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MIC2297
40V PWM Boost Regulator White LED Driver
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 2.5mm x 2.5mm MLFTM-10L package. The MIC2297 has a junction temperature range of -40C to +125C.
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 1A shutdown current Over temperature protection UVLO 2.5mm x 2.5mm MLFTM-10L package -40oC to +125oC junction temperature range
Applications
* * * * * * * PDAs GPS systems Smart phones Mini PCs Digital cameras IP phones LED flashlights
___________________________________________________________________________________________________________
Typical Applications
6.8H-22H 6.8H-22H
VIN
SW OVP
1-Cell Li I on 3V to 4.2V
1F
EN
MIC2297 BRT -42BM L
REF FB
0.47F /50V 1-Ce ll Li I on 3V to 4.2V
VIN
SW OVP
1F
PWM
EN
MIC2297 BRT -42BM L
REF FB AGND PGND COMP
0.47F /50V
1F
AGN PGND COMP D
0.1F
10
1F
0.1F
10
10 Series LED Driver with Output OVP
MLF and MicroLeadFrame is a trademark of Amkor Technology
10 Series LED Driver with PWM Brightness Control
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 2005
M9999-050305 (408) 955-1690
Micrel, Inc.
MIC2297
Ordering Information
Part Number MIC2297-15YML MIC2297-42YML Lead Finish Pb-Free Pb-Free Output Over Voltage Protection 15V 42V Junction Temperature Range -40C to 125C -40C to 125C Package 2.5mm x2.5mm MLFTM-10L 2.5mm x2.5mm MLFTM-10L
Pin Configuration
PGND 1 OVP 2 VIN 3 EN 4 COMP 5 10 SW 9 8 7 6 FB REF BRT AGND
Pin Description
Pin Number 1 2 3 4 5 6 Pin Name PGND OVP VIN EN COMP AGND Pin Function Ground (Return). 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. Supply (Input): Input voltage. Enable (Input): Logic high enables regulator. Logic low shuts down regulator. Compensation Pin. Analog Ground. 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. 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. 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. Switch Node (Output): Internal power BIPOLAR collector. Ground (Return): Backside pad. 2
M9999-050305 (408) 955-1690
7
BRT
8
REF
9 10 EPad May 2005
FB SW GND
Micrel, Inc.
MIC2297
Absolute Maximum Rating (1)
Supply voltage (VIN)........................................................12V Switch voltage (VSW) ........................................ -0.3V to 50V Enable pin voltage (VEN)....................................... -0.3 to VIN FB Voltage (VFB)...............................................................6V VBRT ..................................................................................6V Switch Current (ISW) .........................................................3A Ambient Storage Temperature (TS)............-65C to +150C ESD Rating (3) ................................................................ 2KV
Operating Range (2)
Supply Voltage (VIN).......................................... 2.5V to 10V Maximum Output Voltage (VOUT)....................................40V Junction Temperature Range (TJ)..............-40C to +125C Package Thermal Impedance JA 2.5mm x2.5mm MLFTM-10L lead .................65C/W
Electrical Characteristics
TA=25oC, VIN =VEN = 3.6V, VOUT = 30V, IOUT = 20mA, unless otherwise noted. Bold values indicate -40C TJ 125C. Symbol VIN VUVLO IVIN ISD VFB IFB Parameter Supply Voltage Range Under-Voltage Lockout Quiescent Current Shutdown Current Feedback Voltage Feedback Input Current Line Regulation Load Regulation DMAX ISW VSW ISW VEN Maximum Duty Cycle Switch Current Limit Switch Saturation Voltage Switch Leakage Current Enable Threshold VIN 2.5V VIN = 2.5V, ISW = 0.5A VEN = 0V, VIN = 10V TURN ON TURN OFF 1.5 0.4 VFB = 200mV (not switching) VEN = 0V(4) (+/-5%) (+/6.5%) (Over Temp) VFB = 200mV 2.5V VIN 4.5V 5mA IOUT 20mA 93 1.2 1.7 220 0.01 1 2.5 190 187 -450 0.5 0.5 1 Condition Min 2.5 1.8 2.1 4 0.1 200 Typ Max 10 2.4 7 1 200 213 Units V V mA A mV nA % % % A mV A V
IEN
Enable Pin Current
VEN = 10V VBRT = 0V VBRT = 1V VBRT = 5V VBRT = OPEN 0.185 0.95 0.19 525 MIC2297-42BML (nominal voltage) MIC2297-15BML (nominal voltage) Hysteresis 42 15
20 0.2 1.0 0.2 600 46 16 150 10
40 0.015 0.215 1.05 0.21 675 50 18
A
VREF
Brightness Control Accuracy
V
fSW VOVP
TJ
Oscillator Frequency Over Voltage protection Over-Temperature Threshold Shutdown
KHz V C C
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Notes: 1.
MIC2297
2. 3. 4.
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, JA, and the ambient temperature, TA. The maximum allowable power dissipation will result in excessive die temperature, and the regulator will go into thermal shutdown. This device is not guaranteed to operate beyond its specified operating rating. IC devices are inherently ESD sensitive. Handling precautions required. Human body model. ISD = IVIN.
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Typical Characteristics
6 5.5 5 4.5 4 3.5 3 2 3 Not Switching VFB = 1V 456789 INPUT VOLTAGE (V) 10 97 96 95 2 500
Supply Current vs. Input Voltage
100 99 98
Max Duty Cycle vs. Input Voltage
800
Frequency vs. Input Voltage
700
600
3
456789 INPUT VOLTAGE (V)
10
400 2
3
456789 INPUT VOLTAGE (V)
10
1200 1000 800 600 400 200
Switch Current vs. Switch Voltage
220 215 210 205 200 195
Switch Voltage vs. Input Voltage
20 18 16 14 12
LED Current vs. Input Voltage
VIN = 3.6V 0 0 500 1000 1500 SWITCH CURRENT (mA) 190 2
IFSWITCH = 0.5A 3 456789 INPUT VOLTAGE (V) 10 10 2 3 456789 INPUT VOLTAGE (V) 10
1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 2 3
BRT Voltage vs. Input Voltage
300 280 260 240 220 200 180 160 140 120 100 2
FB Voltage vs. Input Voltage
30 25 20 15 10 5
LED Current vs. BRT Voltage
456789 INPUT VOLTAGE (V)
10
3
456789 INPUT VOLTAGE (V)
10
0
RSENSE 10001200 BRT VOLTAGE (mV)
300 250 200 150 100 50 0
FB Voltage vs. BRT Voltage
Efficiency for 10 LEDs @ 20mA vs. Input Voltage
80 78 76 74 72 L = 6.8H
80 75 70 65 60 55 50 45
Efficiency for 10 LEDs vs. LED Current
VIN = 4.2V VIN = 3.6V VIN = 3.2V
L = 6.8H 5 10 15 20 25 30 LED CURRENT (mA) 35
10001200 BRT VOLTAGE (mV)
70 2
3
456789 INPUT VOLTAGE (V)
10
40 0
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MIC2297
Typical Characteristics (continued)
85 80 75 70 65 60 55 50 45 40 35 0 L = 15H 10 20 30 LED CURRENT (mA) 40
Efficiency for 10 LEDs vs. LED Current
VIN = 4.2V VIN = 3.2V VIN = 3.6V
80 75 70 65 60 55 50 45 40 0
Efficiency for 9 LEDs vs. LED Current
VIN = 4.2V VIN = 3.2V VIN = 3.6V
85 80 75 70 65 60 55 50 45
Efficiency for 9 LEDs vs. LED Current
VIN = 4.2V VIN = 3.2V
VIN = 3.6V
L = 6.8H 5 10 15 20 25 30 LED CURRENT (mA) 35
40 35 0 5
L = 15H 10 15 20 25 30 35 40 LED CURRENT (mA)
35 30 25 20 15 10 5 0 0 20
LED Current vs. Duty Cycle
4V 5V 3V 2V
C1 1F, 16V
L1 = Murata LQH32CN100K11 L1
VIN
SW
MIC2297BML
C2 0.47F, 50V
2V peak PWM
C3 0.22F, 10V PWM
EN REF BRT
OVP C4 0.1F, 10V
COMP
AGND
PGND
FB
PWM = 20kHz RSENSE 40 60 80 DUTY CYCLE (%) 100
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MIC2297
Functional Characteristics
Enable Characteristics
Output Voltage Enable Voltage (10V/div) (2V/div) Output Voltage (100mV/div)
Waveform 1
VIN = 3.6V 10 LEDs @ 5mA L = 15H
VIN = 3.6V 10 LEDs @ 20mA L = 15H COUT = 0.47V
Inductor Current (100mA/div)
Input Current (100mA/div)
SW Voltage (20V/div)
0A
TIME (2ms/div)
TIME (1s/div)
Waveform 2
Output Voltage (100mV/div) Inductor Current (200mA/div) SW Voltage (20V/div)
VIN = 3.6V 10 LEDs @ 20mA L = 15H
0A
TIME (1s/div)
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MIC2297
Block Diagram
FB COMP OVP EN
BRT
1.245V 1.245V
SW
REF
PWM Generator
+ +
600kHz Oscillator Ramp Generator
GND
MIC2297 Block Diagram
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MIC2297
6.8H-22H
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:
VIN
SW OVP
1-Cell Li I on 3V to 4.2V
1F
EN
MIC2297
BRT -42BM L REF FB
0.47F /50V
1F
AGN PGND COMP D
0.1F
10
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:
D = 1- Vin Vout
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:
I out <
Where
Vin I peak Vout 2
I LED =
200mV RFB
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).
I peak =
(Vout - Vin ) Vin
L f
V out
In DCM, the duty cycle is smaller than in continuous conduction mode. In DCM the duty cycle is given by:
D=
f 2 L I out (Vout - Vin ) Vin
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.
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MIC2297
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 overvoltage 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 10 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:
Component Selection
Inductor Inductor selection is a balance between efficiency, stability, cost, size, and rated current. For most applications a 22H 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:
f rhpz =
2 Vin Vout L I out 2
I LED =
BRT 5 * Rsense
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 124k 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:
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:
I LED =
V peak * D
5 * Rsense
Where Vpeak is the peak PWM voltage and D is the duty cycle of the PWM signal.
fz =
1 C Resr 2
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.
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MIC2297
fp =
I out C Vout
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 1F 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:
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:
f zero =
1 2R1C1
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 f pole = 1 2R1C 2
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.
I LED =
0.2V R
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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 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. (c) 2005 Micrel, Incorporated.
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