High-Voltage, Step-Down DC-DC

Controllers in µMAX

______________________________________________________________________________________

11

loss. For 1% error, the current through R2 should be at

least 100 times FB’s input bias current.

Current-Sense-Resistor Selection

The current-sense comparator limits the peak switching

switching current will increase efficiency and reduce

the size and cost of external components. However,

since available output current is a function of the peak

switching current, the peak current limit must not be set

too low.

Set the peak current limit to 1.3 times the maximum

load current by setting the current-sense resistor to:

Inductor Selection

The essential parameters for inductor selection are induc-

tance and current rating. The MAX1744/MAX1745 ope-

rate with a wide range of inductance values. In many

applications, values between 4.7μH and 100μH take best

advantage of the controller’s high switching frequency.

Calculate the minimum inductance value as follows:

where 1μs is the minimum on-time. Inductor values

high inductor values, the MAX1744/MAX1745 begin

continuous-conduction operation at a lower fraction of

the full load (see the

Detailed Description section).

The inductor’s saturation and heating current ratings

must be greater than the peak switching current to pre-

vent overheating and core saturation. Saturation occurs

when the inductor’s magnetic flux density reaches the

maximum level the core can support, and inductance

starts to fall. The heating current rating is the maximum

DC current the inductor can sustain without overheating.

For optimum efficiency, the inductor windings’ resis-

tance should be less than the current-sense resistance.

If necessary, use a toroid, pot-core, or shielded-core

inductor to minimize radiated noise. Table 1 lists induc-

tor types and suppliers for various applications.

External Switching Transistor

The MAX1744/MAX1745 drive a P-channel enhance-

ment-mode MOSFET. The EXT output swings from VH

to IN. Be sure that the MOSFET’s on-resistance is spec-

ified for 5V gate drive or less. Table 1 recommends

MOSFET suppliers.

Four important parameters for selecting a P-channel

MOSFET are drain-to-source breakdown voltage, cur-

drain-to-source breakdown voltage rating should be at

with a maximum continuous drain-current rating higher

than the peak current limit:

The Qg specification should be 80nC or less to ensure

fast drain voltage rise and fall times, and reduce power

specifies all of the capacitances associated with charging

the MOSFET gate. EXT pin rise and fall times vary with dif-

ferent capacitive loads, as shown in the

Typical Operating

reduce power losses while the MOSFET is on. It should

be equal to or less than the current-sense resistor.

LIM MAX

CS MAX

SENSE

I

V

R

)(

)

()

≥=

L=

V- V

(MIN)

IN

OUT

()

()

xs

V

R

CS MIN

CS

1

μ

R=

V

1.3 x I

CS

CS(MIN)

OUT(MAX)

R3

R2

FROM

OUTPUT

TO FB

Figure 5. Adjustable-Output Operation Using the MAX1745