Datasheet

Setting the Output Voltage Using the Feedback Resistor Divider
V =0.6Vx1+
O
()
R1
R2
(7)
C1=
2 x25kHzxR1p
1 1
=
2 x ƒ xR1p
z
(8)
Inductor Selection
DI =V x
L O
DI
L
I max=I max+
L O
V
I
V
O
L x ƒ
2
1-
(9)
TPS62510
www.ti.com ................................................................................................................................................................ SLVS651A – MAY 2006 – REVISED JULY 2009
Table 2. Output Capacitor Selection
L C
O
2.2 µ H ≥ 22 µ F (ceramic capacitor)
3.3 µ H ≥ 22 µ F (ceramic capacitor)
(1)
(1) For output currents < 800 mA, a 10- µ F output capacitor is sufficient.
The external resistor divider sets the output voltage of the converter.
The output voltage is calculated as:
with:
R1 + R2 ≤ 1 M Ω , and the internal reference voltage is V
ref
typical = 0.6 V.
To keep the operating quiescent current to a minimum, a high impedance feedback divider is selected with
R1 + R2 ≤ 1 M Ω . The sum of R1 and R2 should not be greater than 1 M Ω to avoid possible noise related
regulation issues. A feedforward capacitor is needed across the upper feedback resistor to place a zero at a
frequency of 25 kHz in the control loop. After selecting the feedback resistor values, the feedforward capacitor is
calculated as:
with:
R1 = upper resistor of voltage divider, and C1 = upper capacitor of voltage divider.
Select the capacitor value that is closest to the calculated value. This capacitor is only needed when setting the
output voltage with the external divider.
For high efficiencies, the inductor should have a low dc resistance to minimize conduction losses. Especially at
high switching frequencies where the core material has a higher impact on the efficiency. The inductor value
determines the inductor ripple current. The larger the inductor value, the smaller the inductor ripple current, and
the lower the conduction losses of the converter. However, larger inductor values cause slower load transient
response. Usually, the inductor ripple current as calculated in Equation 9 , should be around 20% of the average
output current.
To avoid saturation of the inductor, the inductor should be rated at least for the maximum output current of the
converter plus the inductor ripple current calculated in Equation 9 :
with:
f = Switching frequency (1.5 MHz typical)
L = Inductor value
Δ I
L
= Peak-to-Peak inductor ripple current
I
L
max = Maximum Inductor current
The highest inductor current occurs at maximum V
I
.
A more conservative approach is to select the inductor current rating just for the maximum typical switch current
limit of the converter of 2 A. See Table 3 for inductor recommendations.
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