RT8856 datasheet(21/23 Pages) RICHTEK

21 / 23 page

RT8856

DS8856-03

June 2011

www.richtek.com

Users can use the same NTC thermistor for both thermal

throttling and current limit setting as shown in Figure 9.

Just divide the ROC1b into RTTa and RTTb, and write the

VNTC equation at thermal throttling temperature TT

°C :

RTTa + RTTb = ROC1b

(26)

OC2

TTb

OC2

OC1b

OC1a

NTC, TT C

// R

0.8 V

=×

(27)

Solving (26) and (27) for RTTa and RTTb as :

RTTb = 4 x (ROC1a // RNTC, TT°C )−ROC2

(28)

RTTa = ROC1b

− RTTb

(29)

NTC

RT8856

0.8 x VCC

VRTT

CMP

VCC

ROC1b

ROC2

ROC1a

NTC

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 VVSEN with another resistor. By choosing the appropriate

ratio of these two resistors, current monitor gain can be

set and VCM will be 1V with maximum output current.

Maximum value of VCM is clamped at 1.15V.

=×

× ×

LOAD

DROOP

CMSET

(30)

Inductor Selection

The switching frequency and ripple current determine the

inductor value as follows :

OUT(MIN)

MIN

Ripple

V(1 D

)

×−

=×

(31)

where N is the total number of phases. DMIN is the minimum

duty at highest input voltage VIN.

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 :

PD(MAX) = (TJ(MAX)

− TA) / θJA

where TJ(MAX) is the maximum junction temperature, TA is

the ambient temperature, and

θJAis the junction to ambient

thermal resistance.

RT8856 Datasheet(PDF) 21 Page - Richtek Technology Corporation

RT8856 Datasheet(HTML) 21 Page - Richtek Technology Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL