Datasheet

5
LTC1872
OPERATIO
U
(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
set as if V
ITH
/RUN = 1V (at low duty cycles) even though
the voltage at the I
TH
/RUN pin is at a lower value. If the
inductor’s average current is greater than the load require-
ment, the voltage at the I
TH
/RUN pin will drop. When the
I
TH
/RUN voltage goes below 0.85V, the sleep signal goes
high, turning off the external MOSFET. The sleep signal
goes low when the I
TH
/RUN voltage goes above 0.925V
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
SF = I
OUT
/I
OUT(MAX)
(%)
1872 F02
0 70 80 90 1006010 20 30 40 50
I
RIPPLE
= 0.4I
PK
AT 5% DUTY CYCLE
I
RIPPLE
= 0.2I
PK
AT 5% DUTY CYCLE
V
IN
= 4.2V
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.