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LTC1624

APPLICATIONS INFORMATION

Positive-to-Negative Converter: Output Voltage

Programming

Setting the output voltage for a positive-to-negative con-

verter is different from other architectures since the feed-

back voltage is referenced to the LTC1624 ground pin and

the ground pin is referenced to – VOUT. The output voltage

is set by a resistive divider according to the following

formula:

OUT









≈−

−









119

The external resistive divider is connected to the output as

shown in Figure 8.

Positive-to-Negative Converter: Power

MOSFET Selection

One external N-channel power MOSFET must be selected

for use with the LTC1624 for the switch. As in step-down

applications the source of the power MOSFET is con-

nected to the Schottky diode and inductor. The peak-to-

peak gate drive levels are set by the INTVCC voltage. The

gate drive voltage is equal to approximately 5V for VIN >

5.6V and a logic level MOSFET can be used. At VIN voltages

below 5V the INTVCC voltage is equal to VIN – 0.6V and a

sublogic level MOSFET should be used.

Selection criteria for the power MOSFET include the “ON”

resistance RDS(ON), reverse transfer capacitance CRSS,

input voltage and maximum output current. When the

LTC1624 is operating in continuous mode the duty cycle

for the MOSFET is given by:

Main

OUT

Switch Duty Cycle =

OUT

with

VOUT being the absolute value of VOUT.

The MOSFET power dissipation and maximum switch

current are given by:

that the voltage across C1 is constant such that VC1 = VIN

at full load over the entire VIN range. Assuming the enegry

storage in the coupling capacitor C1 must be equal to the

enegry stored in L1, the minimum capacitance of C1 is:

MIN

OUT

IN MIN

()

= () (

)

SEPIC Converter: Duty Cycle Limitations

The minimum on-time of 450ns sets a limit on how high

an input-to-output ratio can be tolerated while not skip-

ping cycles. This only impacts designs when very low

output voltages (VOUT < 2.5V) are needed. Note that a

SEPIC converter would not be appropriate at these low

output voltages. The maximum input voltage is (remem-

ber not to exceed the absolute maximum limit of 36V):

VIN(MAX) = 10.1VOUT + 5V

For DC > 9%

Positive-to-Negative Converter Applications

The LTC1624 can also be used as a positive-to-negative

converter with a grounded inductor shown in Figure 8.

Since the LTC1624 requires a positive feedback signal

relative to device ground, Pin 4 must be tied to the

regulated negative output. A resistive divider from the

negative output to ground sets the output voltage.

Remember not to exceed maximum VIN ratings VIN +

VOUT≤ 36V.

PMAIN = ISW MAX ×

I OUT MAX I + δ RDS ON +

k V IN MAX + VOUT

1.85 C

RSS 200kHz

()

( )

()

()(

)

()

{

Figure 8. Positive-to-Negative Converter

SENSE–

ITH/RUN

VFB

GND

VIN

BOOST

LTC1624

1000pF

100pF

COUT

RSENSE

CIN

–VOUT

VIN

1624 F08

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