Datasheet LT1956, LT1956-5 (Analog Devices) - 8

LT1956/LT1956-5
U U W U APPLICATIO S I FOR ATIO FEEDBACK PIN FUNCTIONS
current through the diode and inductor is equal to the short-circuit current limit of the switch (typically 2A for The feedback (FB) pin on the LT1956 is used to set output the LT1956, folding back to less than 1A). Minimum voltage and provide several overload protection features. switch on time limitations would prevent the switcher The first part of this section deals with selecting resistors from attaining a sufficiently low duty cycle if switching to set output voltage and the remaining part talks about frequency were maintained at 500kHz, so frequency is foldback frequency and current limiting created by the FB reduced by about 5:1 when the feedback pin voltage drops pin. Please read both parts before committing to a final below 0.8V (see Frequency Foldback graph). This does design. The 5V fixed output voltage part (LT1956-5) has not affect operation with normal load conditions; one internal divider resistors and the FB pin is renamed SENSE, simply sees a shift in switching frequency during start-up connected directly to the output. as the output voltage rises. The suggested value for the output divider resistor (see In addition to lower switching frequency, the LT1956 also Figure 2) from FB to ground (R2) is 5k or less, and a operates at lower switch current limit when the feedback formula for R1 is shown below. The output voltage error pin voltage drops below 0.6V. Q2 in Figure 2 performs this caused by ignoring the input bias current on the FB pin is function by clamping the VC pin to a voltage less than its less than 0.25% with R2 = 5k. A table of standard 1% normal 2.1V upper clamp level. This foldback current limit values is shown in Table 1 for common output voltages. greatly reduces power dissipation in the IC, diode and in- Please read the following section if divider resistors are ductor during short-circuit conditions. External synchro- increased above the suggested values. nization is also disabled to prevent interference with fold- back operation. Again, it is nearly transparent to the user R2(V −1.22) R OUT 1= under normal load conditions. The only loads that may be 1.22 affected are current source loads which maintain full load
Table 1
current with output voltage less than 50% of final value. In
OUTPUT R1 % ERROR AT OUTPUT
these rare situations the feedback pin can be clamped above
VOLTAGE R2 (NEAREST 1%) DUE TO DISCRETE 1%
0.6V with an external diode to defeat foldback current limit.
(V) (k
Ω
) (k
Ω
) RESISTOR STEPS
Caution: clamping the feedback pin means that frequency 3 4.99 7.32 + 0.32 shifting will also be defeated, so a combination of high in- 3.3 4.99 8.45 – 0.43 put voltage and dead shorted output may cause the LT1956 5 4.99 15.4 – 0.30 to lose control of current limit. 6 4.75 18.7 + 0.38 The internal circuitry which forces reduced switching 8 4.47 24.9 + 0.20 frequency also causes current to flow out of the feedback 10 4.32 30.9 – 0.54 pin when output voltage is low. The equivalent circuitry is 12 4.12 36.5 + 0.24 shown in Figure 2. Q1 is completely off during normal 15 4.12 46.4 – 0.27 operation. If the FB pin falls below 0.8V, Q1 begins to conduct current and reduces frequency at the rate of
More Than Just Voltage Feedback
approximately 3.5kHz/µA. To ensure adequate frequency The feedback pin is used for more than just output voltage foldback (under worst-case short-circuit conditions), the sensing. It also reduces switching frequency and current external divider Thevinin resistance must be low enough limit when output voltage is very low (see the Frequency to pull 115µA out of the FB pin with 0.44V on the pin (RDIV Foldback graph in Typical Performance Characteristics). ≤ 3.8k). The net result is that reductions in frequency and This is done to control power dissipation in both the IC current limit are affected by output voltage divider imped- and in the external diode and inductor during short-circuit ance. Although divider impedance is not critical, caution conditions. A shorted output requires the switching regu- should be used if resistors are increased beyond the lator to operate at very low duty cycles, and the average suggested values and short-circuit conditions will occur 1956f 8