Datasheet LT3845A (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | High Voltage Synchronous Current Mode Step-Down Controller with Adjustable Operating Frequency |
| Pages / Page | 26 / 8 — PIN FUNCTIONS. TG:. VFB:. IN:. Exposed Pad (SGND) (TSSOP Only):. CC: |
| File Format / Size | PDF / 295 Kb |
| Document Language | English |
PIN FUNCTIONS. TG:. VFB:. IN:. Exposed Pad (SGND) (TSSOP Only):. CC:
Model Line for this Datasheet
Text Version of Document
LT3845A
PIN FUNCTIONS TG:
The TG pin is the bootstrapped gate drive for the top powered from this bias supply. An external diode connected N-Channel MOSFET. Since very fast high currents are driven from VCC to the BOOST pin charges the bootstrapped from this pin, connect it to the gate of the power MOSFET capacitor during the off-time of the main power switch. with a short and wide, typically 0.02” width, PCB trace to Back driving the VCC pin from an external DC voltage minimize inductance. source, such as the VOUT output of the regulator supply,
V
increases overall efficiency and reduces power dissipation
C:
The VC pin is the output of the error amplifier whose voltage corresponds to the maximum (peak) switch in the IC. In shutdown mode this pin sinks 20µA until the current per oscillator cycle. The error amplifier is typically pin voltage is discharged to 0V. configured as an integrator by connecting an RC network
VFB:
The output voltage feedback pin, VFB, is externally from the VC pin to SGND. This circuit creates the dominant connected to the supply output voltage via a resistive divider. pole for the converter regulation control loop. Specific The VFB pin is internally connected to the inverting input integrator characteristics can be configured to optimize of the error amplifier. In regulation, VFB is 1.231V. transient response. When Burst Mode operation is enabled
V
(see Pin 4 description), an internal low impedance clamp on
IN:
The VIN pin is the main supply pin and should be decoupled to SGND with a low ESR capacitor (at least the VC pin is set at 100mV below the burst threshold, which 0.1µF) located close to the pin. limits the negative excursion of the pin voltage. Therefore, this pin cannot be pulled low with a low impedance source.
Exposed Pad (SGND) (TSSOP Only):
The exposed If the VC pin must be externally manipulated, do so through leadframe is internally connected to the SGND pin. Solder a 1kΩ series resistance. the exposed pad to the PCB ground for electrical contact
V
and optimal thermal performance.
CC:
The VCC pin is the internal bias supply decoupling node. Use a low ESR, 1µF or greater ceramic capacitor to decouple this node to PGND. Most internal IC functions are 3845afa 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts
PIN FUNCTIONS TG:
The TG pin is the bootstrapped gate drive for the top powered from this bias supply. An external diode connected N-Channel MOSFET. Since very fast high currents are driven from VCC to the BOOST pin charges the bootstrapped from this pin, connect it to the gate of the power MOSFET capacitor during the off-time of the main power switch. with a short and wide, typically 0.02” width, PCB trace to Back driving the VCC pin from an external DC voltage minimize inductance. source, such as the VOUT output of the regulator supply,
V
increases overall efficiency and reduces power dissipation
C:
The VC pin is the output of the error amplifier whose voltage corresponds to the maximum (peak) switch in the IC. In shutdown mode this pin sinks 20µA until the current per oscillator cycle. The error amplifier is typically pin voltage is discharged to 0V. configured as an integrator by connecting an RC network
VFB:
The output voltage feedback pin, VFB, is externally from the VC pin to SGND. This circuit creates the dominant connected to the supply output voltage via a resistive divider. pole for the converter regulation control loop. Specific The VFB pin is internally connected to the inverting input integrator characteristics can be configured to optimize of the error amplifier. In regulation, VFB is 1.231V. transient response. When Burst Mode operation is enabled
V
(see Pin 4 description), an internal low impedance clamp on
IN:
The VIN pin is the main supply pin and should be decoupled to SGND with a low ESR capacitor (at least the VC pin is set at 100mV below the burst threshold, which 0.1µF) located close to the pin. limits the negative excursion of the pin voltage. Therefore, this pin cannot be pulled low with a low impedance source.
Exposed Pad (SGND) (TSSOP Only):
The exposed If the VC pin must be externally manipulated, do so through leadframe is internally connected to the SGND pin. Solder a 1kΩ series resistance. the exposed pad to the PCB ground for electrical contact
V
and optimal thermal performance.
CC:
The VCC pin is the internal bias supply decoupling node. Use a low ESR, 1µF or greater ceramic capacitor to decouple this node to PGND. Most internal IC functions are 3845afa 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Typical Application Related Parts
