MAX1797 Evaluation Kit

2

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Detailed Description

Input Source

The input source for the MAX1797 EV kit must be

greater than 1.0V for guaranteed startup (0.7V for oper-

ation once started), and less than the output voltage. A

typical input voltage range would be the 2.0V to 3.3V

range of a 2-cell NiCd battery. An input voltage greater

than the selected output voltage (but less than 6V) will

not damage the circuit; however, the MAX1797 output

will nearly equal the input voltage when the device is

not shutdown. In shutdown, the output is high imped-

ance, and the ouput voltage will be 0V if a load is pre-

sent. If OUT is driven by another voltage source, such

as a backup battery, when the MAX1797 is shut down,

the IC presents no load to the backup source.

Once started, the MAX1797 operates from the regulat-

ed output voltage. This means that the input voltage

can fall below the 1.0V minimum guaranteed startup

voltage. Typically, the regulated output will be main-

tained even if the input voltage drops to 0.7V.

Shutdown Jumper Selection

The MAX1797 EV kit features a shutdown mode that

reduces quiescent current to <1µA to preserve battery

life. In shutdown, the output of the boost converter goes

to high impedance and is fully disconnected from the

input.

Using the Low-Battery Detector

The MAX1797 has an additional comparator useful for

monitoring the input source’s voltage level. Resistors R4

and R5 are connected as a voltage-divider between the

VIN pad and the MAX1797 LBI pin. Note that a PC trace

across R5 shorts the LBI pin to VIN when this function is

not used. Cut the trace before installing R5. Refer to the

Low-Battery Detection section of the MAX1797 data

sheet for instructions on selecting values for R4 and R5.

Output Voltage Selection

The MAX1797 is initially set for a 3.3V output by connect-

ing the FB pin to OUT, or set to 5V by connecting the FB

pin to GND. However, by adding external resistors R1

and R2, the output can be adjustable from 2.0V to 5.5V.

Cut the trace shorting resistor R1, and install output volt-

age-divider resistors R1 and R2. The Selecting the

Output Voltage section of the MAX1797 data sheet gives

instruction for calculating R1 and R2 values.

Table 1. Jumper JU1 Functions

(Shutdown Mode)

SHUNT

LOCATION

SHDN PIN

MAX1797

OUTPUT

1 and 2

Connected to VIN,

SHDN = high

Shutdown mode,

2 and 3

Connected to GND,

SHDN = low

MAX1797

enabled,