LT3437 LINER [Linear Technology], LT3437 Datasheet - Page 10

• Manufacturer Part Number: LT3437
• Description: High Voltage 500mA, 200kHz Step-Down Switching Regulator with 100uA Quiescent Current
• Manufacturer: LINER [Linear Technology]

LT3437
APPLICATIO S I FOR ATIO
10
function reduces input current surge by regulating switch
current via the V
rate (dV/dt) at the output. A capacitor (C1 in Figure 2) from
the C
dV/dt. When the feedback voltage is below 0.4V, the V
will rise, resulting in an increase in switch current and
output voltage. If the dV/dt of the output causes the current
through the C
reduced resulting in a constant dV/dt at the output. As the
feedback voltage increases, I
increased dV/dt until the soft-start function is defeated
with 0.9V present at the FB pin. The soft-start function
does not affect operation during normal load conditions.
However, if a momentary short (brown out condition) is
present at the output which causes the FB voltage to drop
below 0.9V, the soft-start circuitry will become active.
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 LT3437 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 input capacitor values in the range of
2.2µF to 10µF are suitable for most applications. If opera-
tion is required close to the minimum input required by the
LT3437, a larger value may be required. This is to prevent
excessive ripple causing dips below the minimum operat-
ing voltage resulting in erratic operation.
Input voltage transients caused by input voltage steps, or
by hot plugging the LT3437 to a pre-powered source such
as a wall adapter, can exceed maximum V
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
I
RIPPLE RMS
SS
pin to the output determines the maximum output
(
SS
)
C
capacitor to exceed I
=
pin to maintain a constant voltage ramp
I
OUT
V
U
IN
V
U
OUT
CSS
(
V
increases, resulting in an
IN
W
–
CSS
V
OUT
, the V
)
IN
ratings. The
C
U
voltage is
C
pin
cause the input voltage to swing above the DC level of input
power source and it may exceed the maximum voltage
rating of the input capacitor and LT3437. All input voltage
transient sequences should be observed at the V
the LT3437 to ensure that absolute maximum voltage
ratings are not violated.
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 to critically damp the
resonant circuit formed by the input lead inductance and
the input capacitor. The typical values of ESR will fall in the
range of 0.5Ω to 2Ω and capacitance will fall in the range
of 5µF to 50µF.
If tantalum capacitors are used, values in the 22µF to
470µF 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.
OUTPUT CAPACITOR
The output capacitor is normally chosen by its effective
series resistance (ESR) because this is what determines
output ripple voltage. To get low ESR takes volume, so
physically smaller capacitors have higher ESR. The ESR
range for typical LT3437 applications is 0.05Ω to 0.2Ω. A
typical output capacitor is an AVX type TPS, 100µF at 10V,
with a guaranteed ESR less than 0.1Ω. This is a “D” size
surface mount solid tantalum capacitor. TPS capacitors
are specially constructed and tested for low ESR, so they
give the lowest ESR for a given volume. The value in
microfarads is not particularly critical, and values from
22µF to greater than 500µF work well, but you cannot
cheat Mother Nature on ESR. If you find a tiny 22µF solid
tantalum capacitor, it will have high ESR and output ripple
voltage could be unacceptable. Table 2 shows some
typical solid tantalum surface mount capacitors.
IN
pin of
3437f