RT8800 Datasheet, PDF (17/26 Pages) Richtek Technology Corporation – General Purpose 2/3-Phase PWM Controller for High-Density Power Supply

RT8800/B
GM
70
60
GM3
50
GM2
40
30
20
GM1
10
0
0
20
40
60
80
100
V_C (mV)
Figure 10. The Linearity of GMx
Figure 11 shows the time sharing technique of GM
amplifier. We apply test signal at phase 3 and observe the
waveforms at both pins of GM amplifier. The waveforms
show time sharing mechanism and the perfomance of GM
to hold both input pins equal when the shared time is on.
Time Sharing of GM
CH1:(2V/Div)
CH2:(50mV/Div)
CH3:(50mV/Div)
PWM3
VISP3
For some case with preferable current ratio instead of
current balance, the corresponding technique is provided.
Due to different physical environment of each channel, it
is necessary to slightly adjust current loading between
channels. Figure 12. shows the application circuit of GM
for current ratio requirement. Applying KVL along L+DCR
branch and R1+C//R2 branch:
L ^dIL + DCR x IL = R1⎜⎛ ^VC + C ^dVC ⎟⎞ + VC
dt
⎝ ^{R2 dt} ⎠
= R1x C ^dVC + ^{R1+ R2} VC
dt
R2
For VC = ^R2 DCR x IL
R1+ R2
Look for its corresponding conditions:
L ^dIL + DCR x IL = (R1//R2) x C x DCR x ^dIL + DCR x IL
dt
dt
Let ^L = (R1//R2) x C
DCR
Thus if ^L = (R1//R2) x C
DCR
Then VC = ^R2 x DCR x IL
R1+ R2
With internal current balance function, this phase would
share (R₁+R₂)/R₂ times current than other phases.
Figure 13 &14 show different settings for the power stages.
I_L
1.5uH 1m
VISP3
and
VICOMMON
VICOMMON
Time (1μs/Div)
Figure 11
Current Ratio Setting
I_L
L
DCR
C
R1
+ _VC -
R2
Figure 12. Application circuit for current ratio setting
3k
1uF
3k
Figure 13. GM3 Setting for current ratio function
I_L
1.5uH 1m
1.5k
1uF
Figure 14. GM1,2 Setting for current ratio function
DS8800/B-08 April 2011
www.richtek.com
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