LT1766/LT1766-5
16
1766fc
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 LT1766 and force the switching
current into a tight local loop, thereby minimizing EMI.
The RMS ripple current can be calculated from:
IIVVVV
RIPPLE RMS
OUT OUT IN OUT IN
()
=
()
–/
2
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 2.2μF
to 20μF are suitable for most applications. If operation is
required close to the minimum input required by the output
of the LT1766, 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 LT1766 circuit is powered up you
may need to check for input voltage transients.
The input voltage transients may be caused by input volt-
age steps or by connecting the LT1766 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 LT1766.
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 criti-
cally 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 0.5Ω 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.
CATCH DIODE
Highest effi ciency 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 signifi cant 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 generally
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 attempt to
get it to recover. Then, when the diode has fi nally turned
off, some tens of nanoseconds later, the V
SW
node volt-
age ramps up at an extremely high dV/dt, perhaps 5 to
even 10V/ns! With real world lead inductances, the V
SW
node can easily overshoot the V
IN
rail. This can result in
poor RFI behavior and if the overshoot is severe enough,
damage the IC itself.
The suggested catch diode (D1) is an International Rectifi er
10MQ060N Schottky. It is rated at 1.5A average forward
current and 60V reverse voltage. Typical forward voltage
is 0.63V at 1A. The diode conducts current only during
switch off time. Peak reverse voltage is equal to regulator
input voltage. Average forward current in normal operation
can be calculated from:
I
IVV
V
DAVG
OUT IN OUT
IN
()
–
=
()
This formula will not yield values higher than 1.5A with
maximum load current of 1.5A. The only reason to
consider a larger diode is the worst-case condition of a
high input voltage and shorted output. With a shorted
condition, diode current will increase to a typical value
of 2A, determined by peak switch current limit. This is
safe for short periods of time, but it would be prudent to
check with the diode manufacturer if continuous operation
under these conditions must be tolerated.
APPLICATIONS INFORMATION
