RT8855 데이터 시트보기 (PDF) - Richtek Technology
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RT8855 Datasheet PDF : 18 Pages
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RT8855
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
8.7V
4.2V
SVC xx
SVD xx
VDD or
VDDNB
PGOOD
Valid
Valid
Vboot
PWROK
T1
T2
T3
Figure 4. SVI-Mode Power-sequencing Diagram
CORE Section- Output Current Sensing
The RT8855 provides a low input offset current-sense
amplifier (CSA) to monitor the continuous output current
of each phase for VCORE. Output current of CSA (IX[n]) is
used for current balance and active voltage position as
shown in Figure 5. In this inductor current sensing topology,
RS and CS must be set according to the equation below :
L
DCR
= RS
× CS
Then the output current of CSA will follow the equation
below :
IX
=
[IL
×DCR −
VOFS-CSA + 235nA ×(RCSP
RCSN
− RCSN )]
235nA is the typical value of the CSA input offset current.
VOFS-CSA is the input offset. Usually, “VOFS-CSA + 235nA x
(RCSP − RCSN)” is negligible except at very light load and
the equation can be simplified as the equation below :
IX
=
IL ×DCR
RCSN
www.richtek.com
12
L DCR
CSA: Current Sense Amplifier
RS
CS
ISP RCSP
235nA
+
IX
VOFS_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 RT8855 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. RT8855 detects the voltage
of ISN4, ISN3 and ISN2 at rising edge of POR. At the
rising edge, RT8855 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
Connect a resistor (RT) from the RT pin to GND can program
the switching frequency of each phase. Figure 6 shows
the relationship between the resistance and switching
frequency.
1200
Frequency vs. RRT
1000
800
600
400
200
0
0
40
80 120 160 200 240 280
RRT (kkΩoh) m)
Figure 6. RRT vs. Phase switching Frequency.
DS8855-01 April 2011
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