XRP6670 Datasheet, PDF (9/13 Pages) Exar Corporation – 3A 300kHz-2.5MHz Synchronous Step Down Regulator

XRP6670
3A 300kHz-2.5MHz Synchronous Step Down Regulator
voltages. Note that example inductors shown
in tables 3 and 4 are Wurth shielded inductors.
VOUT(V) ∆IL(p-p)(A)
L(µH)
Inductor
Example
3.3
0.76
2.8
0.81
2.5
0.84
1.8
0.76
1.5
0.70
1.2
0.62
1.1
0.57
1.5
74437346015
1.5
74437346015
1.5
74437346015
1.5
74437346015
1.5
74437346015
1.5
74437346015
1.5
74437346015
Table 3: Suggested Inductor Values
for f=1MHz, VIN=5V and IOUT=2.7A
VOUT(V) ∆IL(p-p)(A)
L(µH)
Inductor
Example
2.5
0.41
1.8
0.54
1.5
0.54
1.2
0.51
1.1
0.49
1.5
74437346015
1.5
74437346015
1.5
74437346015
1.5
74437346015
1.5
74437346015
Table 4: Suggested Inductor Values
for f=1MHz, VIN=3.3V and IOUT=2.7A
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 calculate the C_OUT
based on the unloading transient requirement
from:
'
Where:
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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 (1.5A).
V_transient is the maximum permissible voltage
transient corresponding to the load step
mentioned above. V_transient is typically specified
from 3% to 5% of V_OUT.
ESR of the capacitor has to be selected such
that the output voltage ripple requirement
∆V_OUT, nominally 1% of V_OUT, is met. Voltage
ripple ∆V_OUT is mainly composed of two
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
because of their low ESR which is typically in
the range of 5mΩ. Therefore ∆V_OUT is mainly
capacitive. For ceramic capacitors calculate
the ∆V_OUT from:
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Where:
∆I_L is from table 2 or 3
C_OUT is the value calculated above
f_s is nominal switching frequency
If tantalum or electrolytic capacitors are used
then ∆V_OUT is essentially a function of ESR:
!
!"_# 78
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
than the regulator’s maximum input voltage.
Calculate the capacitor’s current rating from:
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Where:
>? 1 ?
I_OUT is regulator’s maximum current (3A)
D is duty cycle (D=V_OUT/V_IN)
Calculate the C_IN capacitance from:
"
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© 2013 Exar Corporation
9/13
Rev. 1.0.1