LTC3532_15 Datasheet, PDF (10/16 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter

LTC3532
U
OPERATIO
be minimized. Note that if the load current applied during
forced Burst Mode operation (BURST pin is grounded)
exceeds the current that can be supplied, the output voltage
will start to droop and the IC will automatically come out
of Burst Mode operation and enter fixed frequency mode,
raising V_OUT. Once regulation is achieved, the IC will then
enter Burst Mode operation once again, and the cycle will
repeat, resulting in about 4% output ripple. Note that Burst
Mode operation is inhibited during soft-start.
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 of operation, even
at the same load current. To prevent this, the LTC3532
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 3 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 SHDN/SS pin is brought above 1V typical, the IC is
enabled but the EA duty cycle is clamped from V_C. A de-
tailed diagram of this function is shown in Figure 5. The
components R_SS and C_SS provide a slow ramping voltage
on SHDN/SS to provide a soft-start function. To ensure
that V_C is not being clamped, SHDN/SS must be raised
above 2.4V. To enable Burst Mode operation, SHDN/SS
must be raised to within 0.5V of V_IN.
V_IN
SOFT-START
CLAMP
TO PWM
V_CI
COMPARATORS
15μA
ERROR AMP
1.22V
FB
9
V_C ^CP1
10
V_OUT
R1
R2
CHIP
ENABLE
SHDN/SS
8
R_SS
C_SS
ENABLE SIGNAL
3532 F05
1V
Figure 5. Soft-Start Circuitry
3532fc
10