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LTC1871

1871fe

APPLICATIONS INFORMATION

Programming the Operating Frequency

The choice of operating frequency and inductor value is

a tradeoff between efﬁciency and component size. Low

frequency operation improves efﬁciency by reducing

MOSFET and diode switching losses. However, lower

frequency operation requires more inductance for a given

amount of load current.

The LTC1871 uses a constant frequency architecture that

can be programmed over a 50kHz to 1000kHz range with

a single external resistor from the FREQ pin to ground, as

shown in Figure 1. The nominal voltage on the FREQ pin is

0.6V, and the current that ﬂows into the FREQ pin is used

to charge and discharge an internal oscillator capacitor. A

graph for selecting the value of RT for a given operating

frequency is shown in Figure 6.

INTVCC Regulator Bypassing and Operation

An internal, P-channel low dropout voltage regulator pro-

duces the 5.2V supply which powers the gate driver and

logic circuitry within the LTC1871, as shown in Figure 7.

The INTVCC regulator can supply up to 50mA and must be

bypassed to ground immediately adjacent to the IC pins

with a minimum of 4.7μF tantalum or ceramic capacitor.

Good bypassing is necessary to supply the high transient

currents required by the MOSFET gate driver.

For input voltages that don’t exceed 7V (the absolute

maximum rating for this pin), the internal low dropout

regulator in the LTC1871 is redundant and the INTVCC pin

can be shorted directly to the VIN pin. With the INTVCC

pin shorted to VIN, however, the divider that programs the

regulated INTVCC voltage will draw 10μA of current from

the input supply, even in shutdown mode. For applications

that require the lowest shutdown mode input supply cur-

rent, do not connect the INTVCC pin to VIN. Regardless of

whether the INTVCC pin is shorted to VIN or not, it is always

necessary to have the driver circuitry bypassed with a

4.7μF tantalum or low ESR ceramic capacitor to ground

immediately adjacent to the INTVCC and GND pins.

In an actual application, most of the IC supply current is

used to drive the gate capacitance of the power MOSFET.

As a result, high input voltage applications in which a

large power MOSFET is being driven at high frequencies

can cause the LTC1871 to exceed its maximum junction

Figure 6. Timing Resistor (RT) Value

FREQUENCY (kHz)

100

300

1000

1871 F06

100

200

1000

900

800

700

600

500

400

Figure 7. Bypassing the LDO Regulator and Gate Driver Supply

1.230V

P-CH

5.2V

DRIVER

GATE

CVCC

4.7μF

CIN

INPUT

SUPPLY

2.5V TO 30V

GND

PLACE AS CLOSE AS

POSSIBLE TO DEVICE PINS

1871 F07

INTVCC

VIN

GND

LOGIC

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