RT8868A Datasheet, PDF (15/21 Pages) Richtek Technology Corporation – 4/3/2/1-Phase PWM Controller for AMD AM2/AM2+/AM3 CPUs

RT8868A
VCC12 ^9.6V
VCC5 ^4.6V
POR
EN
VID(1)/PVI xx
PVI mode
(6-bits)
xx
Valid
8.7V
4.2V
VDD
PGOOD
PWROK
T2
T1
Figure 3. PVI-Mode Power-sequencing Diagram
VCC12 ^9.6V
VCC5 ^4.6V
POR
EN
VID(1)/PVI xx
SVC xx
SVD xx
VDD or
VDDNB
PGOOD
PWROK
Valid
Valid
Vboot
T1
T2
T3
8.7V
4.2V
Figure 4. SVI-Mode Power-sequencing Diagram
CORE Section Output Current Sensing
The RT8868A provides a low input offset Current Sense
Amplifier (CSA) to monitor the continuous output current
of each phase for V_CORE. Output current of CSA (I_X[n]) is
used for current balance and active voltage position as
shown in Figure 5. In this inductor current sensing topology,
R_S and C_S must be set according to the equation below :
L
DCR
= R_S
× C_S
Then the output current of CSA will follow the equation
below :
I_X
=
[I_L
×DCR −
V_OFS-CSA + 235nA × (R_CSP
R_CSN
− R_CSN )]
235nA is the typical value of the CSA input offset current.
V_OFS-CSA is the input offset. Usually, “V_OFS-CSA + 235nA x
(R_CSP − R_CSN)” is negligible except at very light load and
the equation can be simplified as the equation below :
I_X
=
I_L ×DCR
R_CSN
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS8868A-00 May 2012
L _DCR
CSA: Current Sense Amplifier
R_S
C_S
ISP ^RCSP
235nA
+
I_X
V_{OFS_CSA}
+
-
-
ISN ^RCSN
235nA
Figure 5. Current Sensing Circuit
CORE Section Phase Detection
The number of the operational phases is determined by
the internal circuitry that monitors the ISNx voltages during
start up. Normally, the RT8868A operates as a 4-phase
PWM controller. Pull ISN4 and ISP4 to 5VCC programs
3-phase operation, pull ISN3 and ISP3 to 5VCC programs
2-phase operation, and pull ISN2 and ISP2 to 5VCC
programs 1-phase operation. The RT8868A detects the
voltage of ISN4, ISN3 and ISN2 at rising edge of POR. At
the rising edge, the RT8868A detects whether the voltage
of ISN4, ISN3 and ISN2 are higher than “VCC5-1V”
respectively to decide how many phases should be active.
Phase detection is only active during start up. Once POR
= high, the number of operational phases is determined
and latched.
CORE Section Switching Frequency
Connecting a resistor (R_RT) from the RT pin to GND
programs the switching frequency of each phase. Figure
6 shows the relationship between the resistance and
switching frequency.
1200
1000
800
600
400
200
0
0
40
80 120 160 200 240 280
R_RT (k Ωoh) m)
Figure 6. R_RT vs. Phase switching Frequency
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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