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FR9855-Preliminary 0.3-JAN-2016

FR9855

85T

Functional Pin Description

Pin

Name

Pin No.

(SOP-8EP)

Pin No.

(TDFN3x3-10)

Pin Function

1

1

This pin includes enable the converter on/off, and select operation mode (The mode setting,

please refer to the following page 11). Connect VIN with a 100k

Ω resistor for self-startup and

operate in power saving mode.

FB

2

2

Voltage feedback input pin. Connect FB and VOUT with a resistive voltage divider. This IC

senses feedback voltage via FB and regulates it at 0.765V.

PG

3

x

Open drain power good output

.

SS

4

4

Soft-start pin. This pin controls the soft-start period. Connect a capacitor from SS to GND to

set the soft-start period.

GND

5

5

Ground pin.

LX

6

6,7

Power switching node. Connect an external inductor to this switching node.

BST

7

8

High side gate drive boost pin. A capacitor rating between 10nF~100nF must be connected

from this pin to LX. It can boost the gate drive to fully turn on the internal high side NMOS.

VIN

8

9,10

Power supply input pin. Placed input capacitors as close as possible from VIN to GND to

avoid noise influence.

Exposed

Pad

9

11

Ground pin. The exposed pad must be soldered to a large PCB area and connected to GND

for maximum power dissipation.

REG

x

3

Internal regulator output. Connect a 1uF capacitor to GND to stabilize the internal regulator

voltage.

Block Diagram

Logic

Control

Driver

Logic

BST

LX

OTP

UVLO

High-Side

MOSFET

Low-Side

MOSFET

FB

UVLO

&

POR

VIN

VCC

GND

OTP

Internal

Regulator

VCC

0.5M

SHDN

Vref

FB

0.9 x Vref.

On Time

Generator

Off Time

Generator

SS

6µA

LX

Comparator

PG

(SOP-8 EP only)

Cycle by Cycle

Current Limit

OCP

REG

(TDFN-10 only)

Figure 4. Block Diagram of FR9855