Datasheet LT3991, LT3991-3.3, LT3991-5 (Analog Devices) - 7
| Manufacturer | Analog Devices |
| Description | 55V, 1.2A Step-Down Regulator with 2.8μA Quiescent Current |
| Pages / Page | 24 / 7 — Typical perForMance characTerisTics TA = 25°C, unless otherwise noted. … |
| File Format / Size | PDF / 398 Kb |
| Document Language | English |
Typical perForMance characTerisTics TA = 25°C, unless otherwise noted. Transient Load Response,. Load Current Stepped from
Model Line for this Datasheet
Text Version of Document
LT3991/LT3991-3.3/LT3991-5
Typical perForMance characTerisTics TA = 25°C, unless otherwise noted. Transient Load Response, Load Current Stepped from Switching Waveforms; Switching Waveforms; Full 0.5A to 1A Burst Mode Operation Frequency Continuous Operation
VOUT 100mV/ V V SW SW DIV 5V/DIV 5V/DIV IL I 500mA/DIV L I 500mA/DIV L 500mA/ DIV VOUT VOUT 20mV/DIV 20mV/DIV 10µs/DIV 3991 G25 5µs/DIV 3971 G26 1µs/DIV 3971 G27 VIN = 48V, VOUT = 3.3V VIN = 48V, VOUT = 3.3V VIN = 48V, VOUT = 3.3V COUT = 47µF ILOAD = 20mA ILOAD = 1A COUT = 47µF COUT = 47µF
pin FuncTions BD (Pin 1):
This pin connects to the anode of the boost
FB (Pin 6, LT3991 Only):
The LT3991 regulates the FB pin diode. The BD pin is normally connected to the output. to 1.19V. Connect the feedback resistor divider tap to this
BOOST (Pin 2):
This pin is used to provide a drive volt- pin. Also, connect a phase lead capacitor between FB and age, higher than the input voltage, to the internal bipolar VOUT. Typically this capacitor is 10pF. NPN power switch.
VOUT (Pin 6, LT3991-3.3/LT3991-5 Only):
The LT3991-
SW (Pin 3):
The SW pin is the output of an internal power 3.3 and LT3991-5 regulate the VOUT pin to 3.3V and 5V switch. Connect this pin to the inductor, catch diode, and respectively. This pin connects to the internal 10MΩ boost capacitor. feedback divider that programs the fixed output voltage.
V SS (Pin 7):
A capacitor and a series resistor are tied between
IN (Pin 4):
The VIN pin supplies current to the LT3991’s internal circuitry and to the internal power switch. This SS and ground to slowly ramp up the peak current limit pin must be locally bypassed. of the LT3991 on start-up. The soft-start capacitor is only actively discharged when EN is low. The SS pin is released
EN (Pin 5):
The part is in shutdown when this pin is low when the EN pin goes high. Float this pin to disable soft- and active when this pin is high. The hysteretic threshold start. The soft-start resistor has a typical value of 100k. voltage is 1.005V going up and 0.975V going down. The EN threshold is only accurate when V
RT (Pin 8):
A resistor is tied between RT and ground to IN is above 4.3V. If VIN is lower than 4.3V, ground EN to place the part in shutdown. set the switching frequency. Tie to VIN if shutdown feature is not used. 3991fa 7 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts
Typical perForMance characTerisTics TA = 25°C, unless otherwise noted. Transient Load Response, Load Current Stepped from Switching Waveforms; Switching Waveforms; Full 0.5A to 1A Burst Mode Operation Frequency Continuous Operation
VOUT 100mV/ V V SW SW DIV 5V/DIV 5V/DIV IL I 500mA/DIV L I 500mA/DIV L 500mA/ DIV VOUT VOUT 20mV/DIV 20mV/DIV 10µs/DIV 3991 G25 5µs/DIV 3971 G26 1µs/DIV 3971 G27 VIN = 48V, VOUT = 3.3V VIN = 48V, VOUT = 3.3V VIN = 48V, VOUT = 3.3V COUT = 47µF ILOAD = 20mA ILOAD = 1A COUT = 47µF COUT = 47µF
pin FuncTions BD (Pin 1):
This pin connects to the anode of the boost
FB (Pin 6, LT3991 Only):
The LT3991 regulates the FB pin diode. The BD pin is normally connected to the output. to 1.19V. Connect the feedback resistor divider tap to this
BOOST (Pin 2):
This pin is used to provide a drive volt- pin. Also, connect a phase lead capacitor between FB and age, higher than the input voltage, to the internal bipolar VOUT. Typically this capacitor is 10pF. NPN power switch.
VOUT (Pin 6, LT3991-3.3/LT3991-5 Only):
The LT3991-
SW (Pin 3):
The SW pin is the output of an internal power 3.3 and LT3991-5 regulate the VOUT pin to 3.3V and 5V switch. Connect this pin to the inductor, catch diode, and respectively. This pin connects to the internal 10MΩ boost capacitor. feedback divider that programs the fixed output voltage.
V SS (Pin 7):
A capacitor and a series resistor are tied between
IN (Pin 4):
The VIN pin supplies current to the LT3991’s internal circuitry and to the internal power switch. This SS and ground to slowly ramp up the peak current limit pin must be locally bypassed. of the LT3991 on start-up. The soft-start capacitor is only actively discharged when EN is low. The SS pin is released
EN (Pin 5):
The part is in shutdown when this pin is low when the EN pin goes high. Float this pin to disable soft- and active when this pin is high. The hysteretic threshold start. The soft-start resistor has a typical value of 100k. voltage is 1.005V going up and 0.975V going down. The EN threshold is only accurate when V
RT (Pin 8):
A resistor is tied between RT and ground to IN is above 4.3V. If VIN is lower than 4.3V, ground EN to place the part in shutdown. set the switching frequency. Tie to VIN if shutdown feature is not used. 3991fa 7 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Related Parts
