MAX8743(2004) 데이터 시트보기 (PDF) - Maxim Integrated

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MAX8743
(Rev.:2004)

Dual, High-Efficiency, Step-Down Controller with High Impedance in Shutdown

Maxim Integrated 

Dual, High-Efficiency, Step-Down

Controller with High Impedance in Shutdown

Inductor Selection

The switching frequency (on-time) and operating point

(% ripple or LIR) determine the inductor value as follows:

L=

VOUT (VIN - VOUT)

VIN × f × LIR × I LOAD(MAX)

Example: ILOAD(MAX) = 8A, VIN = 15V, VOUT = 1.8V,

f = 300kHz, 25% ripple current or LIR = 0.25:

L=

1.8V (15V - 1.8V)

= 2.3µH

15V × 345kHz × 0.25 × 8A

Find a low-loss inductor with 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) + [(LIR / 2) ✕ ILOAD(MAX)]

Transient Response

The inductor ripple current also impacts transient-

response performance, especially at low VIN - VOUT dif-

ferentials. Low inductor values allow the inductor

current to slew faster, replenishing charge removed

from the output filter capacitors by a sudden load step.

The amount of output sag is also a function of the maxi-

mum duty factor, which can be calculated from the on-

time and minimum off-time:

VSAG

=

(∆I LOAD(MAX))2 × L

2 × CF × DUTY (VIN(MIN) - VOUT)

where:

DUTY =

K (VOUT + 0.075V) VIN

K (VOUT + 0.075V) VOUT + min off - time

where minimum off-time = 400ns typ (Table 4).

The amount of overshoot during a full-load to no-load

transient due to stored inductor energy can be calculat-

ed as:

VSOAR = L ✕ IPEAK2 / (2 x COUT x VOUT)

where IPEAK is the peak inductor current.

Determining the Current Limit

For most applications, set the MAX8743 current limit by

the following procedure:

1) Determine the minimum (valley) inductor current

(IL(MIN)) under conditions when VIN is small, VOUT is

large, and load current is maximum. The minimum

inductor current is ILOAD minus half the ripple cur-

rent (Figure 4).

2) The sense resistor determines the achievable cur-

rent-limit accuracy. There is a trade-off between cur-

rent-limit accuracy and sense-resistor power

dissipation. Most applications employ a current-

sense voltage of 50mV to 100mV. Choose a sense

resistor such that:

RSENSE = Current-Limit Threshold Voltage / IL(MIN)

Extremely cost-sensitive applications that do not

require high-accuracy current sensing can use the on-

resistance of the low-side MOSFET switch in place of

the sense resistor by connecting CS_ to LX_ (Figure

7a). Use the worst-case value for RDS(ON) from the

MOSFET data sheet, and add a margin of 0.5%/°C for

the rise in RDS(ON) with temperature. Use the calculat-

ed RDS(ON) and IL(MIN) from step 1 above to determine

the current-limit threshold voltage. If the default 50mV

threshold is unacceptable, set the threshold value as in

step 2 above.

In all cases, ensure an acceptable current limit consid-

ering current-sense and resistor accuracies.

MAX8743

LX

DL

CS

MAX8743

LX

DL

CS

a)

b)

Figure 7. Current-Sense Configurations

18 ______________________________________________________________________________________

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