Datasheet

LTC3534
11
3534fb
For more information www.linear.com/LTC3534
In Burst Mode operation the typical maximum average
output currents in the three operating regions, buck, four
switch, and boost are given by:
I
OUT(MAX)BURST–BUCK
≈ 100mA;
Burst Mode operation – buck region: V
IN
> V
OUT
I
OUT(MAX)BURST–FOUR_SWITCH
≈ 125mA;
Burst Mode operation – four switch region: V
IN
≈ V
OUT
I
OUT(MAX)BURST−BOOST
≈
125• V
IN
V
OUT
mA;
Burst Mode operation – boost region: V
IN
< V
OUT
The efficiency below 1mA becomes dominated primarily
by the quiescent current. The Burst Mode operation ef
-
ficiency is given by:
Efficiency ≅
η•I
LOAD
25µA +I
LOAD
where h is typically 90% during Burst Mode operation.
A graph of Burst Mode operation maximum output current
vs V
IN
(for V
OUT
= 5V) is provided in the Typical Perfor-
mance Characteristics section.
Burst Mode Operation to Fixed Frequency Transient
Response
In Burst Mode operation, the compensation network is
not used and V
C
is disconnected from the error amplifier.
During long periods of Burst Mode operation, leakage
currents in the external components or on the PC board
could cause the compensation capacitor to charge (or
discharge), which could result in a large output transient
when returning to fixed frequency mode operation, even
at the same load current. To prevent this, the LTC3534
incorporates an active clamp circuit that holds the voltage
on V
C
at an optimal voltage during Burst Mode operation.
This minimizes any output transient when returning to
fixed frequency mode operation. For optimum transient
response, Type III compensation is also recommended
to broad band the control loop and roll off past the two
pole response of the output LC filter. (See Closing the
Feedback Loop).
Soft-Start
The soft-start function is combined with shutdown. When
the RUN/SS pin is brought above 1V typical, the LTC3534
is
enabled but the error amplifier duty cycle is clamped
from V
C
. A detailed diagram of this function is shown in
Figure 3. The components R
SS
and C
SS
provide a slow
ramping voltage on RUN/SS to provide a soft-start func
-
tion. To ensure that V
C
is not being clamped, RUN/SS
must be raised to 2.4V or above. The IC must be enabled
(even with a soft-start) commanding PWM mode. Once
the LTC3534 is in regulation, then Burst Mode operation
can be commanded.
operaTion
–
+
15
14
13
2
11
V
IN
C
SS
R
SS
RUN/SS
1V
FB
R1
R2
C
P1
V
C
V
CI
V
OUT
1V
TO PWM
COMPARATORS
CHIP
ENABLE
ENABLE
SIGNAL
3534 F03
–
+
ERROR
AMP
Figure 3. Soft-Start Circuitry