Datasheet
14
LTC3440
3440fb
APPLICATIO S I FOR ATIO
WUUU
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
1.22V
R1
R2
3440 F08
FB
9
V
C
C
P1
C
Z1
R
Z
V
OUT
10
C
P2
–
+
ERROR
AMP
Figure 7. Error Amplifier with Type I Compensation
1.22V
R1
R2
3440 F07
FB
9
V
C
C
P1
V
OUT
10
–
+
ERROR
AMP
f
eRC
Hz
Which is extremely close to DC
f
RC
Hz
f
RC
Hz
f
RC
Hz
POLE
P
ZERO
ZP
ZERO
Z
POLE
ZP
1
3
1
1
1
2
1
2
2
1
232 1
1
2
1
21
1
2
≈
π
=
π
=
π
=
π
•• • •
•• •
•• •
•• •
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
C
pin slams to the upper clamp level. This condition
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
C2
10nF
C1
4.7nF
R2
1M
R1
1M
V
OUT
V
IN
SOFT-START
SO/SS
M2
NMOS
VN2222
M1
NMOS
VN2222
D1
1N4148
3440 F09