ltc3440 Linear Technology Corporation, ltc3440 Datasheet - Page 14

• Manufacturer Part Number: ltc3440
• Description: Micropower Synchronous Buck-boost Dc/dc Converter
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3440
14
Short-Circuit Improvements
The LTC3440 is current limited to 2.7A peak to protect the
IC from damage. At input voltages above 4.5V a current
limit condition may produce undesirable voltages to the IC
due to the series inductance of the package, as well as the
Figure 8. Error Amplifier with Type III Compensation
Figure 7. Error Amplifier with Type I Compensation
f
Which is extremely close to DC
f
f
f
POLE
POLE
ZERO
ZERO
ERROR
AMP
1
2
2
1
≈
=
=
=
+
–
ERROR
2
2
AMP
2
2
U
• •
• •
• •
• •
π
π
+
π
π
–
FB
V
10
9
C
1.22V
32
R
R
R
1
1
U
1
Z
1
Z
e
FB
V
10
1
3
•
9
•
•
C
C
1.22V
C
C
R
•
Z
P
Z
R
P
C
1
1
2
1
P2
Hz
C
Hz
•
P1
C
Hz
W
P1
C
P
1
V
3440 F08
Hz
OUT
V
OUT
R1
R2
3440 F07
R1
R2
U
C
Z1
traces and external components. Following the recom-
mendations for output voltage >4.3V and input voltage
>4.5V will improve this condition. Additional short-circuit
protection can be accomplished with some external cir-
cuitry.
In an overload or short-circuit condition the LTC3440
voltage loop opens and the error amp control voltage on
the V
forces boost mode operation in order to attempt to provide
more output voltage and the IC hits a peak switch current
limit of 2.7A. When switch current limit is reached switches
B and D turn on for the remainder of the cycle to reverse
the volts • seconds on the inductor. Although this prevents
current run away, this condition produces four switch
operation producing a current foldback characteristic and
the average input current drops. The IC is trimmed to
guarantee greater than 1A average input current to meet
the maximum load demand, but in a short-circuit or
overload condition the foldback characteristic will occur
producing higher peak switch currents. To minimize this
affect during this condition the following circuits can be
utilized.
Restart Circuit
For a sustained short-circuit the circuit in Figure 9 will
force a soft-start condition. The only design constraint is
that R2/C2 time constant must be longer than the soft-
start components R1/C1 to ensure start-up.
Figure 9. Soft-Start Reset Circuitry for a Sustained Short-Circuit
C
SOFT-START
pin slams to the upper clamp level. This condition
SO/SS
4.7nF
C1
V
IN
R1
1M
VN2222
NMOS
M2
10nF
1N4148
C2
D1
VN2222
NMOS
3440 F09
M1
R2
1M
V
OUT
3440fb