Pin Description

Pin

Name

Function

1

Cap1+

Positive terminal for the first charge

pump capacitor.

2

Cap1−

Negative terminal for the first charge

pump capacitor.

3

Cap2+

Positive terminal for the second charge

pump capacitor.

4

Cap2−

Negative terminal for the second charge

pump capacitor.

5V

low

In Step-Up mode, this will be the input

terminal. In Step-Down mode, this will

be the output terminal.

6

Gnd

Ground

7V

high

In Step-Down mode, this will be the

input terminal. In Step-Up mode, this

will be the output terminal.

8

Enable

Active high CMOS logic level Enable

Input. Connect to Voltage Input terminal

to enable the IC. Connect to Ground

(Pin 6) to disable.

Detailed Operation

OPERATING PRINCIPLE

The LM3351 is a charge-pump voltage converter that pro-

vides a voltage conversion ratio of 3/2 in step-up mode and

a conversion ratio of 2/3 in the step-down mode. Thus it can

be used in the step-down mode to provide a 3.3V output

from a regulated 5V input or in the step-up mode to provide

a 5V output from a regulated 3.3V input. Other values of in-

put voltages can be used as long as they are within the lim-

its.

The LM3351 contains an array of CMOS switches which are

operated in a certain sequence to provide the step-up or

step-down of the input supply. An internal RC oscillator pro-

vides the timing signals.

Energy transfer and storage are provided by four inexpen-

sive ceramic capacitors. The selection of these capacitors is

explained in the Capacitor Selection section under Applica-

tion Information.

V

DS100146-7

DS100146-8

Output Source Impedance vs

Temperature (Step-Down)

DS100146-9

Switching Frequency vs

Temperature

DS100146-10

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