Datasheet

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SLVS426 − MAY 2002
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18
calculated to achieve the desired output voltage. In the EVM design, the value of R1 is determined as
R01A = 27 kΩ and R01B = 1.8 kΩ for V
O
1, R03A = 47 kΩ and R03B = 1.8 kΩ for V
O
2, R14A = 10 kΩ and
R14B = 1.2 kΩ for V
O
3, and R18 = 6.8 k + 820 Ω for V
O
4 considering stability. For V
O
1:
R05 +
(27 k ) 1.8 k) 0.85
3.3 * 0.85
+ 9.99 kW
Therefore, use 10 kΩ.
In a same manner, R07 = R11 = R19 = 10 kΩ as follows.
R07 +
(47 k ) 1.8 k) 0.85
5 * 0.85
+ 10.00 kW
R11 +
(10 k ) 1.2 k) 0.85
1.8 * 0.85
+ 10.02 kW
R19 +
(6.8 k ) 820) 0.85
1.5 * 0.85
+ 9.96 kW
The values of R01B, R03B, R14B and R19 are chosen so that the calculated values of R05, R07, R11, and R19
are standard value resistors and the V
O
setpoint maintains the highest precision. This is best accomplished by
combining two resistor values. If a standard value resistor can not be applied, use a value for R01A, R03A,
R14A, and R18 that is just slightly less than the desired total. A small resistor value in the range of tens or
hundreds of ohms for R01B, R03B, R14B and R18 can then be added to generate the desired final value.
OUTPUT INDUCTOR SELECTION
The required value for the output filter inductor can be calculated by using the equation below, assuming the
magnitude of the ripple current is 20 % of the maximum output current:
L
(out)
+
VIN * V
O
0.2 I
O
V
O
VIN
1
f
S
Where L
(out)
is output filter inductor value (H), VIN is the input voltage (V), I
O
is the maximum output current
(A), f
s
is the switching frequency (Hz).
Example : VIN = 8 V; V
O
= 3.3 V; I
O
= 4 A; f
s
= 300 kHz.
Then, L
(out)
= 8.1 µH.
If faster output response is required for a sudden transition of the load, smaller inductance value is
recommended.
OUTPUT INDUCTOR RIPPLE CURRENT
The output inductor current can affect not only the efficiency, but also the output voltage ripple. The equation
is exhibited below:
I
(ripple)
+
VIN * V
O
* I
O
ǒ
r
DS(on)
) R
L
Ǔ
L
(out)
V
O
VIN
1
f
S
where I
(ripple)
is the peak-to-peak ripple current (A) through the inductor; Io is the output current; r
DS(on)
is the
on-time resistance of MOSFET (Ω); R
L
is the inductor dc resistance (Ω). From the equation, it can be seen that
the current ripple can be adjusted by changing the output inductor value.
Example: VIN = 8 V; V
O
= 3.3 V; I
O
= 4 A; rDS(on) = 25 mΩ; R
L
= 10 mΩ; f
s
= 300 kHz; L
(out)
= 4 µH.
Then, the ripple current I
(ripple)
= 1.57 A