MIC3263 Datasheet, PDF (21/32 Pages) Micrel Semiconductor – Six-Channel WLED Driver for Backlighting Applications with Flicker-Free Dimming

Micrel, Inc.
MIC3263
Equation 3 shows that s = ^D'2RLOAD is a right-half plane
L
zero (f_RHPZ):
Equation 4:
RHP
Zero→ f_RHPZ
=
D'2R_LOAD
2πL
The loop bandwidth should be about 1/10 of the f_RHPZ to
ensure stability. From Equation 3, it is shown that there is
only the single pole due to R_LOADC_OUT. 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.
To measure G_VC(S), tie CRV to a DC voltage source. Tie
CRV to the steady state voltage that CRV will operate
usually between 1V and 2.4V. By connecting CRV to a
constant DC voltage, this effectively opens the CRV
control loop and allows the measurement of the boost
control loop. G_VC(S) can be calculated with a computer
using the above equation. From the bode plot of G_VC(S)
find what the gain of G_VC(s) is at 1/10 of f_RHPZ or less. Next
design the error amp gain G_EA(s) so the loop gain at the
cross over frequency T(f_CO) is 0db where f_CO =1/10 of f_RHPZ
or lower.
Error Amp
The error amp is a g_m type and the gain – G_EA(S) – is:
Equation 5:
⎛⎛
1 ⎞⎞
G_ea (s)
=
g_m
⎜ ^Zo
⎝
II
⎜ ^R COMP
⎝
+
sC _COMP
⎟⎟
⎠⎠
g_m = 0.056mA/V and Z_O = 5MΩ. The error amplifier zero is
1
fZero = _{2πRCOMPCCOMP} . Set the f_CO at the mid band
where G_EA(f_CO) = g_m × R_COMP. At f_ZERO × 10 the phase boost
is near its maximum.
40
20
0
-20
-40
-60
-80
-100
1.E+02
Fzero
Midband Gain
Gain
Phase
1.E+03
1.E+04
Freq
1.E+05
1.E+06
Figure 14. Error Amp Transfer Function
Figure 15. Internal Error Amp and External Compensation
Example 1
Conditions: V_IN = 12V, V_OUT = 29V, I_OUT = 0.18A, L = 22μH,
C_OUT = 4.7μF R_LOAD = V_OUT/I_OUT = 161Ω. When V_CRV =
1.8V, the f_RHPZ is:
f_RHPZ
=
D'2R_LOAD
2πL
= 162kHz
Figure 16 shows a plot of:
G_vc (s)
=
V_OUT (s)
V_CONTROL (s)
=
⎛
⎜⎝
1
Ri
⎞⎛
⎟⎠ ⎜⎝
D'R_LOAD
2
⎞
⎟⎠
⎛
⎜⎜⎝1-
sL
D'2R_LOAD
⎞
⎟⎟⎠
⎛⎜⎝₁₊
sR_LOADC_OUT
2
⎞
⎟⎠
January 2010
21
M9999-012110