RT8868A 데이터 시트보기 (PDF) - Richtek Technology
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RT8868A Datasheet PDF : 21 Pages
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RT8868A
of the short circuit OCP is designed 1.5 times of normal
OCP level. Hence, the equation of short circuit OCP
threshold is :
ILX(MAX), short
=
1.5
x
ILX(MAX) =
3
×
VIMAX
RIMAX
×
RCSNX
DCRX
,
and the delay of short circuit OCP is 20μs. When short
circuit OCP is triggered, the RT8868A will turn off both
high side MOSFET and low side MOSFET of all channels.
CORE Section Over Voltage Protection (OVP)
The over voltage protection monitors the output voltage
via the FB pin. Once VFB exceeds 1.8V, OVP is triggered
and latched for VCORE section. RT8868A will try to turn
on each low side MOSFET and turn off each high side
MOSFET to protect CPU.
NB Section Output Current Sensing
The RT8868A provides low input offset Current Sense
Amplifier (CSA) to monitor the continuous output current
of NB section. Output current of CSA (IX_NB) is used for
over current detection as shown in Figure 12. In this
inductor current sensing topology, RS_NB and CS_NB must
be set according to the equation below :
LNB
DCRNB
= RS_NB × CS_NB
Then the output current of CSA will follow the equation
below :
IX_NB
=
IL_NB ×DCRNB
RCSN_NB
CSA: Current Sense Amplifier
IX_NB
+
-
LNB DCRNB
RS_NB CS_NB
RCSN_NB
Figure 12. Current Sensing Circuit for NB Section
NB Section Over Current Protection (OCP)
NB section uses an external resistor RIMAX_NB connected
to IMAX_NB pin to generate a reference current IMAX_NB
for over current protection as depicted in Figure 13.
IIMAX_NB
=
VIMAX_NB
RIMAX_NB
where VIMAX_NB is typically 1.6V. The OCP comparator
compares the sensed phase current IX_NB with the reference
current. Equivalently, the maximum phase NB current
ILX_NB(MAX) is calculated as below :
1
4
×
IIMAX_NB
=
1
8
×
IX_NB
IX_NB
=
2 ×IIMAX_NB
=
2×
VIMAX_NB
RIMAX_NB
ILX_NB(MAX)
=
IX_NB
×
RCSN_NB
DCRNB
=
2×
VIMAX_NB
RIMAX_NB
×
RCSN_NB
DCRNB
Once IX_NB is larger than 2 x IIMAX_NB, OCP of NB section
is triggered and latched. Then, the RT8868A will turn off
both high side MOSFET and low side MOSFET of NB
section. A 100μs delay is used in OCP detection circuit
to prevent false trigger.
Except the normal OCP function described above, there
is another short circuit OCP function especially designed
for short circuit protection. Since short circuit may cause
catastrophic damage over a very short period, the short
circuit OCP should have a very short delay for triggering
OCP latch. Also to prevent false trigger, the trigger level
of the short circuit OCP is designed 1.5 times of normal
OCP level of NB section. Hence, the equation of NB
section short circuit OCP threshold is :
ILX_NB(MAX), short = 1.5 x ILX_NB(MAX)
=
3×
VIMAX_NB
RIMAX_NB
×
R CSN_NB
DCRNB
,
and the delay of short circuit OCP of NB section is 20μs.
When short circuit OCP is triggered at NB section, the
RT8868A will turn off both high side MOSFET and low
side MOSFET of NB section.
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
18
is a registered trademark of Richtek Technology Corporation.
DS8868A-00 May 2012
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