XRP7675 Datasheet, PDF (8/12 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
APPLICATION INFORMAT ION
SETTING THE OUTPUT V OLTAGE
Use an external resistor divider to se t the
output voltage. Program the output voltage
from:
ܴͳൌܴʹൈ൬Ͳܸ _Ǥͻ^ை௎_ʹܸ^்_{ͷെͳ൰}
VOUT(V)
3.3
2.5
1.8
1.5
1.2
Æ©,L(p - p) (A)
L(µH)
Inductor
Example
0.7
4.9
744314 490
0.8
4.9
744314 490
0.7
4.9
744314 490
0.6
4.9
744314 490
0.5
4.9
744314 490
Table 2: Suggested inductor values
for VIN=5V and IOUT=3 A
Where:
R1 is the resistor between V _OUT and FB
R2 is the resistor between FB and GND
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0.925V is the nominal feedback voltage .
OUTPUT INDUCTOR
Select the output inductor for inductance L, DC
current rating I _DC and saturation current rating
I _SAT. I _DC should be larger than regulator output
current. I _SAT, as a rule of thumb, should be
50% higher than the regulator output current.
Since the regulator is rated at 2A th
en I _DC•$
and I _SAT•$Calculate the inductance from:
ܮൌሺܸ_ூேെܸ_{ை௎்}ሻ൬߂_{ܫ௅ൈܸ}^ை_{݂௦௎ൈ}^்_{ܸூே}൰
Where:
OUTPUT CAPACITOR C_OUT
Select the output capacitor for voltage rating,
capacitance C _OUT and Equivalent Series
Resistance ESR . The voltage rating, as a rule
of thumb, should be at least twice the output
voltage. When calculating
the required
capacitance, usually the overriding
requirement is current load - step transient. If
the unloading transient (i.e., when load
transitions from a high to a low current ) is
met, then usually the loading transient (when
load transitions from a low to a high
current )
is met as well. Therefore c alculate the C _OUT
based on the unload ing transient requirement
from:
Ü¥_{ை௎்}ൌܮൈቆሺܸ_{ை௎்àµ}^ܫு_ܸ^{௜௧௛௚௥ଶ௔െ}_௡^Ü«_{௦௅ሻ௜௢ଶ௘௪ଶെ௡ܸ௧ை௎்ଶ}ቇ
Where:
Æ©,_L is peak - to - peak inductor current ripple
nominally set to 30% - 40% of I _OUT
f_S is nominal switching frequency (340kHz)
As an example , inductor values for several
common output voltages are shown in table
s1
and 2 .
L is the inductance calculated in the preceding
step
I _High is the value of load - step prior to
unloading. This is nominally set equal to
regulator current rating ( 3A).
I _Low is the value of load - step after unloading.
This is nominally set equal to 50% of regulator
current rating ( 1.5 A).
VOUT(V)
5.0
3.3
2.5
1.8
1.5
1.2
Æ©,L( p - p ) (A)
L( µ H)
Inductor
Example
0.9
10
744314101
0.7
10
744314101
0.6
10
744314101
0.6
7.6
744314 760
0.5
7.6
744314 760
0.6
4.9
744314 490
Table 1: Suggested inductor values
for VIN=12V and IOUT= 3A
V_transient is the maximum permissible voltage
transient corresponding to the load step
mentioned above. V _transient is typically specified
fr om 3% to 5% of V_OUT.
ESR of the capacitor ha s to be selected such
that the output voltage ripple requirement
Æ©9_OUT, nominally 1% of V _OUT, is met. Voltage
ripple Æ©9_OUT is mainly composed of t wo
components : the resistive ripple due to ESR
and capacitive ripple due to
C_OUT charge
transfer. For applications requiring low
voltage
ripple , ceramic capacitors are recommended
© 2013 Exar Corporation
8/ 12
Rev. 1.0.0