XRP6670 Datasheet, PDF (8/13 Pages) Exar Corporation – 3A 300kHz-2.5MHz Synchronous Step Down Regulator

XRP6670
3A 300kHz-2.5MHz Synchronous Step Down Regulator
R2 is the resistor between FB and GND
0.800V is the nominal feedback voltage
A resistor selection guide for common values
of V_OUT is shown in table 1.
VOUT
1.1V
1.2V
1.5V
1.8V
2.5V
2.8V
3.3V
R1(kΩ)
R2(kΩ)
100
267
100
200
105
120
120
95.3
100
47
75
30
75
24
Table 1: Resistor Selection
PROGRAMMING THE FREQUENCY
Use resistor R_OSC between SHDN/RT and GND
pins to program the switching frequency. A
graph of nominal frequency versus R_OSC is
shown in figure 16.
DUTY-CYCLE LIMITATION
XRP6670 has a “Minimum On-Time”
specification of 150ns which imposes a
restriction on minimum duty-cycle (see table
2)
F (MHz)
TTYP(ns)
TMIN(ns) Duty-cycleMIN
1.0
1000
800
2.0
500
400
0.19
0.38
Table 2: Minimum duty-cycle arising
from “Minimum On-Time)
For example if frequency is set at 2MHz then
typical switching period is 500ns. Allowing a
±20% uncertainty, minimum period is 400ns
and corresponding minimum duty-cycle is
0.38. Recall that for a buck regulator duty-
cycle=V_OUT/V_IN. Therefore when operating at
2MHz with V_IN of 5V, a V_OUT≤1.9V is not
possible (5V x 0.38 = 1.9V).
Fig. 16: Frequency versus R_OSC
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 3A then I_DC≥3A
and I_SAT≥4.5A.
Please note that “Peak Switch Current” is
rated at 3.2A minimum. Therefore applications
that require an output current of 3A should
limit the peak-to-peak inductor current ripple
to ∆I_L≤0.4A. In the following we will use the
common practice of ∆I_L≤1A. Therefore worst-
case maximum output current will be limited
to I_OUT=3.2A-0.5A=2.7A.
Calculate the inductance from:
The following equation closely fits the
empirical data and can be used to select R_OSC
for a given frequency.
.
.
Where:
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∆I_L is peak-to-peak inductor current ripple
nominally set to ≤30% of I_OUT
f_S is nominal switching frequency
As an example, inductor values corresponding
to 5V_IN/1MHz and 3.3V_IN/1MHz are shown in
tables 3 and 4 for several common output
© 2013 Exar Corporation
8/13
Rev. 1.0.1