RT8859M
Power Ready (POR) Detection
Current
Mirror
During start-up, the RT8859M will detect the voltage at
the voltage input pins : VCC, EN, DVD and DVDA. When
VCC > 4.24V, VDVD > 1.06V and VDVDA > 1.06V, the
RT8859M will recognize the power state of system to be
ready (POR = high) and wait for enable command at the
EN pin. After POR = high and VEN > 0.7V, the RT8859M
will enter start-up sequence for both CORE rail and AXG
rail. If the voltage at any voltage pin drops below low
threshold (POR = low), the RT8859M will enter power down
sequence and all the functions will be disabled.Normally,
connecting system VTT (1.05V) to the EN pin and power
stage VIN (12V, through a voltage divider) to theDVDpin is
recommended. 2ms (max) after the chip has been
enabled, the SVIDcircuitry will be ready. All the protection
latches (OVP, OCP, UVP) will be cleared only after POR
= low. The condition of VEN = low will not clear these
latches.
2.14V
+
-
IBIAS
53.6k
Figure 2. IBIAS Setting
ICCMAX, ICCMAXA and TMPMAX
The RT8859M provides ICCMAX, ICCMAXAandTMPMAX
pins for platform users to set the maximum level of output
current or VR temperature : ICCMAX for CORE VR max
current, ICCMAXAforAXGVR max current, and TMPMAX
for CORE VR max temperature.
To set ICCMAX, ICCMAXA and TMPMAX, platform
designers should use resistive voltage divider on these
three pins. The current of the divider should be several
milliamps to avoid noise effect. The 3 items share the
same algorithms : the ADC divides 5V into 255 levels.
Therefore, the LSB = 5 / 255 = 19.6mV, which means
19.6mV applied to ICCMAX pin equals to 1A setting. For
example, if the maximum level of temperature is desired
to be 120°C, the voltage applied to TMPMAX should be
120 x 19.6mV = 2.352V. The ADC circuit inside these
three pins will decode the voltage applied and store the
maximum current/temperature setting into ICC_Max and
Temp_Max registers. TheADC monitors and decodes the
voltage at these three pins only ONCE after power up.
After ADC decoding (only once), a 128μA current will be
generated at the ICCMAXApin for internal use. Make sure
the voltage at the ICCMAXA pin is greater than 1.55V to
guarantee proper functionality.
CMP
VCC
DVD
+
-
4.24V
1.06V
CMP
+
-
POR
CMP
CMP
EN
+
-
Chip EN
0.7V
DVDA
+
-
1.06V
Figure 1. Power Ready (POR)Detection
Precise Reference Current Generation
The RT8859M includes complicated analog circuits inside
the controller. These analog circuits need very precise
reference voltage/current to drive these analog devices.
The RT8859M will auto generate a 2.14V voltage source
at the IBIAS pin, and a 53.6kΩ resistor is required to be
connected between IBIAS and analog ground. Through
this connection, the RT8859M will generate a 40μA current
from the IBIAS pin to analog ground, and this 40μAcurrent
will be mirrored inside the RT8859M for internal use.Note
that other types of connection or other values of resistance
applied at the IBIAS pin may cause failure of the
RT8859M's functions, such as slew rate control, OFS
accuracy, etc. In other words, the IBIAS pin can only be
connected with a 53.6kΩ resistor toGND. The resistance
accuracy of this resistor is recommended to be 1% or
higher.
V
CC
ICCMAX
A/D
Converter
ICCMAXA
TMPMAX
I
Figure 3. ADC Pins Setting
Copyright 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
30
DS8859M-05 July 2012
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
