5

LTC1872

OPERATIO

(Refer to Functional Diagram)

Comparator OVP guards against transient overshoots

> 7.5% by turning off the external N-channel power

MOSFET and keeping it off until the fault is removed.

Burst Mode Operation

The LTC1872 enters Burst Mode operation at low load

currents. In this mode, the peak current of the inductor is

inductor’s average current is greater than the load require-

high, turning off the external MOSFET. The sleep signal

and the LTC1872 resumes normal operation. The next

oscillator cycle will turn the external MOSFET on and the

switching cycle repeats.

Undervoltage Lockout

To prevent operation of the N-channel MOSFET below safe

input voltage levels, an undervoltage lockout is incorpo-

rated into the LTC1872. When the input supply voltage

drops below approximately 2.0V, the N-channel MOSFET

and all circuitry is turned off except the undervoltage

block, which draws only several microamperes.

Figure 2. Maximum Output Current vs Duty Cycle

DUTY CYCLE (%)

110

100

90

80

70

60

50

40

30

20

10

1872 F02

0

70 80 90 100

60

10

20 30

40 50

AT 5% DUTY CYCLE

AT 5% DUTY CYCLE

Overvoltage Protection

The overvoltage comparator in the LTC1872 will turn the

external MOSFET off when the feedback voltage has risen

7.5% above the reference voltage of 0.8V. This compara-

tor has a typical hysteresis of 20mV.

Slope Compensation and Inductor’s Peak Current

The inductor’s peak current is determined by:

I

V

R

PK

ITH

SENSE

=

−

()

07

10

.

when the LTC1872 is operating below 40% duty cycle.

However, once the duty cycle exceeds 40%, slope com-

pensation begins and effectively reduces the peak induc-

tor current. The amount of reduction is given by the curves

in Figure 2.

Short-Circuit Protection

Since the power switch in a boost converter is not in series

with the power path from input to load, turning off the

switch provides no protection from a short-circuit at the

output. External means such as a fuse in series with the

boost inductor must be employed to handle this fault

condition.