Applications Information

Operating Principle

The LM3355 is designed to provide a step-up/step-down

voltage regulation in battery powered systems. It combines

switched capacitor circuitry, reference, comparator, and

shutdown logic in a single 10-pin MSOP package. The

LM3355 can provide a regulated voltage between 1.8V and

4.1V from an input voltage between 2.5V and 5.5V. It can

supply a load current up to 50 mA.

As shown in

Figure 1, the LM3355 employs two feedback

loops to provide regulation in the most efficient manner

possible. The first loop is from V

COMP, the AND gate G

switch array. The comparator’s output is high when V

less than the reference V

gating the clock to the switch array. In this manner, charge is

transferred to the output only when needed. The second

loop controls the gain configuration of the switch array. This

loop consists of the comparator, the digital control block, the

phase generator, and the switch array. The digital control

block computes the most efficient gain from a set of five

gains based on inputs from the A/D and the comparator. The

gain signal is sent to the phase generator which then sends

the appropriate timing and configuration signals to the switch

array. This dual loop provides regulation over a wide range of

loads efficiently.

Since efficiency is automatically optimized, the curves for

V

mance Characteristics section exhibit small variations. The

reason is that as input voltage or output load changes, the

digital control loops are making decisions on how to optimize

efficiency. As the switch array is reconfigured, small varia-

tions in output voltage and efficiency result. In all cases

where these small variations are observed, the part is oper-

ating correctly; minimizing output voltage changes and opti-

mizing efficiency.

Charge Pump Capacitor Selection

A 0.33 µF ceramic capacitor is suggested for C1 and C2. To

ensure proper operation over temperature variations, an

X7R dielectric material is recommended.

Maximum V

DS200219-32

Maximum V

DS200219-30

DS200219-3

FIGURE 1. Block Diagram

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