RT8855 데이터 시트보기 (PDF) - Richtek Technology

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RT8855

4/3/2/1-Phase PWM Controller for AMD AM2/AM2+ CPUs

Richtek Technology 

RT8855

1

4

× IIMAX

=

1

8

×

IX(MAX)

IX(MAX)

=

2 ×IIMAX

=

2

×

VIMAX

RIMAX

ILX(MAX)

=

IX(MAX)

×

RCSNX

DCRX

=

2

×

VIMAX

RIMAX

×

RCSNX

DCRX

Once IX is larger than 2 x IIMAX, OCP of CORE section is

triggered and latched. Then, RT8855 will turn off both high

side MOSFET and low side MOSFET of all channels. A

100us 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, this short-

circuit-OCP should have a very short delay for triggering

OCP latch. Also to prevent false trigger, the trigger level

of short-circuit-OCP is designed 1.5 times of normal OCP

level. Hence, the equation of short-circuit-OCP is :

ILX(MAX), short

=

1.5

x

ILX(MAX) =

3

×

VIMAX

RIMAX

×

RCSNX

DCR X

,

and the delay of short-curcuit-OCP is 20us. when short-

circuit-OCP is triggered, the RT8855 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. RT8855 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 RT8855 provides low input offset current-sense

amplifier (CSA) to monitor the continuous output current

of NB scetion. 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

DS8855-01 April 2011

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 typical 1.6V. 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 × RCSN_NB

RIMAX_NB DCRNB

Once IX_NB is larger than 2 x IIMAX_NB, OCP of NB section

is triggered and latched. Then, RT8855 will turn off both

high side MOSFET and low side MOSFET of NB section.

A 100us 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, this short-

circuit-OCP should have a very short delay for triggering

OCP latch. Also to prevent false trigger, the trigger level

of short-circuit-OCP is designed 1.5 times of normal OCP

level of NB section. Hence, the equation of NB section

short-circuit-OCP is :

ILX_NB(MAX), short = 1.5 x ILX_NB(MAX)

=

3×

VIMAX_NB

RIMAX_NB

×

RCSN_NB

DCRNB

,

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