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LT3570 Datasheet(PDF) 13 Page - Linear Technology

Part No. LT3570
Description  1.5A Buck Converter, 1.5A Boost Converter and LDO Controller
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT3570 Datasheet(HTML) 13 Page - Linear Technology

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LT3570
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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:
IIN2(RMS) =IOUT2
VOUT2 VIN2 –VOUT2
()
VIN2
<
IOUT2
2
and is largest when VIN2 = 2 • VOUT2 (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
magnitudes 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 example
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 VIN2 and GND pins on the IC
for optimal noise immunity.
A final 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 plugging
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:
IRIPPLE = 0.3 • VIN1
VOUT1 –VIN1
f•L • VOUT1
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:
ID(AVG) =IOUT1
VIN1 –VOUT1
VIN1
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 voltage.
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
VR (V)
IAVE (A)
VF AT 1A (mV)
On Semiconductor
MBRM120E
20
1
530
MBRM140
40
1
550
Diodes Inc.
B120
20
1
500
B130
30
1
500
International Rectifier
10BQ030
30
1
420
APPLICATIONS INFORMATION


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