A7121A
DC-DC CONVERTER BUCK (STEP-DOWN)
1MHz, 2.0A, SYNCHRONOUS
AiT Semiconductor Inc.
www.ait-ic.com
Application Information
Setting the Output Voltage
Figure 1 above shows the basic application circuit with A7121A. The external resistor sets the output voltage
according to the following equation:
R2
R1
V
OUT = 0.6V X 1+
R1=150kΩ for all outputs; R2= 100kΩ for VOUT=1.0V, R2=150kΩ for VOUT =1.2V, R2=300kΩ for VOUT =1.8V,
and R2=475kΩ for VOUT =2.5V, R2=604kΩ for VOUT =3.0V, and R2=680kΩ for VOUT =3.3V. (Use 1% tolerance
resistor)
Inductor Selection
For most designs, the A7121A operates with inductors of 1μH to 4.7μH. Low inductance values are physically
smaller but require faster switching, which results in some efficiency loss. The inductor value can be derived
from the following equation:
V
OUT x (VIN − VOUT
)
L =
V
IN x ΔI x fOSC
L
WhereΔIL is inductor Ripple Current. Large value inductors lower ripple current and small value inductors
result in high ripple currents. Choose inductor ripple current approximately 35% of the maximum load current
2000mA, or ΔIL=700mA.
For output voltages above 2.0V, when light-load efficiency is important, the minimum recommended inductor
is 2.2μH. For optimum voltage-positioning load transients, choose an inductor with DC series resistance in the
50mΩ to 150mΩ range. For higher efficiency at heavy loads (above 500mA), or minimal load regulation (but
some transient overshoot), the resistance should be kept below 100mΩ. The DC current rating of the inductor
should be at least equal to the maximum load current plus half the ripple current to prevent core saturation.
Table 1 lists some typical surface mount inductors that meet target applications for the A7121A.
Vendor
Sunlord
P/N
L(μH) DCR(mΩ)
I
Rating (A)
2.95
5.5
I
sat (A)
4.9
9
Size(L*W*H), (mm)
4.0 x 4.0 x 3.0
5.7 x 5.4 x 3.0
4.3 x 4.3 x 2.0
SWPA4030S2R2NT
2.2
2.2
2.2
30
29
Mag.Layers MMD-05CZ-2R2M-M2
Mag.Layers MND-04BZN2R2M-XG
23.5
8
3.5
Table 1 Recommended Inductor
REV1.3
- JUL 2013 RELEASED, NOV 2016 UPDATED -
- 10 -
AITSEMI [ AiT Semiconductor ]
