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LTM4703

Rev. 0

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true buck regulator, and the maximum output current

depends upon the input voltage. Figure 15 and Figure 16

illustrate how much current the LTM4703 delivers for a

given input voltage.

VIN

LTM4703

VIN

VOUT

GND

NEGATIVE

OUTPUT VOLTAGE

4703 F02

Figure 2. The LTM4703 Can Be Used to Generate a

Negative Voltage

Note that the Figure 2 configuration requires that any

load current transient directly affects the transient voltage

onto the LTM4703 GND, as shown in Figure 3, fast load

transients can disrupt the LTM4703’s operation or even

cause damage. Carefully evaluate whether the negative

buck configuration is suitable for the application.

VIN

LTM4703

VIN

VOUT

GND

FAST LOAD TRANSIENT

OUTPUT TRANSIENT RESPONSE

4703 F03

Figure 3. Any Output Voltage Transient

Appears on LTM4703 GND

The CIN and COUT capacitors in Figure 4 form an AC

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

plugged or rises quickly, the resultant VOUT is a positive

transient, 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 location

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

system where the LTM4703 is far from the load, placing

the Schottky diode closest to the most sensitive load com-

ponent may be the best design choice. Carefully evaluate

whether the negative buck configuration is suitable for the

application. When generating a negative output, connect

BIAS to LTM4703 GND.

CIN

COUT

LTM4703

VIN

VOUT

GND

OPTIONAL

SCHOTTKY

DIODE

AC DIVIDER

VIN

FAST LOAD TRANSIENT

OUTPUT TRANSIENT RESPONSE

4703 F04

Figure 4. A Schottky Diode Can Limit the Transient

Caused by a Fast Rising VIN to Safe Levels

Shorted Input Protection

Care must be taken in systems where the output is held

high when the power input to the LTM4703 is absent. This

may occur in battery charging applications or in battery

backup systems where a battery or some other supply is

diode OR-ed with the LTM4703’s output. If the PVIN pin

is allowed to float and the RUN pin is held high (either

by a logic signal or because it is connected to VIN), then

the LTM4703’s internal circuitry pulls its quiescent cur-

rent 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 essen-

tially zero. However, if the PVIN pin is grounded while the

output is held high, parasitic diodes inside the LTM4703

can pull large currents from the output through the PVIN

pin. Figure 5 shows a circuit that runs only when the

input voltage is present and protects against a shorted

or reversed input.

LTM4703

RUN

VIN

4703 F05

Figure 5. The Input Diode Prevents a Shorted Input from

Discharging a Backup Battery Connect to the Output.

It Also Protects the Circuit from a Reversed Input. The

LTM4703 Only Runs When the Input is Present

APPLICATIONS INFORMATION

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