RT8856 Datasheet, PDF (21/23 Pages) Richtek Technology Corporation – Multi-Phase PWM Controller for CPU Core Power Supply

RT8856
Users can use the same NTC thermistor for both thermal
throttling and current limit setting as shown in Figure 9.
Just divide the R_OC1b into R_TTa and R_TTb, and write the
V_NTC equation at thermal throttling temperature TT°C :
R_TTa + R_TTb = R_OC1b
(26)
R_OC2
R_OC2 + R_TTb
+ R_OC1b + R_OC1a // R_NTC,
TT°C
×
V_CC
= 0.8 × V_CC
(27)
Solving (26) and (27) for R_TTa and R_TTb as :
R_TTb = 4 x (R_OC1a // R_{NTC, TT°C} )−R_OC2
R_TTa = R_OC1b − R_TTb
(28)
(29)
RT8856
VRTT
V_CC
R_OC1a
NTC
+
CMP
-
NTC
0.8 x V_CC
R_OC1b
R_OC2
Figure 9. Using single NTC Thermistor for Thermal
Throttling and Current Limit Setting
Current Monitor
The current monitor allows the system to accurately
monitor the CPU's current dissipation and quickly predict
whether the system is about to overheat before the
significantly slower temperature sensor signals an over
temperature alert. The voltage output of CM pin is
proportional to the output current. This pin is connected
to ground with one resistor while CMSET pin is connected
to V_VSEN with another resistor. By choosing the appropriate
ratio of these two resistors, current monitor gain can be
set and V_CM will be 1V with maximum output current.
Maximum value of V_CM is clamped at 1.15V.
V_CM
=
I_LOAD
× R_DROOP
×2×
R_CM
R_CMSET
(30)
Inductor Selection
The switching frequency and ripple current determine the
inductor value as follows :
L_MIN
=
N×
V_OUT(MIN) × (1− D_MIN )
f_SW × I_Ripple
(31)
where N is the total number of phases. D_MIN is the minimum
duty at highest input voltage V_IN.
Higher inductance yields in less ripple current and hence
in higher efficiency. The flaw is the slower transient
response of the power stage to load transients. This might
increase the need for more output capacitors driving the
cost up. Find a low loss inductor having the lowest possible
DC resistance that fits in the allotted dimensions. The
core must be large enough not to saturate at the peak
inductor current.
Output Capacitor Selection
Output capacitors are used to obtain high bandwidth for
the output voltage beyond the bandwidth of the converter
itself. Usually, the CPU manufacturer recommends a
capacitor configuration. Two different kinds of output
capacitors can be found, bulk capacitors closely located
to the inductors and ceramic output capacitors in close
proximity to the load. The latter ones are for mid frequency
decoupling with especially small ESR and ESL values
while the bulk capacitors have to provide enough stored
energy to overcome the low frequency bandwidth gap
between the regulator and the CPU.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
P_D(MAX) = (T_J(MAX) − T_A) / θ_JA
where T_J(MAX) is the maximum junction temperature, T_A is
the ambient temperature, and θ_JA is the junction to ambient
thermal resistance.
DS8856-03 June 2011
www.richtek.com
21