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LT1949 Datasheet(PDF) 5 Page - Linear Technology |
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LT1949 Datasheet(HTML) 5 Page - Linear Technology |
5 / 8 page 5 LT1949 – + – + – + – + + + Σ RAMP GENERATOR 1.24V REFERENCE R BIAS VC gm FB ENABLE 200mV A = 2 FF A1 COMPARATOR A2 COMPARATOR ERROR AMPLIFIER A4 0.06 Ω DRIVER SW GND 1949 BD Q3 Q S 600kHz OSCILLATOR 5 LBO LBI SHDN SHUTDOWN 3 7 1 4 R1 (EXTERNAL) VOUT 8 R2 (EXTERNAL) FB 2 Figure 3. LT1949 Block Diagram The LT1949 is a current mode, fixed frequency step-up DC/DC converter with an internal 1A NPN power transis- tor. Operation can best be understood by referring to the Block Diagram. At the beginning of each oscillator cycle, the flip-flop is set and the switch is turned on. Current in the switch ramps up until the voltage at A2’s positive input reaches the VC pin voltage, causing A2’s output to change state and the switch to be turned off. The signal at A2’s positive input is a summation of a signal representing switch current and a ramp generator (introduced to avoid subharmonic oscil- lations at duty factors greater than 50%). If the load increases, VOUT (and FB) will drop slightly and the error amplifier will drive VC to a higher voltage, causing current in the switch to increase. In this way, the error amplifier drives the VC pin to the voltage necessary to satisfy the load. Frequency compensation is provided by an external series RC network connected between the VC pin and ground. Layout Hints The LT1949 switches current at high speed, mandating careful attention to layout for proper performance. You will not get advertised performance with careless layouts. Figure 4 shows recommended component placement for a boost (step-up) converter. Follow this closely in your PC layout. Note the direct path of the switching loops. Input capacitor C1 must be placed close (< 5mm) to the IC package. As little as 10mm of wire or PC trace from CIN to VIN will cause problems such as inability to regulate or oscillation. The ground terminal of output capacitor C2 should tie close to Pin 4 of the LT1949. Doing this reduces dI/dt in the ground copper which keeps high frequency spikes to a minimum. The DC/DC converter ground should tie to the PC board ground plane at one place only, to avoid intro- ducing dI/dt in the ground plane. BLOCK DIAGRA OPERATIO |
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