Datasheet

Application information LED7707
26/47
Equation 13b
Where V
IN,min
is the minimum input voltage and I
OUT
is the overall output current.
Note that, the lower the inductor value (and the higher the switching frequency), the higher
the bandwidth can be achieved. The output capacitor is directly involved in the loop of the
boost converter and must be large enough to avoid excessive output voltage drop in case of
a sudden line transition from the maximum to the minimum input voltages.
However a more significant requirement concerns the output voltage ripple.
The output capacitor should be chosen in accordance with the following expression:
Equation 14
where ΔV
OUT, max
is the maximum acceptable output voltage ripple, I
L, peak
is the peak
inductor current, T
OFF
is the off-time of the switching cycle (for an extensive explanation see
Section 6.4.4 on page 33).
Once the output capacitor has been chosen, the R
COMP
can be calculated as:
Equation 15
Where GM = 2.7 S and g
EA
= 375 µS
Equation 15 places the loop bandwidth at f
U
. Then, the C
COMP
capacitor is determined to
place the frequency of the compensation zero 5 times lower than the loop bandwidth:
Equation 16
Where f
Z
= f
U
/5.
In most of the applications an experimental approach is also very valid to compensate the
circuit. A simple technique to optimize different applications is to choose C
COMP
= 4.7 nF
and to replace R
COMP
with a 10 kΩ trimmer adjusting its value to properly damp the output
transient response. Insufficient damping will result in excessive ringing at the output and
poor phase margin.
Figure 18 (a and b) give an example of compensation adjustment for a typical application.
OUT
OUT
I
V
R =
(
)
max,OUT
OFFOUTpeak,L
OUT
V2
TII
C
Δ⋅
⋅−
>
MgG
Cf2
R
EAM
U
COMP
⋅⋅
⋅
⋅
π
=
COMPZ
COMP
Rf2
1
C
⋅⋅π
=