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LT1680CN Datasheet(PDF) 9 Page - Linear Technology |
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LT1680CN Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  9 LT1680 OPERATION Slope Compensation For duty cycles greater than 50%, slope compensation is required to prevent current mode duty cycle instability in the regulator control loop. The LT1680 employs internal slope compensation that is adequate for most applica- tions. However, if additional slope compensation is de- sired, it is available through the SL/ADJ pin. Excessive slope compensation will cause reduction in maximum load current capability and is generally not desirable. APPLICATIONS INFORMATION RSENSE Selection for Input Current Limit RSENSE generates a voltage that is proportional to the inductor current for use by the LT1680 current sense amplifier. The value of RSENSE is based on the required input current. The average current limit function has a typical threshold of 120mV/RSENSE, or: RSENSE = 120mV/ILIMIT Operation with VSENSE common mode voltage below 4.5V may slightly degrade current limit accuracy. See Average Current Limit Threshold Tolerance vs Common Mode Voltage in the Typical Performance Characteristics sec- tion for more information. Output Voltage Programming Output voltage is programmed through a resistor feed- back network to the VFB pin (Pin 7) on the LT1680. This pin is the inverting input of the error amplifier, which is internally referenced to 1.25V. The divider is ratioed to provide 1.25V at the VFB pin when the output is at its desired value. Output voltage is thus set following the relation: VOUT = 1.25V(1 + R2/R1) when an external resistor divider is connected to the output as shown in Figure 1. If high value feedback resistors are used, the input bias current of the VFB pin (1µA maximum) could cause a slight increase in output voltage. A Thevenin resistance at the VFB pin of <5k is recommended. Oscillator Components RCT and CCT The LT1680 oscillator creates a modified sawtooth at its timing node (CT) with a slow charge, rapid discharge characteristic. The discharge time (tDISCH) corresponds to the minimum off time of the PWM controller. This limits maximum duty cycle (DCMAX) to: DCMAX = 1 – (tDISCH)(fO) This relation corresponds to the minimum value of the timing resistor (RCT), which can be determined according to the following relation (RCT vs DCMAX graph appears in the Typical Performance Characteristics section): RCT(MIN) ≈ [(0.8)(10 –3)(1 – DCMAX)]–1 Values for RCT > 15k yield maximum duty cycles above 90%. Given a timing resistor value, the value of the timing capacitor (CCT) can then be determined for desired oper- ating frequency (fO) using the relation: C f R R CT O CT CT ≈ () ( ) ()+() () 1 100 10 185 175 2 5 10 3 375 9 3 /– /. . .– . / – – A plot of Operating Frequency vs RCT and CCT is shown in Figure 2. Typical 100kHz operational values are CCT = 1000pF and RCT = 16.9k. LT1680 VOUT VFB 6 7 R1 R2 1680 F01 SGND Figure 1. Programming LT1680 Output Voltage |
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