MIC3223 Datasheet, PDF (18/24 Pages) Micrel Semiconductor – High Power Boost LED Driver with Integrated FET

Micrel, Inc.
MIC3223
This greatly simplifies the compensation.
One needs only to get a bode plot of the transfer function
of the control to output G_vc(s) with a network analyzer
and/or calculate it. From the bode plot find what the gain of
G_vc(s) is
at
f = ^RHPZ . Next design the error
10
amp gain
G_ea(s) so the loop gain at the cross over frequency T(f_co) is
0 db where fco = ^RHPZ or less.
10
Error Amp
Figure 11. Internal Error Amp and External Compensation
The error amp is a gm type and the gain G_ea(s) is
Eq. (21)
G_ea (s)
=
g_m
⎜⎛
⎜⎝
Z_O
||
⎜⎛
⎜⎝
R_comp
+
1
sC_comp
⎟⎞ ^⎟⎞
⎟⎠ _⎟⎠
g_m
=
0.8mA
V
and Z_o = 1.2MΩ.
The zero is
f_zero
=
1
2R_compC_comp
=
f_co
10
=
R_HPZ
100
.
Error Amp Gain and Phase
60
40
Gain
20
0
-20
Phase
-40
-60
-80
1.E+02
1.E+03 1.E+04 1.E+05
FREQUENCY (Hz)
1.E+06
Set the fco at the mid band where G_ea(f_co) = gm × R_comp. At
f_zero × 10 the phase boost is near its maximum.
Figure 12. Error Amp Transfer Function
January 2010
18
M9999-011510-A