LTC1772BES6 LINER [Linear Technology], LTC1772BES6 Datasheet - Page 5

• Manufacturer Part Number: LTC1772BES6
• Description: Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23
• 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 external P-channel power
MOSFET and keeping it off until the fault is removed.
Low Load Current Operation
Under very light load current conditions, the I
voltage will be very close to the zero current level of 0.85V.
As the load current decreases further, an internal offset at
the current comparator input will assure that the current
comparator remains tripped (even at zero load current)
and the regulator will start to skip cycles, as it must, in
order to maintain regulation. This behavior allows the
regulator to maintain constant frequency down to very
light loads, resulting in less low frequency noise genera-
tion over a wide load current range.
Figure 2 illustrates this result for the circuit of Figure 1
using both an LTC1772 in Burst Mode operation and an
LTC1772B (non-Burst Mode operation). At an output
current of 100mA, the Burst Mode operation part exhibits
an output ripple of approximately 60mV
non-Burst Mode operation part has an output ripple of
only 20mV
ment is even greater. This comes at a tradeoff of lower
efficiency for the non-Burst Mode operation part (see
Figure 1). Also notice the constant frequency operation of
the LTC1772B, even at 5% of maximum output current.
20mV
AC
/DIV
P-P
. At lower output current levels, the improve-
V
V
I
OUT
V
LTC1772 Burst Mode Operation.
IN
OUT
OUT
= 3.6V
= 100mA
= 2.5V
U
Ripple for Figure 1 Circuit Using
(Refer to Functional Diagram)
5 s/DIV
Figure 2. Output Ripple Waveforms for the Circuit of Figure 1.
P-P
, whereas the
TH
1772 F02a
/RUN pin
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 external 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 LTC1772B. 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.
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator will be reduced to about 120kHz. 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.
20mV
AC
/DIV
V
V
I
OUT
V
LTC1772B Non-Burst Mode Operation.
IN
OUT
OUT
= 3.6V
= 100mA
= 2.5V
Ripple for Figure 1 Circuit Using
5 s/DIV
LTC1772B
1772 F02b
5