PDF CS2844 Data sheet ( Hoja de datos )

Número de pieza	CS2844
Descripción	Current Mode PWM Control Circuit
Fabricantes	Cherry Semiconductor Corporation
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No Preview Available ! CS2844/CS3844 CS2845/CS3845 Current Mode PWM Control Circuit with 50% Max Duty Cycle Description Features The CS3844/45 provides all the neces- sary features to implement off-line fixed frequency current-mode control with a minimum number of external components. The CS3844 family incorporates a new precision temperature-controlled oscil- lator to minimize variations in fre- quency. An internal toggle flip-flop, which blanks the output every other clock cycle, limits the duty-cycle range to less than 50%. An undervoltage lockout ensures that VREF is stabilized before the output stage is enabled. In the CS2844/CS3844 turn on occurs at 16V and turn off at 10V. In the CS2845/CS3845 turn on is at 8.4V and turn off at 7.6V. Other features include low start-up current, pulse-by-pulse current limit- ing, and a high-current totem pole out- put for driving capacitive loads, such as gate of a power MOSFET. The out- put is low in the off state, consistent with N-channel devices. Absolute Maximum Ratings Supply Voltage (ICC<30mA).........................................................................Self Limiting Supply Voltage (Low Impedance Source) .................................................................30V Output Current..............................................................................................................±1A Output Energy (Capacitive Load) ................................................................................5µJ Analog Inputs (VFB, VSENSE) .........................................................................-0.3V to 5.5V Error Amp Output Sink Current .............................................................................10mA Lead Temperature Soldering Wave Solder (through hole styles only) ...................10 sec. max, 260°C peak Reflow (SMD styles only) ....................60 sec. max above 183°C, 230°C peak Block Diagram VCC Gnd VFB COMP OSC Sense VCC Undervoltage Lock-out 34V 16V/10V (8.4V/7.6V) Error Amplifier - + 2.50V Oscillator Set/ 5.0 Volt Reset Reference R VREF Undervoltage Lockout R Toggle Flip-Flop NOR Internal Bias 2R R S R 1V Current PWM Sensing Latch Comparator ( ) indicates CS-2845/3845 VCC Pwr VREF V O UT Pwr Gnd s Optimized for Offline Control s Temp. Compensated Oscillator s 50% Max Duty-cycle Clamp s VREF Stabilized before Output Stage is Enabled s Low Start-up Current s Pulse-by-pulse Current Limiting s Improved Undervoltage Lockout s Double Pulse Suppression s 1% Trimmed Bandgap Ref. s High Current Totem Pole Output Package Options 8 Lead PDIP & SO Narrow COMP 1 VFB 2 Sense 3 OSC 4 8 VREF 7 VCC 6 VOUT 5 Gnd 14 Lead SO Narrow COMP 1 NC 2 14 VREF 13 NC VFB 3 NC 4 Sense 5 NC 6 12 VCC 11 VCC Pwr 10 VOUT 9 Gnd OSC 7 8 Pwr Gnd 16 Lead SO Wide NC 1 16 NC NC 2 COMP 3 VFB 4 SENSE 5 OSC 6 NC 7 15 VREF 14 VCC 13 VCCPwr 12 VO 11 Gnd 10 Pwr Gnd NC 8 9 NC Rev. 3/17/99 Cherry Semiconductor Corporation 2000 South County Trail, East Greenwich, RI 02818 Tel: (401)885-3600 Fax: (401)885-5786 Email: [email protected] Web Site: www.cherry-semi.com 1 A ® Company 1 page VOSC OSC RESET Toggle F/F Output EA Output Switch Current VCC IO VO Figure 2: Timing Diagram Vupper Vlower ton tC ton = tC toff = tC+2td Figure 3: Duty Cycle parameters. toff td Circuit Description: continued Setting the Oscillator The times Tc and Td can be determined as follows: ( )tc = RTCT ln VREF - Vlower VREF - Vupper ( )td = RTCT ln VREF - IdRT - Vlower VREF - IdRT - Vupper Substituting in typical values for the parameters in the above formulas: then VREF = 5.0V, Vupper = 2.7V, Vlower = 1.0V, Id = 8.3mA, tc ≈ 0.5534RTCT ( )td = RTCT ln 2.3 - 0.0083 RT 4.0 - 0.0083 RT For better accuracy RT should be ≥10kΩ. Grounding High peak currents associated with capacitive loads neces- sitate careful grounding techniques. Timing and bypass capacitors should be connected close to Gnd in a single point ground. The transistor and 5kΩ potentiometer are used to sample the oscillator waveform and apply an adjustable ramp to Sense. 5 5 Page

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