LTC1779 LINER [Linear Technology], LTC1779 Datasheet - Page 5

• Manufacturer Part Number: LTC1779
• Description: 250mA Current Mode Step-Down DC/DC Converter in ThinSOT
• Manufacturer: LINER [Linear Technology]

OPERATIO
up, the corresponding output current trip level follows,
allowing normal operation.
Comparator OVP guards against transient overshoots
> 7.5% by turning off the internal P-channel power
MOSFET and keeping it off until the fault is removed.
Burst Mode Operation
The LTC1779 enters Burst Mode operation at low load
currents. In this mode, the peak current of the inductor is
set as if V
the voltage at the I
inductor’s average current is greater than the load require-
ment, the voltage at the I
I
high, turning off the internal MOSFET. The sleep signal
goes low when the I
and the LTC1779 resumes normal operation. The next
oscillator cycle will turn the internal MOSFET on and the
switching cycle repeats.
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 internal 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 EAMP. Further reduc-
tion 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 LTC1779. When the input supply voltage
drops below approximately 2.0V, the P-channel MOSFET
and all circuitry is turned off except the undervoltage
block, which draws only several microamperes.
TH
/RUN voltage goes below 0.85V, the sleep signal goes
ITH
/RUN = 1V (at low duty cycles) even though
U
TH
TH
/RUN pin is at a lower value. If the
(Refer to Functional Diagram)
/RUN voltage goes above 0.925V
TH
/RUN pin will drop. When the
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator will be reduced to about 100kHz. This lower
frequency allows the inductor current to safely discharge,
thereby preventing current runaway. The oscillator’s fre-
quency will gradually increase to its designed rate when
the feedback voltage again approaches 0.8V.
Overvoltage Protection
As a further protection, the overvoltage comparator in the
LTC1779 will turn the internal MOSFET off when the
feedback voltage has risen 7.5% above the reference
voltage of 0.8V. This comparator has a typical hysteresis
of 30mV.
Slope Compensation and Inductor’s Peak Current
The inductor’s peak current is determined by:
when the LTC1779 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.
I
PK
Figure 2. Maximum Output Current vs Duty Cycle
M V
10
110
100
(
90
80
70
60
50
40
30
20
10
R
0
ITH RUN
SENSE
V
10 20 30 40 50
IN
/
= 4.2V
I
AT 5% DUTY CYCLE
I
AT 5% DUTY CYCLE
RIPPLE
RIPPLE
– . )
= 0.4I
= 0.2I
2
DUTY CYCLE (%)
0 7
PK
PK
60
70 80 90 100
1779 F02
LTC1779
5