Datasheet

Application notes - buck conversion LED5000
18/51 Doc ID 023951 Rev 1
With the power components selected in accordance with Chapter 5.9: Component selection
and given the BW specification, the components composing the compensation network can
be calculated as:
Equation 16
where the term m
C
is represented in Equation 4 , R
LOAD
the equivalent loading resistor
(
Equation 12 ), R
S
the sensing resistor value, G
m
the error amplifier transconductance and
R
CS
the current sense gain ( Table 5: Electrical characteristics )
Equation 17
where K represents the leading position of the F
Z
( Equation 11 ) with respect to the system
bandwidth. In general, a value of 2 gives enough phase margin to the overall small loop
transfer function.
5.7 Example of system design
Design specification:
V
IN
=48 V, V
FW_LED
=3.7 V, n
LED
= 10, r
LED
= 1.1 W , I
LED
= 1 A, I
LED RIPPLE
= 2%
The inductor and capacitor value are dimensioned to meet the I
LED RIPPLE
specification (see
Chapter 5.9.2 for output capacitor and inductor selection guidelines):
L=22 μH, C
OUT
=1.0 μF mlcc (negligible ESR)
In accordance with
Chapter 5.9.1 the sensing resistor value is:
Equation 18
Assuming a system bandwidth of:
Equation 19
the ideal values of the components making up the compensation network is:
Equation 20
Final component selection is based on commercial values and a small capacitor C
P
is
added to reduce noise at the error amplifier output. C
P
slightly decreases the BW and phase
margin.
R
C
1
R
LOAD
T
SW
⋅
L
------------------------------------
m
C
1D–()0.5–⋅[]⋅+
f
P
-----------------------------------------------------------------------------------------------------------
BW R
CS
⋅
G
m
R
S
⋅
--------------------------- -
⋅=
C
C
K
R
C
BW⋅
------------------------=
R
S
200 mV
1 A
-------------------- 200 mΩ==
BW 70 kHz BW
MAX
<=
R
C
43 kΩ= C
C
650 pF=