Datasheet LT1977 (Analog Devices) - 10

LT1977
U U W U APPLICATIO S I FOR ATIO
and Soft-Start Current graphs in Typical Performance VOUT LT1977 Characteristics). SW 2 C1 Frequency foldback is used to control power dissipation in CSS SOFT-START 9 both the IC and in the external diode and inductor during short-circuit conditions. A shorted output requires the 500kHz FOLDBACK R1 switching regulator to operate at very low duty cycles. As OSCILLATOR DETECT a result the average current through the diode and induc- tor is equal to the short-circuit current limit of the switch – FB 12 (typically 2A for the LT1977). Minimum switch on time ERROR AMP R2 limitations would prevent the switcher from operating at a + sufficiently low duty cycle if switching frequency were 1.25V maintained at 500kHz, so frequency is reduced by about VC 4:1 when the FB pin voltage drops below 0.4V (see 11 Frequency Foldback graph). In addition, if the current in 1977 F02 the switch exceeds 1.5 times the current limitations speci-
Figure 2. Feedback Network
fied by the VC pin, due to minimum switch on time, the LT1977 will skip the next switch cycle. As the feedback
Table 2
voltage rises, the switching frequency increases to 500kHz
OUTPUT R1 OUTPUT
with 0.95V on the FB pin. During frequency foldback,
VOLTAGE R2 NEAREST (1%) ERROR
external synchronization is disabled to prevent interfer-
(V) (k
Ω
, 1%) (k
Ω
) (%)
ence with foldback operation. Frequency foldback does 2.5 100 100 0 not affect operation during normal load conditions. 3 100 140 0 3.3 100 165 0.38 In addition to lowering switching frequency the soft-start ramp rate is also affected by the feedback voltage. Large 5 100 300 0 capacitive loads or high input voltages can cause a high 6 100 383 0.63 input current surge during start-up. The soft-start func- 8 100 536 – 0.63 tion reduces input current surge by regulating switch 10 100 698 – 0.25 current via the VC pin to maintain a constant voltage ramp 12 100 866 0.63 rate (dV/dt) at the output. A capacitor (C1 in Figure 2) from 50nA FB pin input current. The suggested value for the the CSS pin to the output determines the maximum output output divider resistor (see Figure 2) from FB to ground dV/dt. When the feedback voltage is below 0.4V, the VC pin (R2) is 100k or less. A formula for R1 is shown below. A will rise, resulting in an increase in switch current and table of standard 1% values is shown in Table 2 for output voltage. If the dV/dt of the output causes the current common output voltages. through the CSS capacitor to exceed ICSS the VC voltage is reduced resulting in a constant dV/dt at the output. As the V – 1.25 R = R OUT 1 2 • feedback voltage increases ICSS increases, resulting in an 1.25 + R2 • 5 n 0 A increased dV/dt until the soft-start function is defeated with 0.9V present at the FB pin. The soft-start function
More Than Just Voltage Feedback
does not affect operation during normal load conditions. The FB pin is used for more than just output voltage However, if a momentary short (brown out condition) is sensing. It also reduces switching frequency and con- present at the output which causes the FB voltage to drop trols the soft-start voltage ramp rate when output voltage below 0.9V, the soft-start circuitry will become active. is below the regulated level (see the Frequency Foldback 1977fa 10