LTC1622
OPERATIO
the I
TH
pin will drop. When the I
TH
voltage goes below
0.12V, the sleep signal goes high, turning off the external
MOSFET. The sleep signal goes low when the I
TH
voltage
rises above 0.22V and the LTC1622 resumes normal
operation. The next oscillator cycle will turn the external
MOSFET on and the switching cycle repeats.
Frequency Synchronization
The LTC1622 can be externally driven by a TTL/CMOS
compatible clock signal up to 750kHz.
Do not
synchronize
the LTC1622 below its maximum default operating fre-
quency of 625kHz as this may cause abnormal operation
and an undesired frequency spectrum. The LTC1622 is
synchronized to the rising edge of the clock. The external
clock pulse width must be at least 100ns and not more
than the period minus 200ns.
Synchronization is inhibited when the feedback voltage is
below 0.3V. This is to prevent inductor current buildup
under short-circuit conditions. Burst Mode operation is
deactivated when the LTC1622 is externally driven by a
clock.
Dropout Operation
When the input supply voltage decreases towards the
output voltage, the rate of change of inductor current
during the ON cycle decreases. This reduction means that
the P-channel MOSFET will remain on for more than one
oscillator cycle since the inductor current has not ramped
up to the threshold set by EA. Further reduction in input
supply voltage will eventually cause the P-channel MOSFET
to be turned on 100%, i.e., DC. The output voltage will then
be determined by the input voltage minus the voltage drop
across the MOSFET, the sense resistor and the inductor.
Undervoltage Lockout
To prevent operation of the P-channel MOSFET below safe
input voltage levels, an undervoltage lockout is incorpo-
rated into the LTC1622. When the input supply voltage
drops below 2V, the P-channel MOSFET and all circuitry is
turned off except the undervoltage block, which draws
only several microamperes.
SF = I
OUT
/I
OUT(MAX)
(%)
6
U
(Refer to Functional Diagram)
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator will be reduced to about 110kHz. This lower
frequency allows the inductor current to safely discharge,
thereby preventing current runaway. The oscillator’s fre-
quency will gradually increase to its nominal value when
the feedback voltage increases above 0.65V. Note that
synchronization is inhibited until the feedback voltage
goes above 0.3V.
Overvoltage Protection
As a further protection, the overvoltage comparator in the
LTC1622 will turn the external MOSFET off when the
feedback voltage has risen 7.5% above the reference
voltage of 0.8V. This comparator has a typical hysteresis
of 35mV.
Slope Compensation and Peak Inductor Current
The inductor’s peak current is determined by:
I
PK
=
10 R
SENSE
(
V
ITH
)
when the LTC1622 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.
110
100
90
80
70
60
50
40
30
20
10
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
1622 F02
I
RIPPLE
= 0.4I
PK
AT 5% DUTY CYCLE
I
RIPPLE
= 0.2I
PK
AT 5% DUTY CYCLE
V
IN
= 4.2V
UNSYNC
Figure 2. Maximum Output Current vs Duty Cycle
LINER [ LINEAR TECHNOLOGY ]
