned out. The BRT pin and REF pin form a voltage
divider off the internal 1.245V reference. The voltage is
such that with nothing connected to the BRT pin, the REF
voltage is 0.2V and the BRT voltage is 1V. The REF
voltage is 1/5 the BRT voltage.
The minimum REF voltage with BRT pulled to ground is
typically 10mV. With a 10Ω sense resistor, the LED
current is typically 1mA with the BRT pin pulled to ground.
An analog DC voltage can be connected to the BRT pin.
The MIC2297 will create an LED current proportional to
the BRT voltage according to the following equation:
sense
LED
R
BRT
I•
=5
Where BRT is the voltage applied to the BRT pin, and
Rsense is the sense resistor used in the LED string. It’s
important to use a 1uF ceramic capacitor on the REF pin
to filter any noise.
An external PWM signal can be applied to the BRT for
dimming. The 1uF REF capacitor and internal BRT 124kΩ
resistor form an RC that filters the voltage to the REF pin.
The LED current is proportional the PWM duty cycle
according to the following equation:
sense
peak
LED
R
DV
I•
•
=5
Where Vpeak is the peak PWM voltage and D is the duty
cycle of the PWM signal.
Component Selection
Inductor
Inductor selection is a balance between efficiency,
stability, cost, size, and rated current. For most
applications a 22µH is the recommended inductor value. It
is usually a good balance between these considerations.
Larger inductance values reduce the peak-to-peak ripple
current, affecting efficiency. This has the effect of reducing
both the DC losses and the transition losses. There is also
a secondary effect of an inductor’s 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 MIC2297 operating
current) is passed through the inductor, higher DCR
inductors will reduce efficiency. 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:
π
2
2
⋅⋅⋅
=
outout
in
rhpz
ILV
V
f
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).
Output Capacitor
Output capacitor selection is also a trade-off between
performance, size, and cost. Increasing output
capacitance will lead to an improved transient response,
but also an increase in size and cost. X5R or X7R
dielectric ceramic capacitors are recommended for
designs with the MIC2297.
The output capacitor sets the frequency of the pole and
zero in the power stage. The zero is given by:
π
2
1
⋅⋅
=
esr
z
RC
f
For ceramic capacitors, the ESR is very small. This puts
the zero at a very high frequency where it can be ignored.
The frequency of the pole caused by the output capacitor
is given by.
π
⋅⋅
=
out
out
p
VC
I
f
Micrel MIC2297
Mach 2008 10
M9999-032708
Reference Capacitor
A 1uF ceramic should be used on the reference pin to
prevent noise from getting into this node. A 1uF ceramic is
needed when a PWM signal is connected to the BRT pin.
Diode Selection
The MIC2297 requires an external diode for operation. A
Schottky 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 and the maximum reverse voltage is rated
greater than the output voltage.
Input capacitor
A minimum 1F ceramic capacitor with an X5R or X7R
dielectric is recommended for designing with the MIC2297.
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
MIC2297, with short traces for good noise performance.
The MIC2297 utilizes a feedback pin to compare the LED
current to an internal reference. The LED current is
adjusted by selecting the appropriate feedback resistor
value. The desired output current can be calculated as
follows:
R
V
I
LED
2.0
=
Compensation
The comp pin is connected to the output of the voltage
error amplifier. The voltage error amplifier is a
transconductance amplifier. Adding a series RC to ground
adds a zero at:
11
2
1
CR
f
zero
π
=
The resistor typically ranges from 10kOhm to 50kOhm.
The capacitor typically ranges from 1nF to 100nF.
Adding a capacitor from comp to ground adds a pole at
21
2
1
CR
f
pole
π
=
This capacitor typically ranges from 100pF to 10nF.
Generally an RC to ground is all that is needed. The RC
should be placed as close as possible to the comp pin.
The capacitor should be a ceramic with a X5R, X7R, or
COG dielectric.
Grounding
Both the AGND and PGND must be connected to the
exposed backside pad. The exposed backside pad also
improves thermal performance. A large ground plane
decreases thermal resistance to ambient air.
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Micrel MIC2297
Mach 2008 11
M9999-032708
Package Information
10-Pin Package MLF
®
(ML)
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
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