LTC3440 Datasheet, PDF (14/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter

LTC3440
APPLICATIO S I FOR ATIO
f_POLE1
≈
2
•
π
1
• 32e³
•R1•
C_P1
Hz
Which is extremely close to DC
f_ZERO1 =
1
2 • π •R_Z
Hz
• C_P1
f_ZERO2
=
1
Hz
2 • π •R1 • C_Z1
f_POLE2
=
1
2 • π •R_Z
• C_P2
Hz
ERROR
AMP
1.22V
FB
9
V_C
C_P1
10
V_OUT
R1
R2
3440 F07
Figure 7. Error Amplifier with Type I Compensation
ERROR
AMP
V_OUT
1.22V
FB
9
R1
C_Z1
V_C
R_Z ^CP1
R2
10
C_P2
3440 F08
Figure 8. Error Amplifier with Type III Compensation
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
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.
V_IN
SOFT-START
SO/SS
C1
4.7nF
R1
1M
M2
NMOS
VN2222
D1
1N4148
C2
10nF
R2
1M
M1
NMOS
VN2222
V_OUT
3440 F09
Figure 9. Soft-Start Reset Circuitry for a Sustained Short-Circuit
3440fb
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