LT3430 Linear Technology, LT3430 Datasheet - Page 14
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LT3430
Manufacturer Part Number
LT3430
Description
High Voltage/ 3A/ 200kHz Step-Down Switching Regulator
Manufacturer
Linear Technology
Datasheet
1.LT3430.pdf
(28 pages)
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LT3430
APPLICATIO S I FOR ATIO
the switching frequency. If the gain of the error amplifier
is high enough at the switching frequency, output ripple
voltage (although smaller for a ceramic output capacitor)
may still affect the proper operation of the regulator. A
filter capacitor C
suggested to control possible ripple at the V
Ceramic” solution is possible for the LT3430 by choosing
the correct compensation components for the given
application.
Example: For V
can be stabilized, provide good transient response and
maintain very low output ripple voltage using the follow-
ing component values: (refer to the first page of this data
sheet for component references) C3 = 4.7 F, R
C
Note 19 for further detail on techniques for proper loop
compensation.
INPUT CAPACITOR
Step-down regulators draw current from the input supply
in pulses. The rise and fall times of these pulses are very
fast. The input capacitor is required to reduce the voltage
ripple this causes at the input of LT3430 and force the
switching current into a tight local loop, thereby minimiz-
ing EMI. The RMS ripple current can be calculated from:
Ceramic capacitors are ideal for input bypassing. At 200kHz
switching frequency, the energy storage requirement of
the input capacitor suggests that values in the range of
4.7 F to 20 F are suitable for most applications. If opera-
tion is required close to the minimum input required by the
output of the LT3430, a larger value may be required. This
is to prevent excessive ripple causing dips below the
minimum operating voltage resulting in erratic operation.
Depending on how the LT3430 circuit is powered up you
may need to check for input voltage transients.
14
C
I
= 22nF, C
RIPPLE RMS
F
= 220pF and C1 = 100 F. See Application
IN
= 8V to 40V, V
F
I
in parallel with the R
OUT
U
V
U
OUT
OUT
V
IN
= 5V at 2A, the LT3430
–
W
V
OUT
C
/C
/
C
C
V
pin. An “All
IN
network is
U
2
C
= 3.3k,
The input voltage transients may be caused by input
voltage steps or by connecting the LT3430 converter to an
already powered up source such as a wall adapter. The
sudden application of input voltage will cause a large
surge of current in the input leads that will store energy in
the parasitic inductance of the leads. This energy will
cause the input voltage to swing above the DC level of input
power source and it may exceed the maximum voltage
rating of input capacitor and LT3430.
The easiest way to suppress input voltage transients is to
add a small aluminum electrolytic capacitor in parallel with
the low ESR input capacitor. The selected capacitor needs
to have the right amount of ESR in order to critically
dampen the resonant circuit formed by the input lead
inductance and the input capacitor. The typical values of
ESR will fall in the range of 0.5 to 2 and capacitance will
fall in the range of 5 F to 50 F.
If tantalum capacitors are used, values in the 22 F to
470 F range are generally needed to minimize ESR and
meet ripple current and surge ratings. Care should be
taken to ensure the ripple and surge ratings are not
exceeded. The AVX TPS and Kemet T495 series are surge
rated. AVX recommends derating capacitor operating
voltage by 2:1 for high surge applications.
CATCH DIODE
Highest efficiency operation requires the use of a Schottky
type diode. DC switching losses are minimized due to its
low forward voltage drop, and AC behavior is benign due
to its lack of a significant reverse recovery time. Schottky
diodes are generally available with reverse voltage ratings
of up to 60V and even 100V, and are price competitive with
other types.
The use of so-called “ultrafast” recovery diodes is gener-
ally not recommended. When operating in continuous
mode, the reverse recovery time exhibited by “ultrafast”
diodes will result in a slingshot type effect. The power
internal switch will ramp up V
IN
current into the diode in an
sn3430 3430is