LM3350 datasheet(5/8 Pages) NSC

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Detailed Operation (Continued)

STEP-UP APPLICATIONS NEED AN EXTRA DIODE

The LM3350 is biased from pin V

high. Thus for step-up appli-

cations, an external Schottky diode (D1) is needed to supply

power to V

high during startup (See Figure 2). Note that during

shutdown, this diode will provide a DC path from V

in to Vout.

The load may therefore continue to draw current from the in-

put voltage source. This Schottky diode is not required for

step-down applications (See

Figure 3).

SHUTDOWN MODE

When ENABLE is a logic low (ground), the LM3350 enters a

low power shutdown mode. In this mode, all circuitry is dis-

abled and therefore, all switching action stops. During shut-

down, the current consumption drops to 250 nA (typical).

When ENABLE is a logic high, (i.e. 3.3V for step-up mode

and 5.0V for step-down mode), the LM3350 returns to nor-

mal operation.

Application Information

CAPACITOR SELECTION

The LM3350 requires four capacitors: an input bypass ca-

pacitor (connected between V

in and ground), an output hold

capacitor (connected between V

out and ground), and two

sampling capacitors (C1 and C2 in

Figures 2, 3). 0.33 µF

(±20%) ceramic chip type capacitors are recommended for

all four capacitors. The usable operating frequency should

be greater than 5 MHz for all capacitors.

The input bypass capacitor (C

in) is the least critical. Its value

should be at least that of the sampling capacitors.

While the recommended sampling capacitor values are 0.33

µF, other values may be selected. Values other than 0.33 µF

affect maximum output current rating and efficiency.

Figure 1

shows typical output impedance (Z

out)

versus capacitor

value.

The output hold capacitor value determines the output ripple.

Increasing the value of the hold capacitor decreases the

ripple. The value of this capacitor (C

out) can be calculated

(approximately) based on the output ripple (

∆V

out) require-

ments from:

where I

load is the load current and fO is the oscillator fre-

quency.

In order to ensure superior performance over the entire oper-

ating temperature range, capacitors made of X7R dielectric

material are suggested. However, capacitors made of other

dielectric materials that still meet the ±20% specification

over the entire temperature range can also be used.

PRECAUTIONS

The LM3350 is not short circuit protected.

DS100002-11

A. Step-Up Mode

DS100002-12

B. Step-Down Mode

FIGURE 1. Z

out vs. C

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