MIC2172/3172 Bias Evaluation Board Steve Chenetz General Des

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MIC2172/3172 Bias Evaluation Board Steve Chenetz General Des

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MIC2172/3172 Bias Evaluation Board
MIC2172/3172
Bias Evaluation Board Steve Chenetz
General Description
This evaluation board uses MIC3172 MIC2172 switching regulator generate positive and/or negative voltages, suitable powering LCDs (liquid crystal displays). evaluation board schematic shown Figure jumper allows easy selection preset output voltages. MIC2172/3172 regulator features switching frequency 100kHz combines switching transistor, control circuitry, reference single package. MIC3172 also features low-current shutdown pin, while MIC2172 features sync external synchronization. This evaluation board demonstrates following: Positive and/or negative voltage output greater than input voltage (boost function). Thermistor-controlled [temperature] adjusted output voltage. (For LCDs that requires power source with negative temperature coefficient.) Potentiometer contrast adjustment. Soft-start circuity. P-channel MOSFET switch-controlled positive output which provides true, zero-voltage shutdown.
Requirements boost evaluation board requires input power source, which should able supply minimum output loaded with resistive active load. active load constant current constant resistance mode. Precautions evaluation board does have input reverse polarity protection. Applying negative voltage terminal (with respect ground) permanently damage components board. maximum input board 16V, limited input capacitor's voltage rating. Exceeding this voltage extended period time will damage capacitor. tantalum capacitors positive negative outputs rated 35V. exceed voltage rating these capacitors.
10µF
22µH COMP SGND
+VOUT
SS14
Enable
MIC3172 PGND1 PGND2
43.2k
22µF
0.1µF
TP2105
SYNC 33pF open 4.75k 0.01µF
22µF
CMSD4448LT1
2.26k 2N7002
SWITCHED VOUT
5.23k
3.65k
0.1µF
SS14
CMPD6263 SS14
100k
MMBZ5222LT1
22µF,
-VOUT
Note Remove switch when using MIC2172 (sync input). Note used with MIC3172 (enable input). Note resistors metal film, unless noted otherwise.
Figure Boost Converter
Micrel, Inc. 1849 Fortune Drive Jose, 95131 (408) 944-0800 (408) 944-0970 http://www.micrel.com
January 2000
MIC2172/3172 Bias Evaluation Board
MIC2172/3172 Bias Evaluation Board
Capacitor diodes comprise soft-start circuit. When MIC3172 starts switch output voltage rises. Energy coupled through causes voltage increase feedback This higher voltage feedback forces regulator turn slower rate. 2.5V zener diode, prevents excessive voltage from appearing feedback pin. Removing will disable soft start circuit. provide compensation stabilize voltage control loop. create zero feedback loop compensate pole formed output capacitance (C3) output resistance. creates pole that compensates zero caused output capacitance output capacitor ESR. MIC3172 (Enable Input) board comes assembled with MIC3172 converter which enable function. Switch used enable (pin high) disable (pin low) converter. MIC2172 (Sync Input) sync function required, control replaced with MIC2172BM. enable switch, must removed proper operation. capacitively couples sync signal Diode prevents sync signal from going more than diode drop below ground. resistor added location adjust switching frequency. Refer MIC2172 data sheet additional information using sync input. Output Voltage Setting Three standard voltages selected placing shorting jumper across proper pins header Placing jumper position replacing with proper value allows user program different output voltage. Make sure output voltage does exceed voltage rating output capacitor (35V) Schottky diode (40V). higher voltages required switch rating MIC3172), output capacitors diodes must replaced. value calculated equation below:
Circuit Operation
Boost Converter Operation During time, internal switching transistor MIC3172 turns current flows through inductor transistor. current through inductor increases, energy stored inductor. voltage across inductor equal (neglecting switch voltage) reverse voltage across diode VOUT. During time, switching transistor turns decreasing current flowing inductor forces inductor voltage reverse polarity. voltage across inductor increases until conducts clamps voltage VOUT VD1. energy inductor discharged into output capacitor load. This generates positive output voltage. negative output voltage developed using capacitordiode inverter. During time, current flows from inductor, through charging VOUT. Diode reversed biased. During time, voltage positive terminal zero. voltage across cannot change instantaneously which forces voltage anode -VOUT. Diode conducts which charges -VOUT VD3. Circuit Features P-channel MOSFET used switch positive output off. Either unswitched +VOUT terminal switched VOUT terminal used. When enable switch off, voltage switched VOUT terminal zero voltage unswitched +VOUT terminal equals input voltage. negative output does need switched since capacitor provides isolation between input negative output. When MIC3172 disabled, negative output will drop zero volts. necessary remove components from board when only polarity output used, however, disconnecting unused output will improve efficiency power supply. Removing will disconnect negative supply. When negative output used removed. remove since feedback loop used voltage regulation senses positive output voltage. output voltage range selected with jumper Within that range, output voltage continuously adjusted using potentiometer This potentiometer used vary contrast display fine tune output voltage. adjusts both positive negative outputs. Thermistor feedback loop power supply which gives output voltage negative temperature coefficient approximately -40mV/°C. Resistor helps linearize thermistor operating temperature range. negative temperature coefficient required, remove replace with 6.65k resistor.
1.24 REQUIV VOUT
1.24
Where: REQUIV Ohms VOUT volts constant resistance with wiper centered. REQUIV
MIC2172/3172 Bias Evaluation Board
January 2000
MIC2172/3172 Bias Evaluation Board
Bill Materials
Item 10K3A1W3 Betatherm ST-22 Dale-Vishay Part Number MIC3172BM TP2105K1 2N7002LT1 C23Y5V1C106ZP TPSE226M035R0300 TAZE105M050CRSZ0000 12065C104MATMA 06033C103MATMA 0603YC104MATMA 06035A330MATMA SS14 CMSD4448 CMPD6263 MMBZ5222LT1 CD54-220MC Manufacturer Micrel Supertex Motorola Tokin Central Semiconductor Central Semiconductor Motorola Sumida Description MIC3172 TP2015, P-channel MOSFET 2N7002, N-channel MOSFET 10µF, ceramic 22µF, tantalum 1µF, tantalum 0.1µF, 0.01µF ceramic 0.1µF, 33pF ceramic Schottky diode, 1A/40V diode diode zener diode, 2.5V inductor, 22µH 43.2k 0603 size, metal film 2.26k 0603 size, metal film 3.65k 0603 size, metal film 5.23k 0603 size, metal film 0603 size, metal film 4.75k 0603 size, metal film 0603 size, metal film 0603 size, metal film potentiometer open 0603 size, metal film 100k 0603 size, metal film thermistor
Layouts
Component-Side Silk Screen
January 2000
MIC2172/3172 Bias Evaluation Board
MIC2172/3172 Bias Evaluation Board
Component-Side Copper
Solder-Side Copper
MICREL INC. 1849 FORTUNE DRIVE JOSE, 95131
(408) 944-0800
(408) 944-0970
http://www.micrel.com
This information believed accurate reliable, however responsibility assumed Micrel infringement patents other rights third parties resulting from use. license granted implication otherwise under patent patent right Micrel Inc. 2000 Micrel Incorporated
MIC2172/3172 Bias Evaluation Board
January 2000

MIC2172/3172 Bias Evaluation Board Steve Chenetz General Des

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