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LTM8003

8003fc

For more information www.linear.com/LTM8003

APPLICATIONS INFORMATION

most versatile way to generate a negative output is to

use a dedicated regulator that was designed to generate

a negative voltage, but using a buck regulator like the

LTM8003 to generate a negative voltage is a simple and

cost effective solution, as long as certain restrictions are

kept in mind.

NEGATIVE

OUTPUT VOLTAGE

LTM8003

VOUT

VIN

8003 F01

GND

VIN

Figure1. The LTM8003 Can Be Used to Generate a Negative Voltage

Figure1showsatypicalnegativeoutputvoltageapplication.

Note that LTM8003 VOUT is tied to system GND and input

power is applied from VIN to LTM8003 VOUT. As a result,

the LTM8003 is not behaving as a true buck regulator,

and the maximum output current depends upon the input

voltage. In the example shown in the Typical Applications

section, there is an attending graph that shows how much

current the LTM8003 can deliver for given input voltages.

FAST LOAD

TRANSIENT

OUTPUT TRANSIENT

RESPONSE

LTM8003

VOUT

VIN

8003 F02

GND

VIN

Figure2. Any Output Voltage Transient Appears on LTM8003 GND

Note that this configuration requires that any load current

transient will directly impress the transient voltage onto

the LTM8003 GND, as shown in Figure 2, so fast load

transients can disrupt the LTM8003’s operation or even

cause damage.

FAST VIN

TRANSIENT

OUTPUT EXPERIENCES

A POSITIVE TRANSIENT

LTM8003

VOUT

VIN

8003 F03

GND

COUT

CIN

OPTIONAL

SCHOTTKY

DIODE

AC DIVIDER

VIN

Figure3. A Schottky Diode Can Limit the Transient Caused by

a Fast Rising VIN to Safe Levels

The CIN and COUT capacitors in Figure 3 form an AC divider

at the negative output voltage node. If VIN is hot-plugged

or rises quickly, the resultant VOUT will be a positive tran-

sient, which may be unhealthy for the application load.

An anti-parallel Schottky diode may be able to prevent

this positive transient from damaging the load. The loca-

tion of this Schottky diode is important. For example, in

a system where the LTM8003 is far away from the load,

placing the Schottky diode closest to the most sensitive

load component may be the best design choice. Care-

fully evaluate whether the negative buck configuration is

suitable for the application. When generating a negative

output voltage, tie BIAS to LTM8003 GND.

Shorted Input Protection

Care needs to be taken in systems where the output is held

high when the input to the LTM8003 is absent. This may

occur in battery charging applications or in battery backup

systems where a battery or some other supply is diode

ORed with the LTM8003’s output. If the VIN pin is allowed

tofloatandtheRUNpinisheldhigh(eitherbyalogicsignal

or because it is tied to VIN), then the LTM8003’s internal

circuitry pulls its quiescent current through its internal

power switch. This is fine if your system can tolerate a

few milliamps in this state. If you ground the RUN pin, the

internal current drops to essentially zero. However, if the

VIN pin is grounded while the output is held high, parasitic

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