XRP7675 Datasheet, PDF (9/12 Pages) Exar Corporation – 3A 18V Synchronous PFM/PWM Step-Down Regulator

XRP7 6 7 5
3 A 1 8 V Sy n ch r on ou s PFM/ PW M St ep - Dow n Reg u lat or
because of their low ESR which is typically in
the range of PŸ. Therefore Ʃ9_OUT is mainly
capa citive. For ceramic capacitors c alculate
the Æ©9_OUT from:
߂ܸ_{ை௎்}ൌͺൈ_{߂ܥை}^ܫ_{௎௅்ൈ݂௦}
Where:
Æ©,_L is from table 1 or 2 in previous section
C_OUT is the value calculated above
f_s is nominal switching frequency (340kHz)
If tantalum or electrolytic capacitors are used
WKHQV_O_UTÆ©is essentially a function of ESR:
߂ܸ_{ை௎்}ൌ߂ܫ_à¯àµˆÜ´ÜµÜ§
EXTERNAL BOOTSTRAP D IODE
A low - cost diode , such as 1N4148 , may
provide higher efficiency when the input
voltage is 5V or the output is 5V or 3.3V.
Circuit configuration is shown in figures 16 and
17. The external bootstrap diode is also
recommended where duty cycle
larger than 65%.
(V _OUT/V _IN ) is
V_IN = 5V
IN
BS
XRP7 6 7 5
SW
1N4148
10nF
INPUT CAPACITOR C_IN
Fig. 16 : Optional external bootstrap diode
Select the input capacitor for voltage rating,
where input voltage is fixed 5V
RMS current rating and capacitance. The
voltage rating should be at least 50% higher
than the regul ator ¶Vmaximum input voltage.
1N4148
BS
&DOFXODWHWKHFDSDFLWRU¶VFXUUHQWUDWLQJIURP
ܫ_{஼ூǡோேெௌ}ൌܫ_{ை௎்}ൈඥܦൈሺͳെܦሻ
XRP7 6 7 5
10nF
V_OUT = 5V
or 3.3V
Where:
SW
I _OUT LVUHJXODWRU¶VPD[LPXPFXUUHQW$
C_OUT
D is duty cycle (D=V _OUT/V _IN )
Calculate the C _IN capacitance from:
ܥ_ூேൌ^{ܫை௎்ൈ݂}_௦^{ܸൈை௎}_{ܸூ்}^ൈ_ேଶ^{ሺൈܸூ}_߂^ே_{ܸെூே}^{ܸை௎்ሻ}
Fig. 17 : Optional external bootstrap diode
where output voltage is 5V or 3.3V
LOOP COMPENSATION
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 SW pin to improve
the regulator efficiency. See Table 3.
Part Number
Voltage/Current
Rating
Vend or
B130
SK13
MBRS130
30V/1A
30V/1A
30V/1A
Diodes, Inc.
Diodes, Inc.
International
Rectifier
Table 3: Optional Schottky diode
XRP7675 utilizes current - mode con trol . This
allows using a minimum of external
components to compensate the regulator. In
general only two components are needed: RC
and CC. Proper compensation of the regulator
(determining RC and CC) results in optimum
transient response. In terms of powe r 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 and 34kHz. The corresponding
phase - margin should be between 45 degrees
and 65 degrees. An important char acteristic of
current - mode buck regulator is its dominant
pole. The frequency of the dominant pole is
given by:
© 2013 Exar Corporation
9/ 12
Rev. 1.0.0