XRP7665 Datasheet, PDF (9/12 Pages) Exar Corporation – 3A 18V Synchronous Step-Down Converter

XRP7 6 6 5
3 A 1 8 V Sy n ch r o n o u s St ep - Do w n Co n v er t er
If tantalum or electrolytic capacitors are used
WKHQV_O_UTÆ©is essentially a function of ESR:
߂ܸ_{ை௎்}ൌ߂ܫ_à¯àµˆÜ´ÜµÜ§
Circuit configuration is shown in figures 16 and
17. The external bootstrap diode is also
recommended where dut y cycle (V _OUT/V _IN ) is
larger than 65%.
INPUT CAPACITOR C_IN
Select the input capacitor for voltage rating,
RMS current rating and capacitance. The
voltage rating should be at least 50% higher
1N4148
than the regulator ¶Vmaximum input voltage.
V_IN = 5V
&DOFXODWHWKHFDSDFLWRU¶VFXUUHQWUD^IW^N LQJIU^BSRP
ܫ_{஼ூǡோேெௌ}ൌܫ_{ை௎்}ൈඥܦൈሺͳെܦሻ
XRP7 6 6 5
10nF
Where:
SW
I _OUT LVUHJXODWRU¶VPD3[AL) PXPFXUUHQW
D is duty cycle (D=V _OUT/V _IN )
Fig. 16 : Optional external bootstrap diode
where input voltage is fixed at 5V
Calculate the C _IN capacitance from:
ܥ_ூேൌ^{ܫை௎்ൈ݂}_௦^{ܸൈை௎}_{ܸூ்}^ൈ_ேଶ^{ሺൈܸூ}_߂^ே_{ܸെூே}^{ܸை௎்ሻ}
Where:
Æ©9_IN is the permissible input voltage ripple,
nominally set at 1% of V _IN
OPTIONAL SCHOTTKY DIODE
An optional Schottky diode may be paralleled
between the GND pin and S W pin to improve
the regulator efficiency. Examples are shown
in Table 3.
Part Number
B130
SK13
MBRS130
Voltage/Current
Rating
30V/1A
30V/1A
30V/1A
Vend or
Diodes, Inc.
Diodes, Inc.
International
Rectifier
Table 3. Optional Schottky diode
EXTERNAL BOOTSTRAP DIODE
A low - cost diode , such as 1N4148 , is
recommended for higher efficiency when
the
input voltage is 5V or the output is 5V or 3.3V.
BS
XRP7 6 6 5
SW
1N4148
10nF
V_OUT = 5V
or 3.3V
C_OUT
Fig. 17 : Optional external bootstrap diode
where output voltage is 5V or 3.3V
LOOP COMPENSATION
XRP7665 utilizes current - mode control . This
allows using a minimum of external
components to compensate the regulator. In
general only two components are needed: RC
and CC. Proper compens ation of the regulator
(determining RC and CC) results in optimum
transient response. In terms of power supply
control theory, t he goals of compensation are
to choose RC and CC such that the regulator
loop gain has a crossover frequency fc
between 15kHz an d 34kHz. The corresponding
phase - margin should be between 45 degrees
and 65 degrees. An important characteristic of
current - mode buck regulator is its dominant
pole. The frequency of the dominant pole is
given by:
© 2013 Exar Corporation
9/ 12
Rev. 2.0. 1