Datasheet

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SLVS289A − MARCH 2000 − REVISED OCTOBER 2000
25
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
droop compensation
Droop compensation with the offset resistor divider from V
OUT
to the VSENSE is used to keep the output voltage
in range during load transients by increasing the output voltage setpoint toward the upper tolerance limit during
light loads and decreasing the voltage setpoint toward the lower tolerance limit during heavy loads. This allows
the output voltage to swing a greater amount and still remain within the tolerance window. The maximum droop
voltage is set with R6 and R7:
V
DROOP
ǒ
max
Ǔ
+ V
IOUT
ǒ
max
Ǔ
R6
R6 ) R7
Where V
DROOP(max)
is the maximum droop voltage; V
IOUT(max)
is the maximum V
IOUT
that reflects the
maximum output current (full load); R6 is the bottom resistor of the divider connected to the DROOP
pin, R7
is the top resistor.
The offset voltage is set to be half of the maximum droop voltage higher than the nominal output voltage, so
the whole droop voltage range is symmetrical to the nominal output voltage. The formula for setting the offset
voltage is:
V
OFFSET
+
1
2
V
DROOP
ǒ
max
Ǔ
+ V
O
ǒ
R12
R10 ) R12
Ǔ
Where V
OFFSET
is the desired offset voltage; V
DROOP(max)
is the droop voltage at full load; V
O
is the nominal
output voltage; R10 is the top resistor of the offset resistor divider, and R12 is the bottom one.
Therefore, with the setup above, at light load, the output voltage is:
V
O
ǒ
NO LOAD
Ǔ
+ V
O
ǒ
nom
Ǔ
) V
OFFSET
+ V
O
ǒ
nom
Ǔ
)
1
2
V
DROOP
And, at full load, the output voltage is:
V
O
ǒ
FULL LOAD
Ǔ
+ V
O
ǒ
nom
Ǔ
* V
OFFSET
+ V
O
ǒ
nom
Ǔ
*
1
2
V
DROOP
output inductor ripple current
The output inductor current ripple can affect not only the efficiency, but also the output voltage ripple. The
equation for calculating the inductor current ripple is exhibited in the following:
I
ripple
+
V
IN
* V
OUT
* I
OUT
ǒ
r
DS(on)
) R
L
Ǔ
L
OUT
D Ts
Where I
ripple
is the peak-to-peak ripple current (A) through the inductor; V
IN
is the input voltage (V); V
OUT
is the
output voltage (V); I
OUT
is the output current; r
DS(on)
is the on-time resistance of MOSFET (Ω); R
L
is the output
inductor equivalent series resistance; D is the duty cycle; and Ts is the switch cycle (S). From the equation, it
can be seen that the current ripple can be adjusted by changing the output inductor value.
Example:
V
IN
= 5 V; V
OUT
= 1.8 V; I
OUT
= 5 A; r
DS(on)
= 10 mΩ; R
L
= 5 mΩ; D = 0.36; Ts = 5 µs; L
OUT
= 6 µH
Then, the ripple I
ripple
= 1 A.