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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 appropriate voltage ratings for their applications. The MIC2291 is available in low-profile, Thin SOT23 5-pin and 8-pin 2mm x 2mm MLF(R) package options. The MIC2291 has a junction temperature range of -40C to +125C. Data sheets and support documentation can be found on Micrel's web site at www.micrel.com.
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 1A shutdown current Over temperature protection UVLO Low-profile Thin SOT23-5 package option 2mm x 2mm MLF(R) package option -40C to +125C junction temperature range
Applications
* * * * * * * Photo Flash LED driver Cell phones PDAs GPS systems Digital cameras IP phones LED flashlights
Typical Application
10H 100mA
10H 100mA
MIC2291xD5
5
MIC2291-15xML
1
1-Cell Li Ion 3V to 4.2V
VIN EN
SW FB GND
2
VIN
SW OVP FB GND 95mV 0.22F
1F
4 3
0.22F ceramic 95mV
1-Cell Li Ion 3V to 4.2V
1F
EN
Thin SOT23 Flash LED Driver
PowerPAK is a trademark of Siliconix, Inc. MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc.
2mm x 2mm Flash LED Driver with Output OVP
Micrel Inc. * 2180 Fortune Drive * San Jose, CA 95131 * USA * tel +1 (408) 944-0800 * fax + 1 (408) 474-1000 * http://www.micrel.com
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MIC2291
Ordering Information
Part Number MIC2291BD5 MIC2291YD5 MIC2291-15BML MIC2291-15YML MIC2291-34BML MIC2291-34YML
Notes: * ** Under bar / Over bar symbol may not be to scale. Over bar symbol located after Pin 1 identifier.
Marking* SSAA SSAA STA STA** STC STC**
Overvoltage Protection 15 15 34 34
Junction Temp. Range -40 to +125C -40 to +125C -40 to +125C -40 to +125C -40 to +125C -40 to +125C
Package 5-Pin Thin SOT23 5-Pin Thin SOT23 8-Pin 2mm x 2mm MLF(R) 8-Pin 2mm x 2mm MLF(R) 8-Pin 2mm x 2mm MLF
(R) (R)
Lead Finish Standard Pb-Free Standard Pb-Free Standard Pb-Free
8-Pin 2mm x 2mm MLF
Pin Configuration
FB GND SW 1 2 3
OVP VIN EN PGND SW FB NC
1 2 3 4
8 7 6
4 EN
5 VIN
AGND
EP
5
5-Pin TSOT23 (D5)
(R) 8-Pin 2mm x 2mm MLF (ML)
Pin Description
Pin Number TSOT23-5 1 2 3 6 Pin Number 2x2 MLF-8 7 SW GND FB Switch node (Output): Internal power BIPOLAR collector. Ground (Return): Ground. Feedback (Input): Output voltage sense node. Connect the cathode of the LED to this pin. Connect current set resistor from this pin to ground. Enable (Input): Logic high (1.5V) enables regulator. Logic low (0.4V) shuts down regulator. Supply (Input): Input Voltage. Overvoltage protection (Input): Connect to the output to clamp the maximum output voltage. Analog ground. Internally connected to ground. Power ground. No connect (no internal connection to die). Ground (Return): Exposed backside pad. Pin Name Pin Name
4 5
3 2 1 4 8 5 EP
EN VIN OVP AGND PGND NC GND
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Absolute Maximum Ratings(1)
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) .........................-65C to +150C ESD Rating(3) .................................................................. 2kV
Operating Ratings(2)
Supply Voltage (VIN).......................................... 2.5V to 10V Junction Temperature (TJ) ........................ -40C to +125C Package Thermal Resistance 2x2 MLF-8 (JA) .................................................93C/W Thin SOT23-5 (JA)..........................................256C/W
Electrical Characteristics(4)
TA = 25C, VIN = VEN = 3.6V; VOUT = 10V; IOUT = 40mA, bold values indicate -40C< TJ < +125C, unless noted.
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(7) Load Regulation(7) DMAX ISW VSW ISW VEN IEN fSW VOVP TJ
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, 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. 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.
Condition
Min 2.5 1.8
Typ 2.1 2.8 0.1
Max 10 2.4 5 1 100 1
Units V V mA A mV nA % % % A mV
VFB > 200mV, (not switching) VEN = 0V(5) (5%) VFB = 95mV 3V VIN 5V 5mA IOUT 40mA 85 ISW = 1.0A VEN = 0V, VSW = 10V TURN ON TURN OFF VEN = 10V(6) 1.05 MIC2291BML- 15 only MIC2291BML- 34 only Hysteresis 13 30 1.5 90
95 -450 0.5 0.5 90 1.2 550 0.01
Maximum Duty Cycle Switch Current Limit Switch Saturation Voltage Switch Leakage Current Enable Threshold Enable Pin Current Oscillator Frequency Overvoltage Protection Overtemperature Threshold Shutdown
5 0.4
A V V A MHz V V C C
20 1.2 14 32 150 10
40 1.35 16 34
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Typical Characteristics
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MIC2291
Functional Characteristics
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MIC2291
Functional Diagram
VIN FB OVP* EN
OVP* SW gm VREF 95mV PWM Generator
1.2MHz Oscillator *OVP available on MLF(R) package option only
Ramp Generator
GND
Figure 1. MIC2291 Block Diagram
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.
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:
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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MIC2291
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.
D1 1A/40V Schottky
Over Voltage Protection For MLF(R) 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. Component Selection
VIN
10H MIC2291-34xML
VOUT
VIN 1-Cell Li Ion EN
SW OVP FB GND R2 3xLED
C2 1F
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.
Inductor Inductor selection is a balance between efficiency, stability, cost, size and rated current. For most applications a 10H 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 = VOUT VIN x L x IOUT x 2
2
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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VIN
Output Capacitor A 1F 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 cur-rent, the maximum output current and the maximum reverse voltage is rated greater than the output voltage. Input Capacitor A minimum 1F 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. 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;
R2 = VREF ILED
VIN PWM EN
SW FB GND
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.
VIN
VIN EN
SW FB GND 5.11k 49.9k DC Equivalent
Where VREF 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.
Figure 4. Continuous Dimming
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Package Information
5-Pin Thin SOT23 (D5)
8-Pin 2mm x 2mm MLF(R) (ML)
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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. (c) 2004 Micrel, Incorporated.
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