LTC1872 datasheet(13/36 Pages) LINER

13 / 36 page

LTC1871

1871fe

APPLICATIONS INFORMATION

Figure 8a. Programming the Turn-On and Turn-Off Thresholds Using the RUN Pin

Boost Converter: Duty Cycle Considerations

For a boost converter operating in a continuous conduction

mode (CCM), the duty cycle of the main switch is:

VO + VD –VIN

VO + VD

where VD is the forward voltage of the boost diode. For

converters where the input voltage is close to the output

voltage, the duty cycle is low and for converters that develop

a high output voltage from a low voltage input supply,

the duty cycle is high. The maximum output voltage for a

boost converter operating in CCM is:

VO(MAX) =

VIN(MIN)

1– DMAX

(

)

–VD

The maximum duty cycle capability of the LTC1871 is

typically 92%. This allows the user to obtain high output

voltages from low input supply voltages.

Boost Converter: The Peak and Average Input Currents

The control circuit in the LTC1871 is measuring the input

current (either by using the RDS(ON) of the power MOSFET

or by using a sense resistor in the MOSFET source), so

the output current needs to be reﬂected back to the input

in order to dimension the power MOSFET properly. Based

on the fact that, ideally, the output power is equal to the

input power, the maximum average input current is:

IIN(MAX) =

IO(MAX)

1– DMAX

The peak input current is:

IIN(PEAK) = 1+

•

IO(MAX)

1– DMAX

The maximum duty cycle, DMAX, should be calculated at

minimum VIN.

Figure 8c. External Pull-Up Resistor On

RUN Pin for “Always On” Operation

Figure 8b. On/Off Control Using External Logic

–

RUN

COMPARATOR

VIN

RUN

INPUT

SUPPLY

OPTIONAL

FILTER

CAPACITOR

–

GND

1871 F8a

BIAS AND

START-UP

CONTROL

1.248V

μPOWER

REFERENCE

–

RUN

COMPARATOR

1.248V

1871 F08b

RUN

EXTERNAL

LOGIC CONTROL

–

RUN

COMPARATOR

VIN

RUN

INPUT

SUPPLY

–

GND

1.248V

1871 F08c

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