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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

The absolute worst case for MOSFET power dissipation

occurs under heavy overloads that are greater than

ILOAD(MAX) but are not high enough to exceed the cur-

rent limit. To protect against this possibility, “overde-

sign” the circuit to tolerate:

ILOAD = ILIMIT(HIGH) + (LIR / 2) ✕ ILOAD(MAX)

where ILIMIT(HIGH) is the maximum valley current

allowed by the current-limit circuit, including threshold

tolerance and on-resistance variation. If short-circuit

protection without overload protection is adequate,

enable overvoltage protection, and use ILOAD(MAX) to

calculate component stresses.

Choose a Schottky diode (D1) having a forward voltage

low enough to prevent the Q2 MOSFET body diode

from turning on during the dead time. As a general rule,

a diode having a DC current rating equal to 1/3 of the

load current is sufficient. This diode is optional and can

be removed if efficiency is not critical.

Applications Information

Dropout Performance

The output voltage adjust range for continuous-conduc-

tion operation is restricted by the nonadjustable 500ns

(max) minimum off-time one-shot. For best dropout per-

formance, use the slower on-time settings. When work-

ing with low input voltages, the duty-cycle limit must be

calculated using the worst-case values for on- and off-

times. Manufacturing tolerances and internal propaga-

tion delays introduce an error to the TON K-factor. This

error is greater at higher frequencies (Table 4). Also,

keep in mind that transient-response performance of

buck regulators operating close to dropout is poor, and

bulk output capacitance must often be added (see the

VSAG equation in the Design Procedure section).

The absolute point of dropout is when the inductor cur-

rent ramps down during the minimum off-time (∆IDOWN)

as much as it ramps up during the on-time (∆IUP). The

ratio h = ∆IUP / ∆IDOWN is an indicator of ability to slew

the inductor current higher in response to increased

load and must always be greater than 1. As h ap-

proaches 1, the absolute minimum dropout point, the

inductor current is less able to increase during each

switching cycle, and VSAG greatly increases unless

additional output capacitance is used.

A reasonable minimum value for h is 1.5, but this may

be adjusted up or down to allow trade-offs between

VSAG, output capacitance, and minimum operating

voltage. For a given value of h, calculate the minimum

operating voltage as follows:

VIN(MIN) = [(VOUT + VDROP1) / {1 - (tOFF(MIN) ✕ h / K)}]

+ VDROP2 - VDROP1

where VDROP1 and VDROP2 are the parasitic voltage

drops in the discharge and charge paths (see the On-

Time One-Shot (TON) section), tOFF(MIN) is from the

Electrical Characteristics, and K is taken from Table 4.

The absolute minimum input voltage is calculated with

h = 1.

If the calculated VIN(MIN) is greater than the required

minimum input voltage, reduce the operating frequency

or add output capacitance to obtain an acceptable

VSAG. If operation near dropout is anticipated, calcu-

late VSAG to ensure adequate transient response.

Dropout Design Example:

VOUT = 1.8V

fSW = 600kHz

K = 1.63µs, worst-case K = 1.4175µs

tOFF(MIN) = 500ns

VDROP1 = VDROP2 = 100mV

h = 1.5

VIN(MIN) = (1.8V + 0.1V) / [1 - (0.5µs ✕ 1.5) / 1.4175µs]

+ 0.1V - 0.1V = 3.8V

Calculating again with h = 1 gives an absolute limit of

dropout:

VIN(MIN) = (1.8V + 0.1V) / [1 - (0.5µs ✕ 1) / 1.4175µs]

+ 0.1V - 0.1V = 2.8V

Therefore, VIN must be greater than 2.8V, even with very

large output capacitance, and a practical input voltage

with reasonable output capacitance would be 3.8V.

Fixed Output Voltages

The MAX8743’s Dual-Mode operation allows the selec-

tion of common voltages without requiring external

components (Figure 8). Connect FB1 to GND for a fixed

1.8V output or to VCC for a 1.5V output, or connect FB1

directly to OUT1 for a fixed 1V output.

Connect FB2 to GND for a fixed 2.5V output or to OUT2

for a fixed 1V output.

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