RT8855 데이터 시트보기 (PDF) - Richtek Technology
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RT8855 Datasheet PDF : 18 Pages
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RT8855
and the delay of short-curcuit-OCP of NB section is 20us.
When short-circuit-OCP is triggered at NB section, the
RT8855 will turn off both high side MOSFET and low side
MOSFET of NB section.
VIN
PSOC2P
Latch
EN
5VCC
(1) 5VCC for (4 phase to 2 phase)
(2) 3.3V for (4 phase to 1 phase)
VDDIO
HS
PWM
Controller
ILX
LX_NB DCRNB
OCP Comparator
LS
RX_NB CX_NB
1.6V
1/4IIMAX_NB
1/8IX_NB
+
+
48
GM
-
RCSN_NB
-
IIMAX_NB
IX_NB
VIMAX_NB
RIMAX_NB
RT8855 NB section
Figure 13. Over Current Protection for NB section.
PSIA Control
PS
PSI
(From I2C)
PSI
(Active Low)
Figure 14. Power-Saving-Mode Circuit.
Table 5. PSI Strategy
PS pin
Pull-Up to 3.3V
Pull-Up to 5V
PSI Strategy
Phase number is set to 1 while
PSI is asserted.
Phase number is set to 2 while
PSI is asserted.
NB Section- Over Voltage Protection (OVP)
The over voltage protection monitors the output voltage
via the FB_NB pin. Once VFB_NB exceeds 1.8V, OVP is
triggered and latched for NB section. RT8855 will try to
turn on low side MOSFET and turn off high side MOSFET
to protect NB.
Power Saving Indicator (PSI)
This is an active-low flag that can be set by the CPU to
allow the regulator to enter Power-Saving mode to
maximize the system efficiency when in light-load
conditions. The status of the flag is communicated to the
controller through either the SVI bus or PS pin. RT8855
monitors the PS pin to define the PSI strategy that is the
action performed by the controller when PSI is asserted.
According Figure 14, by programming different voltage on
PS pin, it configures the controller to operate in one or
two phases condition when PSI is asserted. Pulling-up
PS pin to 3.3V through a resistor, the controller operates
in only 1 phase configuration. If the 3.3V is changed to
5V, RT8855 operates in 2 phase configuration. When PSI
is de-asserted, the controller will return to the original
configuration. The PSI strategy is summarized as shown
in Table 5.
PCB Layout Guideline
Careful PCB layout is critical to achieve low switching
losses and clean, stable operation. The high-power
switching power stage requires particular attention. Follow
these guidelines for optimum PCB layout.
Place the power components first, that includes power
MOSFETs, input and output capacitors, and inductors. It
is important to have a symmetrical layout for each power
train, preferably with the controller located equidistant from
each. Symmetrical layout allows heat to be dissipated
equally across all power trains. Great attention should be
paid for routing the UGATE, LGATE, and PHASE traces
since they drive the power train MOSFETs using short,
high current pulses. It is important to size them as large
and as short as possible to reduce their overall impedance
and inductance. Extra care should be given to the LGATE
traces in particular since keeping their impedance and
inductance low helps to significantly reduce the possibility
of shoot-through.
When placing the MOSFETs try to keep the source of the
upper MOSFETs and the drain of the lower MOSFETs
and as close as possible. Input Bulk capacitors should
be placed close to the drain of the upper MOSFETs and
and the source of the lower MOSFETs and .
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16
DS8855-01 April 2011
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