MAX17000A_08 Datasheet, PDF(24/32 Page) Maxim Integrated Products – Complete DDR2 and DDR3 Memory Power-Management Solution

English

English German Russian Spanish Italian Polish Chinese Japanese Korean French Portuguese	Language :

MAX17000A_08 Datasheet, PDF (24/32 Pages) Maxim Integrated Products – Complete DDR2 and DDR3 Memory Power-Management Solution

◁

Complete DDR2 and DDR3 Memory

Power-Management Solution

stresses and thus drives the selection of input

capacitors, MOSFETs, and other critical heat-con-

tributing components. Most notebook loads gener-

ally exhibit ILOAD = ILOAD(MAX) x 80%.

â¢ Switching Frequency: This choice determines the

basic trade-off between size and efficiency. The

optimal frequency is largely a function of maximum

input voltage, due to MOSFET switching losses that

are proportional to frequency and VIN2. The opti-

mum frequency is also a moving target, due to

rapid improvements in MOSFET technology that are

making higher frequencies more practical.

â¢ Inductor Operating Point: This choice provides

trade-offs between size vs. efficiency and transient

response vs. output noise. Low inductor values pro-

vide better transient response and smaller physical

size, but also result in lower efficiency and higher

output noise due to increased ripple current. The

minimum practical inductor value is one that causes

the circuit to operate at the edge of critical conduc-

tion (where the inductor current just touches zero

with every cycle at maximum load). Inductor values

lower than this grant no further size-reduction bene-

fit. The optimum operating point is usually found

between 20% and 50% ripple current.

Inductor Selection

The switching frequency and operating point (% ripple

current or LIR) determine the inductor value as follows:

fSW

VIN - VOUT

Ã ILOAD(MAX)

LIR â â

ââ

VOUT

VIN

â â

Find a low-loss inductor having the lowest possible DC

resistance that fits in the allotted dimensions. Ferrite

cores are often the best choice, although powdered

iron is inexpensive and can work well at 200kHz. The

core must be large enough not to saturate at the peak

inductor current (IPEAK):

IPEAK

ILOAD(MAX)

Ã âââ1+

LIR â

2 â â

Setting the Valley Current Limit

The minimum current-limit threshold must be high

enough to support the maximum load current when the

current limit is at the minimum tolerance value. The val-

ley of the inductor current occurs at ILOAD(MAX) minus

half the ripple current; therefore:

ILIMIT(LOW)

ILOAD(MAX)

âââ1-

LIR â

2 â â

where ILIMIT(LOW) equals the minimum current-limit

threshold voltage divided by the output sense element

(inductor DCR or sense resistor).

The valley current limit is fixed at 17mV (min) across the

CSH to CSL differential input.

Special attention must be made to the tolerance and

thermal variation of the on-resistance in the case of DCR

sensing. Use the worst-case maximum value for RDCR

from the inductor data sheet, and add some margin for

the rise in RDCR with temperature. A good general rule

is to allow 0.5% additional resistance for each degree

Celsius of temperature rise, which must be included in

the design margin unless the design includes an NTC

thermistor in the DCR network to thermally compensate

the current-limit threshold.

The current-sense method (Figure 7) and magnitude

determine the achievable current-limit accuracy and

power loss. The sense resistor can be determined by:

RSENSE = VLIMIT/ILIMIT

MAX17000A DL

PGND1

INPUT (VIN)

CIN

SENSE RESISTOR

LESL

RSENSE

REQ

CEQ

COUT

CEQREQ =

LESL

RSENSE

CSH

CSL

A) OUTPUT SERIES RESISTOR SENSING

Figure 7a. Current-Sense Configurations (Sheet 1 of 2)

24 ______________________________________________________________________________________

▷