Datasheet LTC1624 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | High Efficiency SO-8 N-Channel Switching Regulator Controller |
| Pages / Page | 28 / 10 — APPLICATIONS INFORMATION. (a). (b). Output Voltage Programming. (c). … |
| File Format / Size | PDF / 413 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. (a). (b). Output Voltage Programming. (c). Figure 3. I. TH/ RUN Pin Interfacing
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Text Version of Document
LTC1624
U U W U APPLICATIONS INFORMATION
pin through a small Schottky diode (like a Central 3.3V I OR 5V ITH/RUN TH/RUN CMDSH-3) to VOUT as shown in Figure 10. However, for D1 3.3V and other lower voltage regulators, additional cir- cuitry is required to derive boost supply power from the C C C C output. R R C C For low input voltage operation (VIN < 7V), a Schottky diode can be connected from V
(a) (b)
IN to BOOST to increase the external MOSFET gate drive voltage. Be careful not to ITH/RUN exceed the maximum voltage on BOOST to SW pins R1 of 7.8V. D1
Output Voltage Programming
CC C1 The output voltage is set by a resistive divider according RC to the following formula:
(c)
1624 F03 R2 V 1.19V OUT = 1 +
Figure 3. I
R 1
TH/ RUN Pin Interfacing
The external resistive divider is connected to the output as Soft start can be implemented by ramping the voltage on shown in Figure 2, allowing remote voltage sensing. When ITH/RUN during start-up as shown in Figure 3(c). As the using remote sensing, a local 100Ω resistor should be voltage on ITH/RUN ramps from 1.19V to 2.4V the internal connected from L1 to R2 to prevent V peak current limit is also ramped at a proportional linear OUT from running away if the sense lead is disconnected. rate. The peak current limit begins at approximately 10mV/RSENSE (at VITH/RUN = 1.4V) and ends at: VOUT L1 160mV/R R2 SENSE (VITH/RUN = 2.4V) VFB The output current thus ramps up slowly, charging the LTC1624 100pF R1 output capacitor. The peak inductor current and maximum GND output current are as follows: 1624 F02 IL(PEAK) = (VITH/RUN – 1.3V)/(6.8RSENSE)
Figure 2. Setting the LTC1624 Output Voltage
IOUT(MAX) = ILPEAK – ∆IL/ 2
ITH/RUN Function
with ∆IL = ripple current in the inductor. The ITH/RUN pin is a dual purpose pin that provides the During normal operation the voltage on the ITH/RUN pin loop compensation and a means to shut down the LTC1624. will vary from 1.19V to 2.4V depending on the load current. Soft start can also be implemented with this pin. Soft start Pulling the ITH/RUN pin below 0.8V puts the LTC1624 into reduces surge currents from VIN by gradually increasing a low quiescent current shutdown (IQ < 30µA). This pin can the internal current limit. Power supply sequencing can be driven directly from logic as shown in Figures 3(a) also be accomplished using this pin. and 3(b). An internal 2.5µA current source charges up the external
Efficiency Considerations
capacitor CC. When the voltage on ITH/RUN reaches 0.8V the LTC1624 begins operating. At this point the error The percent efficiency of a switching regulator is equal to amplifier pulls up the I the output power divided by the input power times 100%. TH/ RUN pin to its maximum of 2.4V (assuming V It is often useful to analyze individual losses to determine OUT is starting low). 10
U U W U APPLICATIONS INFORMATION
pin through a small Schottky diode (like a Central 3.3V I OR 5V ITH/RUN TH/RUN CMDSH-3) to VOUT as shown in Figure 10. However, for D1 3.3V and other lower voltage regulators, additional cir- cuitry is required to derive boost supply power from the C C C C output. R R C C For low input voltage operation (VIN < 7V), a Schottky diode can be connected from V
(a) (b)
IN to BOOST to increase the external MOSFET gate drive voltage. Be careful not to ITH/RUN exceed the maximum voltage on BOOST to SW pins R1 of 7.8V. D1
Output Voltage Programming
CC C1 The output voltage is set by a resistive divider according RC to the following formula:
(c)
1624 F03 R2 V 1.19V OUT = 1 +
Figure 3. I
R 1
TH/ RUN Pin Interfacing
The external resistive divider is connected to the output as Soft start can be implemented by ramping the voltage on shown in Figure 2, allowing remote voltage sensing. When ITH/RUN during start-up as shown in Figure 3(c). As the using remote sensing, a local 100Ω resistor should be voltage on ITH/RUN ramps from 1.19V to 2.4V the internal connected from L1 to R2 to prevent V peak current limit is also ramped at a proportional linear OUT from running away if the sense lead is disconnected. rate. The peak current limit begins at approximately 10mV/RSENSE (at VITH/RUN = 1.4V) and ends at: VOUT L1 160mV/R R2 SENSE (VITH/RUN = 2.4V) VFB The output current thus ramps up slowly, charging the LTC1624 100pF R1 output capacitor. The peak inductor current and maximum GND output current are as follows: 1624 F02 IL(PEAK) = (VITH/RUN – 1.3V)/(6.8RSENSE)
Figure 2. Setting the LTC1624 Output Voltage
IOUT(MAX) = ILPEAK – ∆IL/ 2
ITH/RUN Function
with ∆IL = ripple current in the inductor. The ITH/RUN pin is a dual purpose pin that provides the During normal operation the voltage on the ITH/RUN pin loop compensation and a means to shut down the LTC1624. will vary from 1.19V to 2.4V depending on the load current. Soft start can also be implemented with this pin. Soft start Pulling the ITH/RUN pin below 0.8V puts the LTC1624 into reduces surge currents from VIN by gradually increasing a low quiescent current shutdown (IQ < 30µA). This pin can the internal current limit. Power supply sequencing can be driven directly from logic as shown in Figures 3(a) also be accomplished using this pin. and 3(b). An internal 2.5µA current source charges up the external
Efficiency Considerations
capacitor CC. When the voltage on ITH/RUN reaches 0.8V the LTC1624 begins operating. At this point the error The percent efficiency of a switching regulator is equal to amplifier pulls up the I the output power divided by the input power times 100%. TH/ RUN pin to its maximum of 2.4V (assuming V It is often useful to analyze individual losses to determine OUT is starting low). 10
