Datasheet LTC1735-1 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | High Efficiency Synchronous Step-Down Switching Regulator |
| Pages / Page | 28 / 9 — OPERATIO. (Refer to Functional Diagram). Foldback Current, Short-Circuit … |
| File Format / Size | PDF / 356 Kb |
| Document Language | English |
OPERATIO. (Refer to Functional Diagram). Foldback Current, Short-Circuit Detection. and Short-Circuit Latchoff
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Text Version of Document
LTC1735-1
U OPERATIO (Refer to Functional Diagram)
The main control loop is shut down by pulling Pin 2 (RUN/ continuous operation and assists in controlling voltage SS) low. Releasing RUN/SS allows an internal 1.2µA regulation. If the output voltage is not within 7.5% of its current source to charge soft-start capacitor CSS. When nominal value the PGOOD open-drain output will be CSS reaches 1.5V, the main control loop is enabled with the pulled low and Burst Mode operation will be disabled. ITH voltage clamped at approximately 30% of its maximum value. As CSS continues to charge, ITH is gradually re-
Foldback Current, Short-Circuit Detection
leased allowing normal operation to resume. If VOUT has
and Short-Circuit Latchoff
not reached 70% of its final value when CSS has charged The RUN/SS capacitor, C to 4.1V, latchoff can be invoked as described in the SS, is used initially to limit the inrush current of the switching regulator. After the con- Applications Information section. troller has been started and been given adequate time to The internal oscillator can be synchronized to an external charge up the output capacitors and provide full load clock applied though a series resistor to the PGOOD pin current, CSS is used as a short-circuit time-out circuit. If and can lock to a frequency between 90% and 130% of its the output voltage falls to less than 70% of its nominal nominal rate set by capacitor C output voltage, C OSC. SS begins discharging on the assumption that the output is in an overcurrent and/or short-circuit An overvoltage comparator OV guards against transient condition. If the condition lasts for a long enough period overshoots (> 7.5%) as well as other more serious as determined by the size of the C conditions that may overvoltage the output. In this case, SS, the controller will be shut down until the RUN/SS pin voltage is recycled. This the top MOSFET is turned off and the bottom MOSFET is built-in latchoff can be overridden by providing a current turned on until the overvoltage condition is cleared. > 5µA at a compliance of 5V to the RUN/SS pin. This Foldback current limiting for an output shorted to ground current shortens the soft-start period but also prevents net is provided by amplifier A. As VOSENSE drops below 0.6V, discharge of CSS during an overcurrent and/or short- the buffered ITH input to the current comparator is gradually circuit condition. Foldback current limiting is activated pulled down to a 0.86V clamp. This reduces peak inductor when the output voltage falls below 70% of its nominal current to about 1/4 of its maximum value. level whether or not the short-circuit latchoff circuit is enabled.
Low Current Operation INTV
The LTC1735-1 has three low current modes controlled
CC/EXTVCC Power
by the PGOOD pin. Burst Mode operation is selected when Power for the top and bottom MOSFET drivers and most the PGOOD pin is above 0.8V (typically tied through a of the internal circuitry of the LTC1735-1 is derived from resistor to INTV the INTV CC). During Burst Mode operation, if the CC pin. When the EXTVCC pin is left open, an error amplifier drives the I internal 5.2V low dropout regulator supplies the INTV TH voltage below 0.86V, the CC buffered I power from V TH input to the current comparator will be IN. If EXTVCC is raised above 4.7V, the clamped at 0.86V. The inductor current peak is then held internal regulator is turned off and an internal switch at approximately 20mV/R connects EXTV SENSE (about 1/4 of maximum CC to INTVCC. This allows a high efficiency output current). If I source, such as the primary or a secondary output of the TH then drops below 0.5V, the Burst Mode comparator B will turn off both MOSFETs to maxi- converter itself, to provide the INTVCC power. Voltages up mize efficiency. The load current will be supplied solely by to 7V can be applied to EXTVCC for additional gate drive the output capacitor until I capability. TH rises above the 60mV hysteresis of the comparator and switching is resumed. To provide clean start-up and to protect the MOSFETs, Burst Mode operation is disabled by comparator F when undervoltage lockout is used to keep both MOSFETs off the PGOOD pin is brought below 0.8V. This forces until the input voltage is above 3.5V. 9
U OPERATIO (Refer to Functional Diagram)
The main control loop is shut down by pulling Pin 2 (RUN/ continuous operation and assists in controlling voltage SS) low. Releasing RUN/SS allows an internal 1.2µA regulation. If the output voltage is not within 7.5% of its current source to charge soft-start capacitor CSS. When nominal value the PGOOD open-drain output will be CSS reaches 1.5V, the main control loop is enabled with the pulled low and Burst Mode operation will be disabled. ITH voltage clamped at approximately 30% of its maximum value. As CSS continues to charge, ITH is gradually re-
Foldback Current, Short-Circuit Detection
leased allowing normal operation to resume. If VOUT has
and Short-Circuit Latchoff
not reached 70% of its final value when CSS has charged The RUN/SS capacitor, C to 4.1V, latchoff can be invoked as described in the SS, is used initially to limit the inrush current of the switching regulator. After the con- Applications Information section. troller has been started and been given adequate time to The internal oscillator can be synchronized to an external charge up the output capacitors and provide full load clock applied though a series resistor to the PGOOD pin current, CSS is used as a short-circuit time-out circuit. If and can lock to a frequency between 90% and 130% of its the output voltage falls to less than 70% of its nominal nominal rate set by capacitor C output voltage, C OSC. SS begins discharging on the assumption that the output is in an overcurrent and/or short-circuit An overvoltage comparator OV guards against transient condition. If the condition lasts for a long enough period overshoots (> 7.5%) as well as other more serious as determined by the size of the C conditions that may overvoltage the output. In this case, SS, the controller will be shut down until the RUN/SS pin voltage is recycled. This the top MOSFET is turned off and the bottom MOSFET is built-in latchoff can be overridden by providing a current turned on until the overvoltage condition is cleared. > 5µA at a compliance of 5V to the RUN/SS pin. This Foldback current limiting for an output shorted to ground current shortens the soft-start period but also prevents net is provided by amplifier A. As VOSENSE drops below 0.6V, discharge of CSS during an overcurrent and/or short- the buffered ITH input to the current comparator is gradually circuit condition. Foldback current limiting is activated pulled down to a 0.86V clamp. This reduces peak inductor when the output voltage falls below 70% of its nominal current to about 1/4 of its maximum value. level whether or not the short-circuit latchoff circuit is enabled.
Low Current Operation INTV
The LTC1735-1 has three low current modes controlled
CC/EXTVCC Power
by the PGOOD pin. Burst Mode operation is selected when Power for the top and bottom MOSFET drivers and most the PGOOD pin is above 0.8V (typically tied through a of the internal circuitry of the LTC1735-1 is derived from resistor to INTV the INTV CC). During Burst Mode operation, if the CC pin. When the EXTVCC pin is left open, an error amplifier drives the I internal 5.2V low dropout regulator supplies the INTV TH voltage below 0.86V, the CC buffered I power from V TH input to the current comparator will be IN. If EXTVCC is raised above 4.7V, the clamped at 0.86V. The inductor current peak is then held internal regulator is turned off and an internal switch at approximately 20mV/R connects EXTV SENSE (about 1/4 of maximum CC to INTVCC. This allows a high efficiency output current). If I source, such as the primary or a secondary output of the TH then drops below 0.5V, the Burst Mode comparator B will turn off both MOSFETs to maxi- converter itself, to provide the INTVCC power. Voltages up mize efficiency. The load current will be supplied solely by to 7V can be applied to EXTVCC for additional gate drive the output capacitor until I capability. TH rises above the 60mV hysteresis of the comparator and switching is resumed. To provide clean start-up and to protect the MOSFETs, Burst Mode operation is disabled by comparator F when undervoltage lockout is used to keep both MOSFETs off the PGOOD pin is brought below 0.8V. This forces until the input voltage is above 3.5V. 9
