RT8152C datasheet(22/24 Pages) RICHTEK

22 / 24 page

RT8152C/D

DS8152C/D-04

April 2011

www.richtek.com

Users can use the same NTC thermistor for both thermal

throttling and current limit setting as shown in Figure 13.

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

VNTC equation at thermal throttling temperature TT

°C :

RTTa + RTTb = ROC1b

(24)

OC2

TTb

OC2

OC1b

OC1a

NTC, TT

V0.8 V

// R

×=

(25)

Solving (27) and (28) for R

TTa and RTTb as:

RTTb = 4 x (ROC1a // RNTC, TT)

− ROC2

(26)

RTTa = ROC1b

− RTTb

(27)

Figure 13. Using Single NTC Thermistor for Thermal

Throttling and Current Limit Setting

NTC

0.8 x VCC

VRTT

CMP

VCC

RTTa

RTTb

ROC1a

NTC

ROC2

OCSET

Current Monitor

Figure 14 shows the current monitor setting principle.

Current monitor needs to meet IMVP6.5 specification, the

RT8152C/D is based on the relation between RDROOP and

load current to provide an easily setting and high accuracy

current monitor indicator.

The current monitor indication voltage VCM equation is

shown as :

LOAD

DROOP

CMSET

××

(28)

Where ILOAD is the output load current, RDROOP is the load

line setting of applications, RCM and RCMSET is the current

monitor current setting resistor.

To find RCM and RCMSET base on :

CM(MAX)

CMSET

(MAX)

DROOP

R2 I

××

(29)

The VCM(MAX) must be kept equal to 1V, I(MAX) is needed to

follow the setting current of the IMVP6.5 definition with

various CPU. The RDROOP is the load line setting of

applications. The VCM(MAX) is clamped not higher than

1.15V.

There is a example for current monitor, the following design

parameters are given :

I(MAX) = 30A, RDROOP = 3m

Ω,

VCM(MAX) = 1V, RCMSET = 10k

RCM = 55.6k

⇒

Figure 14. Current Monitor Setting Principle

VSEN

VCC_SENSE

Current

Monitor

Generator

RCMSET

CMSET

VCM

RCM

RGND

Inductor Selection

The switching frequency and ripple current determine the

inductor value as follows :

OUT

(MIN)

Ripple MAX

−

=×

(30)

where TON is the UGATE turn on period.

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 be saturated 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 including, bulk capacitors closely

located to the inductors and ceramic output capacitors in

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