MAX797CSLASHD datasheet(20/32 Pages) MAXIM

20 / 32 page

Step-Down Controllers with

Synchronous Rectifier for CPU Power

______________________________________________________________________________________

Three key inductor parameters must be specified:

inductance value (L), peak current (IPEAK), and DC

resistance (RDC). The following equation includes a

constant LIR, which is the ratio of inductor peak-to-

peak AC current to DC load current. A higher value of

LIR allows smaller inductance, but results in higher

losses and ripple. A good compromise between size

and losses is found at a 30% ripple current to load cur-

rent ratio (LIR = 0.3), which corresponds to a peak

inductor current 1.15 times higher than the DC load

current.

VOUT (VIN(MAX) - VOUT)

L = ———————————

VIN(MAX) x f x IOUT x LIR

where:

f = switching frequency, normally 150kHz or

300kHz

IOUT = maximum DC load current

LIR = ratio of AC to DC inductor current,

typically 0.3

The peak inductor current at full load is 1.15 x IOUT if

the above equation is used; otherwise, the peak current

can be calculated by:

VOUT (VIN(MAX) - VOUT)

IPEAK = ILOAD + ———————————

2 x f x L x VIN(MAX)

The inductor’s DC resistance is a key parameter for effi-

ciency performance and must be ruthlessly minimized,

preferably to less than 25m

Ω at IOUT = 3A. If a stan-

dard off-the-shelf inductor is not available, choose a

core with an LI2 rating greater than L x IPEAK2 and wind

it with the largest diameter wire that fits the winding

area. For 300kHz applications, ferrite core material is

strongly preferred; for 150kHz applications, Kool-mu

(aluminum alloy) and even powdered iron can be

acceptable. If light-load efficiency is unimportant (in

desktop 5V-to-3V applications, for example) then low-

permeability iron-powder cores, such as the

Micrometals type found in Pulse Engineering’s 2.1µH

PE-53680, may be acceptable even at 300kHz. For

high-current applications, shielded core geometries

(such as toroidal or pot core) help keep noise, EMI, and

switching-waveform jitter low.

Current-Sense Resistor Value

The current-sense resistor value is calculated accord-

ing to the worst-case-low current-limit threshold voltage

(from the Electrical Characteristics table) and the peak

inductor current. The continuous-mode peak inductor-

current calculations that follow are also useful for sizing

the switches and specifying the inductor-current satu-

ration ratings. In order to simplify the calculation, ILOAD

may be used in place of IPEAK if the inductor value has

been set for LIR = 0.3 or less (high inductor values)

and 300kHz operation is selected. Low-inductance

resistors, such as surface-mount metal-film resistors,

are preferred.

80mV

RSENSE = ————

IPEAK

Input Capacitor Value

Place a small ceramic capacitor (0.1µF) between V+

and GND, close to the device. Also, connect a low-ESR

bulk capacitor directly to the drain of the high-side

MOSFET. Select the bulk input filter capacitor accord-

ing to input ripple-current requirements and voltage rat-

ing, rather than capacitor value. Electrolytic capacitors

that have low enough ESR to meet the ripple-current

requirement invariably have more than adequate

capacitance values. Aluminum-electrolytic capacitors

such as Sanyo OS-CON or Nichicon PL are preferred

over tantalum types, which could cause power-up

surge-current failure, especially when connecting to

robust AC adapters or low-impedance batteries. RMS

input ripple current is determined by the input voltage

and load current, with the worst possible case occur-

ring at VIN = 2 x VOUT:

————————

√VOUT (VIN - VOUT)

IRMS = ILOAD x ——————————

VIN

IRMS = ILOAD / 2 when VIN is 2 x VOUT

Output Filter Capacitor Value

The output filter capacitor values are generally deter-

mined by the ESR (effective series resistance) and volt-

age rating requirements rather than actual capacitance

requirements for loop stability. In other words, the low-

ESR electrolytic capacitor that meets the ESR require-

ment usually has more output capacitance than is

required for AC stability. Use only specialized low-ESR

capacitors intended for switching-regulator applications,

such as AVX TPS, Sprague 595D, Sanyo OS-CON, or

Nichicon PL series. To ensure stability, the capacitor

must meet both minimum capacitance and maximum

ESR values as given in the following equations:

VREF (1 + VOUT / VIN(MIN))

CF > ––––––––––––––––———–––

VOUT x RSENSE x f

RSENSE x VOUT

RESR < ————————

VREF

(can be multiplied by 1.5, see note below)

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