LT1766 Linear Technology, LT1766 Datasheet - Page 14

• Manufacturer Part Number: LT1766
• Description: 5.5V to 60V 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT1766/LT1766-5
APPLICATIO S I FOR ATIO
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 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
2.2 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 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
voltage 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 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
14
I
RIPPLE RMS
I
OUT
U
V
U
OUT
V
IN
–
W
V
OUT
/
V
IN
U
2
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
attempt to get it to recover. Then, when the diode has
finally turned off, some tens of nanoseconds later, the V
node voltage ramps up at an extremely high dV/dt, per-
haps 5 to even 10V/ns ! With real world lead inductances,
the V
result in poor RFI behavior and if the overshoot is severe
enough, damage the IC itself.
The suggested catch diode (D1) is an International Recti-
fier 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:
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.
I
D AVG
(
SW
node can easily overshoot the V
)
I
OUT
V
IN
V
IN
–
V
OUT
IN
current into the diode in an
IN
rail. This can
1766fa
SW