MAX8543EEE datasheet(19/27 Pages) MAXIM

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Step-Down Controllers with Prebias Startup,

Lossless Sensing, Synchronization, and OVP

______________________________________________________________________________________

A good compromise between size and efficiency is an

LIR of 0.3. Once all the parameters are chosen, the

inductor value is determined as follows:

where fS is the switching frequency. Choose a standard-

value inductor close to the calculated value. The exact

inductor value is not critical and can be adjusted to

make trade-offs among size, cost, and efficiency. Lower

inductor values minimize size and cost, but they also

increase the output ripple and reduce the efficiency due

to higher peak currents. On the other hand, higher induc-

tor values increase efficiency, but eventually resistive

losses due to extra turns of wire exceed the benefit

gained from lower AC current levels. This is especially

true if the inductance is increased without also increas-

ing the physical size of the inductor. Find a low-loss

inductor with the lowest possible DC resistance that fits

the allotted dimensions. Ferrite cores are often the best

choice, although powdered iron is inexpensive and can

work well at 300kHz. The chosen inductor’s saturation

current rating must exceed the peak inductor current

determined as:

Setting the Current Limits

Valley Current Limit

The valley current limit employs a current foldback

scheme. The MAX8543 has a fixed valley current-limit

threshold of 130mV, and a fixed foldback ratio (PFB) of

23%. The foldback ratio is the current-limit threshold

when the output is at 0V (output shorted to ground),

divided by the threshold when the output is at its nominal

regulated value. Thus, the minimum output current limit

(ILIM) and maximum short-circuit current (ISC) is calculat-

ed as:

where RDS(ON) is the maximum on-resistance of the

low-side MOSFET at the highest expected operating

junction temperature, and IP-P is the inductor ripple cur-

rent, calculated as:

Ensure that ILIM is equal to or greater than the maxi-

mum load current at peak current limit (see the Peak

Current Limit section):

where 40mV is the maximum current-limit threshold

when the output is shorted (VOUT = 0V).

The MAX8544 has an adjustable valley current limit and

can be selected for foldback with automatic recovery,

or constant current with latch-up. To set the current limit

for foldback mode, connect a resistor from ILIM2 to the

output (RFOBK), and another resistor from ILIM2 to

GND (RILIM). See Figure 6. The values of RFOBK and

RILIM are calculated as follows:

1) First, select the percentage of foldback (PFB). This

percentage corresponds to the current limit when

VOUT equals zero, divided by the current limit when

VOUT equals a nominal voltage. A typical value of

PFB is in the range of 15% to 40%. A lower value of

PFB yields lower short-circuit current. The following

equations are used to calculate RFOBK and RILIM:

where IVALLEY is the value of the inductor valley

current at maximum load (ILOAD(MAX) - 1/2 IP-P),

and RDS(ON) is the maximum on-resistance of the

low-side MOSFET at the highest operating junction

temperature.

ILIM

DS ON

VALLEY

FOBK

OUT

DS ON

VALLEY

××

× −

()×

−×

× −

()

[]

()

FOBK

OUT

×−

()

DS ON

=+ −

004

()

OUT

SIN

− =

−

()×

××

LIM

DS ON

=+ −

011

()

LIR

PEAK

LOAD MAX

()

LIR

OUT

LOAD MAX

×−

××

()

MAX8544

RFOBK

RILIM

OUT

ILIM2

Figure 6. ILIM2 Resistor Connections

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