Datasheet

LTC1772B
6
1772bfa
OPERATION
Main Control Loop
The LTC1772B is a constant frequency current mode
switching regulator. During normal operation, the external
P-channel power MOSFET is turned on each cycle when
the oscillator sets the RS latch (RS1) and turned off when
the current comparator (ICMP) resets the latch. The peak
inductor current at which ICMP resets the RS latch is
controlled by the voltage on the I
TH
/RUN pin, which is the
output of the error amplifi er EAMP. An external resistive
divider connected between V
OUT
and ground allows the
EAMP to receive an output feedback voltage V
FB
. When
the load current increases, it causes a slight decrease in
V
FB
relative to the 0.8V reference, which in turn causes the
I
TH
/RUN voltage to increase until the average inductor
current matches the new load current.
The main control loop is shut down by pulling the I
TH
/RUN
pin low. Releasing I
TH
/RUN allows an internal 0.5μA current
source to charge up the external compensation network.
When the I
TH
/RUN pin reaches 0.35V, the main control
loop is enabled with the I
TH
/RUN voltage then pulled up
to its zero current level of approximately 0.85V. As the
external compensation network continues to charge up,
the corre
sponding output current trip level follows, allow-
ing 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
TH
/RUN pin
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 generation 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 cur-
rent of 100mA, the Burst Mode operation part exhibits
an output ripple of approximately 60mV
P-P
, whereas the
non-Burst Mode operation part has an output ripple of only
20mV
P-P
. At lower output current levels, the improvement
is even greater. This comes at a tradeoff of lower effi ciency
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.
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
Figure 2. Output Ripple Waveforms for the Circuit of Figure 1.
V
OUT
Ripple for Figure 1 Circuit Using
LTC1772 Burst Mode Operation.
V
OUT
Ripple for Figure 1 Circuit Using
LTC1772B Non-Burst Mode Operation.
V
IN
= 3.6V
V
OUT
= 2.5V
I
OUT
= 100mA
5μs/DIV
20mV
AC
/DIV
1772 F02a
V
IN
= 3.6V
V
OUT
= 2.5V
I
OUT
= 100mA
5μs/DIV
20mV
AC
/DIV
1772 F02b
(Refer to Functional Diagram)