Datasheet LT1959 (Analog Devices) - 8

LT1959
U U W U APPLICATIONS INFORMATION FEEDBACK PIN FUNCTIONS More Than Just Voltage Feedback
The feedback (FB) pin on the LT1959 is used to set output The feedback pin is used for more than just output voltage voltage and provide several overload protection features. sensing. It also reduces switching frequency and current The first part of this section deals with selecting resistors limit when output voltage is very low (see the Frequency to set output voltage and the remaining part talks about Foldback graph in Typical Performance Characteristics). foldback frequency and current limiting created by the FB This is done to control power dissipation in both the IC and pin. Please read both parts before committing to a final in the external diode and inductor during short-circuit design. conditions. A shorted output requires the switching regu- The suggested value for the output divider resistor (see lator to operate at very low duty cycles, and the average Figure 2) from FB to ground (R2) is 2.5k or less, and a current through the diode and inductor is equal to the formula for R1 is shown below. The output voltage error short-circuit current limit of the switch (typically 6A for the caused by ignoring the input bias current on the FB pin is LT1959, folding back to less than 3A). Minimum switch on less than 0.1% with R2 = 2.5k. Please read the following time limitations would prevent the switcher from attaining if divider resistors are increased above the suggested a sufficiently low duty cycle if switching frequency were values. maintained at 500kHz, so frequency is reduced by about 5:1 when the feedback pin voltage drops below 0.5V (see Frequency Foldback graph). This does not affect operation R2 V ( . ) OUT − 1 21 R1= with normal load conditions; one simply sees a gear shift 1.21 in switching frequency during start-up as the output voltage rises. LT1959 VSW TO FREQUENCY In addition to lower switching frequency, the LT1959 also SHIFTING OUTPUT 5V operates at lower switch current limit when the feedback 1V Q1 pin voltage drops below 0.8V. Q2 in Figure 2 performs this ERROR function by clamping the VC pin to a voltage less than its AMPLIFIER R1 normal 2.1V upper clamp level. This foldback current limit + 1.21V R3 R4 greatly reduces power dissipation in the IC, diode and 1k 1k FB + inductor during short-circuit conditions. External synchro- – R5 nization is also disabled to prevent interference with 5k foldback operation. Again, it is nearly transparent to the Q2 R2 2.5k user under normal load conditions. The only loads that may TO SYNC CIRCUIT be affected are current source loads which maintain full load current with output voltage less than 50% of final value. In these rare situations the feedback pin can be clamped 1959 F02 above 0.75V with an external diode to defeat foldback cur- VC GND rent limit. Caution: clamping the feedback pin means that frequency shifting will also be defeated, so a combination of high input voltage and dead shorted output may cause
Figure 2. Frequency and Current Limit Foldback
the LT1959 to lose control of current limit. 8