Datasheet LTC3772 (Analog Devices) - 9
| Hersteller | Analog Devices |
| Beschreibung | Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller |
| Seiten / Seite | 20 / 9 — OPERATIO. (Refer to the Functional Diagram). Overvoltage Protection |
| Dateiformat / Größe | PDF / 259 Kb |
| Dokumentensprache | Englisch |
OPERATIO. (Refer to the Functional Diagram). Overvoltage Protection
Modelllinie für dieses Datenblatt
Textversion des Dokuments
LTC3772
U OPERATIO (Refer to the Functional Diagram) Overvoltage Protection
However, once the controller’s duty cycle exceeds 20%, slope compensation begins and effectively reduces the If VFB exceeds its regulation point of 0.8V by more than peak sense voltage by a scale factor given by the curve in 10% for any reason, such as an output short-circuit to a Figure 1. higher voltage, the overvoltage comparator will hold the external P-channel MOSFET off. This comparator has a The peak inductor current is determined by the peak sense typical hysteresis of 40mV. voltage and the on-resistance of the external P-channel MOSFET:
Peak Current Sense Voltage Selection and Slope Compensation (IPRG Pins)
∆V I SENSE MAX ( ) PEAK = When a controller is operating below 20% duty cycle, the RDS ON ( ) peak current sense voltage (between the SENSE+ and SW pins) allowed across the external P-channel MOSFET is
Soft-Start
determined by: The start-up of V OUT is controlled by the LTC3772 internal soft-start. During soft-start, the error amplifier EAMP A(V – 0.7V) ∆V ITH SENSE MAX ( ) = – 0.015 compares the feedback signal VFB to the internal soft-start 10 ramp (instead of the 0.8V reference), which rises linearly where A is a constant determined by the state of the IPRG from 0V to 0.8V in about 0.6ms. This allows the output pins. Floating the IPRG pin selects A = 1.58; tying IPRG to voltage to rise smoothly from 0V to its final value, while VIN selects A = 2.2; tying IPRG to SGND selects A = 0.97. maintaining control of the inductor current. After the The maximum value of V ITH is typically about 1.98V, so the soft-start is timed out, it is disabled until the part is put in maximum sense voltage allowed across the external shutdown again or the input supply is cycled. P-channel MOSFET is 175mV, 100mV or 250mV for the three respective states of the IPRG pin. 100 90 80 70 60 50 40 30 20 10 SF = REDUCTION IN SENSE VOLTAGE (mV) 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 3772 F01
Figure 1. Reduction in Sense Voltage Due to Slope Compensation vs Duty Cycle
3772f 9
U OPERATIO (Refer to the Functional Diagram) Overvoltage Protection
However, once the controller’s duty cycle exceeds 20%, slope compensation begins and effectively reduces the If VFB exceeds its regulation point of 0.8V by more than peak sense voltage by a scale factor given by the curve in 10% for any reason, such as an output short-circuit to a Figure 1. higher voltage, the overvoltage comparator will hold the external P-channel MOSFET off. This comparator has a The peak inductor current is determined by the peak sense typical hysteresis of 40mV. voltage and the on-resistance of the external P-channel MOSFET:
Peak Current Sense Voltage Selection and Slope Compensation (IPRG Pins)
∆V I SENSE MAX ( ) PEAK = When a controller is operating below 20% duty cycle, the RDS ON ( ) peak current sense voltage (between the SENSE+ and SW pins) allowed across the external P-channel MOSFET is
Soft-Start
determined by: The start-up of V OUT is controlled by the LTC3772 internal soft-start. During soft-start, the error amplifier EAMP A(V – 0.7V) ∆V ITH SENSE MAX ( ) = – 0.015 compares the feedback signal VFB to the internal soft-start 10 ramp (instead of the 0.8V reference), which rises linearly where A is a constant determined by the state of the IPRG from 0V to 0.8V in about 0.6ms. This allows the output pins. Floating the IPRG pin selects A = 1.58; tying IPRG to voltage to rise smoothly from 0V to its final value, while VIN selects A = 2.2; tying IPRG to SGND selects A = 0.97. maintaining control of the inductor current. After the The maximum value of V ITH is typically about 1.98V, so the soft-start is timed out, it is disabled until the part is put in maximum sense voltage allowed across the external shutdown again or the input supply is cycled. P-channel MOSFET is 175mV, 100mV or 250mV for the three respective states of the IPRG pin. 100 90 80 70 60 50 40 30 20 10 SF = REDUCTION IN SENSE VOLTAGE (mV) 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 3772 F01
Figure 1. Reduction in Sense Voltage Due to Slope Compensation vs Duty Cycle
3772f 9