Datasheet LT1375, LT1376 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | 1.5A, 500kHz Step-Down Switching Regulators |
| Pages / Page | 28 / 9 — APPLICATIONS INFORMATION. Figure 2. Frequency and Current Limit Foldback. … |
| File Format / Size | PDF / 518 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Figure 2. Frequency and Current Limit Foldback. Table 1. OUTPUT. % ERROR AT OUTPUT. VOLTAGE. (NEAREST 1%)
Model Line for this Datasheet
- LT1375CN8#PBF LT1375CN8-5#PBF LT1375CS8#PBF LT1375CS8#TRPBF LT1375CS8-5#PBF LT1375CS8-5#TRPBF LT1375HVCS8#PBF LT1375HVCS8#TRPBF LT1375HVIS8#PBF LT1375HVIS8#TRPBF LT1375IN8#PBF LT1375IN8-5#PBF LT1375IS8#PBF LT1375IS8#TRPBF LT1375IS8-5#PBF LT1375IS8-5#TRPBF
- LT1376CN8#PBF LT1376CN8-5#PBF LT1376CS#PBF LT1376CS#TRPBF LT1376CS8#PBF LT1376CS8#TRPBF LT1376CS8-5#PBF LT1376CS8-5#TRPBF LT1376HVCS#PBF LT1376HVCS#TRPBF LT1376HVCS8#PBF LT1376HVCS8#TRPBF LT1376HVIS#PBF LT1376HVIS#TRPBF LT1376HVIS8#PBF LT1376HVIS8#TRPBF LT1376IN8#PBF LT1376IN8-5#PBF LT1376IS#PBF LT1376IS#TRPBF LT1376IS8#PBF LT1376IS8#TRPBF LT1376IS8-5#PBF LT1376IS8-5#TRPBF
Text Version of Document
LT1375/LT1376
U U W U APPLICATIONS INFORMATION
VSW LT1375/LT1376 TO FREQUENCY SHIFTING OUTPUT 5V 1.6V Q1 ERROR AMPLIFIER + 2.4V R1 R3 R4 1k 1k FB + – R5 5k Q2 R2 5k VC GND 1375/76 F02
Figure 2. Frequency and Current Limit Foldback
Please read the following if divider resistors are increased equal to the short-circuit current limit of the switch (typi- above the suggested values. cally 2A for the LT1376, folding back to less than 1A). Minimum switch on time limitations would prevent the R2 V ( − . ) switcher from attaining a sufficiently low duty cycle if OUT 2 42 R1= switching frequency were maintained at 500kHz, so fre- 2 4 . 2 quency is reduced by about 5:1 when the feedback pin
Table 1
voltage drops below 1V (see Frequency Foldback graph).
OUTPUT R1 % ERROR AT OUTPUT
This does not affect operation with normal load condi-
VOLTAGE R2 (NEAREST 1%) DUE TO DISCREET 1%
tions; one simply sees a gear shift in switching frequency
(V) (k
Ω
) (k
Ω
) RESISTOR STEPS
during start-up as the output voltage rises. 3 4.99 1.21 + 0.23 3.3 4.99 1.82 + 0.08 In addition to lower switching frequency, the LT1376 also 5 4.99 5.36 + 0.39 operates at lower switch current limit when the feedback 6 4.99 7.32 – 0.5 pin voltage drops below 1.7V. Q2 in Figure 2 performs this 8 4.99 11.5 – 0.04 function by clamping the VC pin to a voltage less than its normal 2.3V upper clamp level. This foldback current limit 10 4.99 15.8 + 0.83 greatly reduces power dissipation in the IC, diode and 12 4.99 19.6 – 0.62 inductor during short-circuit conditions. Again, it is nearly 15 4.99 26.1 + 0.52 transparent to the user under normal load conditions. The only loads which may be affected are current source loads
More Than Just Voltage Feedback
which maintain full load current with output voltage less The feedback (FB) pin is used for more than just output than 50% of final value. In these rare situations the voltage sensing. It also reduces switching frequency and Feedback pin can be clamped above 1.5V with an external current limit when output voltage is very low (see the diode to defeat foldback current limit. Caution: clamping Frequency Foldback graph in Typical Performance Char- the feedback pin means that frequency shifting will also be acteristics). This is done to control power dissipation in defeated, so a combination of high input voltage and dead both the IC and in the external diode and inductor during shorted output may cause the LT1376 to lose control of short-circuit conditions. A shorted output requires the current limit. switching regulator to operate at very low duty cycles, and The internal circuitry which forces reduced switching the average current through the diode and inductor is frequency also causes current to flow out of the feedback 13756fd 9
U U W U APPLICATIONS INFORMATION
VSW LT1375/LT1376 TO FREQUENCY SHIFTING OUTPUT 5V 1.6V Q1 ERROR AMPLIFIER + 2.4V R1 R3 R4 1k 1k FB + – R5 5k Q2 R2 5k VC GND 1375/76 F02
Figure 2. Frequency and Current Limit Foldback
Please read the following if divider resistors are increased equal to the short-circuit current limit of the switch (typi- above the suggested values. cally 2A for the LT1376, folding back to less than 1A). Minimum switch on time limitations would prevent the R2 V ( − . ) switcher from attaining a sufficiently low duty cycle if OUT 2 42 R1= switching frequency were maintained at 500kHz, so fre- 2 4 . 2 quency is reduced by about 5:1 when the feedback pin
Table 1
voltage drops below 1V (see Frequency Foldback graph).
OUTPUT R1 % ERROR AT OUTPUT
This does not affect operation with normal load condi-
VOLTAGE R2 (NEAREST 1%) DUE TO DISCREET 1%
tions; one simply sees a gear shift in switching frequency
(V) (k
Ω
) (k
Ω
) RESISTOR STEPS
during start-up as the output voltage rises. 3 4.99 1.21 + 0.23 3.3 4.99 1.82 + 0.08 In addition to lower switching frequency, the LT1376 also 5 4.99 5.36 + 0.39 operates at lower switch current limit when the feedback 6 4.99 7.32 – 0.5 pin voltage drops below 1.7V. Q2 in Figure 2 performs this 8 4.99 11.5 – 0.04 function by clamping the VC pin to a voltage less than its normal 2.3V upper clamp level. This foldback current limit 10 4.99 15.8 + 0.83 greatly reduces power dissipation in the IC, diode and 12 4.99 19.6 – 0.62 inductor during short-circuit conditions. Again, it is nearly 15 4.99 26.1 + 0.52 transparent to the user under normal load conditions. The only loads which may be affected are current source loads
More Than Just Voltage Feedback
which maintain full load current with output voltage less The feedback (FB) pin is used for more than just output than 50% of final value. In these rare situations the voltage sensing. It also reduces switching frequency and Feedback pin can be clamped above 1.5V with an external current limit when output voltage is very low (see the diode to defeat foldback current limit. Caution: clamping Frequency Foldback graph in Typical Performance Char- the feedback pin means that frequency shifting will also be acteristics). This is done to control power dissipation in defeated, so a combination of high input voltage and dead both the IC and in the external diode and inductor during shorted output may cause the LT1376 to lose control of short-circuit conditions. A shorted output requires the current limit. switching regulator to operate at very low duty cycles, and The internal circuitry which forces reduced switching the average current through the diode and inductor is frequency also causes current to flow out of the feedback 13756fd 9
