Datasheet

LTC3441
9
3441fa
Burst Mode Operation to Fixed Frequency Transient
Response
When transitioning from Burst Mode operation to fixed
frequency, the system exhibits a transient since the modes
of operation have changed. For most systems this tran-
sient is acceptable, but the application may have stringent
input current and/or output voltage requirements that
dictate a broad-band voltage loop to minimize the tran-
sient. Lowering the DC gain of the loop will facilitate the
task (5M from FB to V
C
) at the expense of DC load
regulation. Type 3 compensation is also recommended to
broad band the loop and roll off past the two pole response
of the LC of the converter (see Closing the Feedback Loop).
OPERATIO
U
SOFT-START
The soft-start function is combined with shutdown. When
the SHDN/SS pin is brought above typically 1V, the IC is
enabled but the EA duty cycle is clamped from the V
C
pin.
A detailed diagram of this function is shown in Figure 5.
The components R
SS
and C
SS
provide a slow ramping
voltage on the SHDN/SS pin to provide a soft-start
function.
9
PV
IN
A
4
SW1
6
GND
5
SW2
L
+–
8
V
OUT
D
C
800mA
I
INDUCTOR
0mA
3441 F03
T1
B
dI
dt
V
IN
L
≈
9
PV
IN
A
4
SW1
6
GND
5
SW2
L
–+
8
V
OUT
D
C
800mA
I
INDUCTOR
0mA
3441 F04
T2
B
dI
dt
V
OUT
L
≈ –
–
+
12
11
V
IN
ERROR AMP
1.22V
14μA
FB
R1
R2
C
P1
V
C
V
OUT
1
SHDN/SS
C
SS
1V
ENABLE SIGNAL
R
SS
SOFT-START
CLAMP
TO PWM
COMPARATORS
CHIP
ENABLE
3441 F05
–
+
V
CI
Figure 3. Inductor Charge Cycle During Burst Mode Operation Figure 4. Inductor Discharge Cycle During Burst Mode Operation
Figure 5. Soft-Start Circuitry