Datasheet

LT3570
13
3570fb
into a tight local loop, minimizing EMI. The input capaci-
tor must have low impedance at the switching frequency
to do this effectively and it must have an adequate ripple
current rating. The RMS input current is:
I
IN2(RMS)
=I
OUT2
•
V
OUT2
V
IN2
–V
OUT2
()
V
IN2
<
I
OUT2
2
and is largest when V
IN2
= 2 • V
OUT2
(50% duty cycle).
Considering that the maximum load current is ~1.5A, RMS
ripple current will always be less than 0.75A.
The high frequency of the LT3570 reduces the energy
storage requirements of the input capacitor, so that the
capacitance required is often less than 10µF. The combi-
nation of small size and low impedance (low equivalent
series resistance or ESR) of ceramic capacitors makes
them the preferred choice. The low ESR results in very
low voltage ripple. Ceramic capacitors can handle larger
mag nitudes of ripple current than other capacitor types
of the same value. Use X5R and X7R types.
An alternative to a high value ceramic capacitor is a lower
value along with a larger electrolytic capacitor, for ex ample
a 1µF ceramic capacitor in parallel with a low ESR tantalum
capacitor. For the electrolytic capacitor, a value larger than
10µF will be required to meet the ESR and ripple current
requirements. Because the input capacitor is likely to see
high surge currents when the input source is applied,
tantalum capacitors should be surge rated. The manu-
facturer may also recommend operation below the rated
voltage of the capacitor. Be sure to place the 1µF ceramic
as close as possible to the V
IN2
and GND pins on the IC
for optimal noise immunity.
A fi nal caution is in order regarding the use of ceramic
capacitors at the input. A ceramic input capacitor can
combine with stray inductance to form a resonant tank
circuit. If power is applied quickly (for example by plug ging
the circuit into a live power source), this tank can ring,
doubling the input voltage and damaging the LT3570. The
solution is to either clamp the input voltage or dampen the
tank circuit by adding a lossy capacitor in parallel with the
ceramic capacitor. For details, see Application Note 88.
Boost Input Capacitor Selection
The capacitor of a boost converter is less critical due to
the fact that the input current waveform is triangular and
does not contain large squarewave currents as found in
the output capacitor. Capacitors in the range of 10µF to
100µF with an ESR of 0.3Ω or less work well up to the
full 1.5A switch current. Higher ESR capacitors may be
acceptable at low switch currents. Input capacitor ripple
current for boost converters is:
I
RIPPLE
= 0.3 • V
IN1
•
V
OUT1
–V
IN1
f•L•V
OUT1
Buck Diode Selection
The catch diode (D2 from Figure 1) conducts current only
during switch-off time. Average forward current in normal
operation can be calculated from:
I
D(AVG)
=I
OUT1
•
V
IN1
–V
OUT1
V
IN1
The only reason to consider a diode with a larger current
rating than necessary for nominal operation is for the
worst-case condition of shorted output. The diode current
will then increase to the typical peak switch current.
Peak reverse voltage is equal to the regulator input volt age.
Use a diode with a reverse voltage rating greater than the
input voltage. Table 3 lists several Schottky diodes and
their manufacturers.
Table 3. Schottky Diodes
PART NUMBER V
R
(V) I
AVE
(A) V
F
AT 1A (mV)
On Semiconductor
MBRM120E 20 1 530
MBRM140 40 1 550
Diodes Inc.
B120 20 1 500
B130 30 1 500
International Rectifi er
10BQ030 30 1 420
APPLICATIONS INFORMATION