LT3845 데이터 시트보기 (PDF) - Linear Technology

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LT3845

High Voltage Synchronous Current Mode Step-Down Controller with Adjustable Operating Frequency

Linear Technology 

LT3845

PIN FUNCTIONS

TG: The TG pin is the bootstrapped gate drive for the top

N-Channel MOSFET. Since very fast high currents are driven

from this pin, connect it to the gate of the power MOSFET

with a short and wide, typically 0.02” width, PCB trace to

minimize inductance.

VC: The VC pin is the output of the error amplifier whose

voltage corresponds to the maximum (peak) switch current

per oscillator cycle. The error amplifier is typically config-

ured as an integrator by connecting an RC network from

the VC pin to SGND. This circuit creates the dominant pole

for the converter regulation control loop. Specific integra-

tor characteristics can be configured to optimize transient

response. When Burst Mode operation is enabled (see Pin

4 description), an internal low impedance clamp on the VC

pin is set at 100mV below the burst threshold, which limits

the negative excursion of the pin voltage. Therefore, this

pin cannot be pulled low with a low impedance source. If

the VC pin must be externally manipulated, do so through

a 1kΩ series resistance.

VCC: 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 powered from this bias supply. An external diode con-

nected from VCC to the BOOST pin charges the bootstrapped

capacitor during the off-time of the main power switch.

Back driving the VCC pin from an external DC voltage

source, such as the VOUT output of the regulator supply,

increases overall efficiency and reduces power dissipation

in the IC. In shutdown mode this pin sinks 20μA until the

pin voltage is discharged to 0V.

VFB: The output voltage feedback pin, VFB, is externally

connected to the supply output voltage via a resistive divider.

The VFB pin is internally connected to the inverting input

of the error amplifier. In regulation, VFB is 1.231V.

VIN: The VIN pin is the main supply pin and should be

decoupled to SGND with a low ESR capacitor (at least

0.1μF) located close to the pin.

Exposed Pad (SGND) (TSSOP Only): The exposed lead-

frame is internally connected to the SGND pin. Solder the

exposed pad to the PCB ground for electrical contact and

optimal thermal performance.

3845fd

8

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