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

Power-Good Output (PGOOD)

The PGOOD window comparator continuously monitors

the output voltage for both overvoltage and undervolt-

age conditions. In shutdown, standby, and soft-start,

PGOOD is actively held low. After a digital soft-start

has terminated, PGOOD is released when the output is

within 10% of the error-comparator threshold. The

PGOOD output is a true open-drain type with no para-

sitic ESD diodes. Note that the PGOOD window detec-

tor is independent of the output overvoltage and

undervoltage protection (UVP) thresholds.

Output Overvoltage Protection

The output voltage can be continuously monitored for

overvoltage. When overvoltage protection is enabled, if

the output exceeds the overvoltage threshold, overvolt-

age protection is triggered and the DL low-side gate-

drivers are forced high. This activates the low-side

MOSFET switch, which rapidly discharges the output

capacitor and reduces the input voltage.

Note that DL latching high causes the output voltage to

dip slightly negative when energy has been previously

stored in the LC tank circuit. For loads that cannot tol-

erate a negative voltage, place a power Schottky diode

across the output to act as a reverse polarity clamp.

Connect OVP to GND to enable the default trip level of

114% of the nominal output. To adjust the overvoltage-

protection trip level, apply a voltage from 1V (100%) to

1.8V (180%) at OVP. Disable the overvoltage protection

by connecting OVP to VCC.

The overvoltage trip level depends on the internal or

external output-voltage feedback divider and is restrict-

ed by the output-voltage adjustment range (1V to 5.5V)

and by the absolute maximum rating of OUT_. Setting

the overvoltage threshold higher than the output-volt-

age adjustment range is not recommended.

Output Undervoltage Protection

The output voltage can be continuously monitored for

undervoltage. When undervoltage protection is

enabled (UVP = VCC), if the output is less than 70% of

the error-amplifier trip voltage, undervoltage protection

is triggered. If an undervoltage protection threshold is

set, the DL low-side gate driver is forced low and the

outputs float. Connect UVP to GND to disable under-

voltage protection.

Note the nonstandard logic levels if actively driving

UVP (see the Electrical Characteristics).

Design Procedure

Firmly establish the input voltage range and maximum

load current before choosing a switching frequency

and inductor operating point (ripple-current ratio). The

primary design trade-off lies in choosing a good

switching frequency and inductor operating point, and

the following four factors dictate the rest of the design:

1) Input Voltage Range. The maximum value

(VIN(MAX)) must accommodate the worst-case high

AC adapter voltage. The minimum value (VIN(MIN))

must account for the lowest battery voltage after

drops due to connectors, fuses, and battery selector

switches. Lower input voltages result in better effi-

ciency.

2) Maximum Load Current. There are two values to

consider. The peak load current (ILOAD(MAX)) deter-

mines the instantaneous component stresses and

filtering requirements, and thus drives output capac-

itor selection, inductor saturation rating, and the

design of the current-limit circuit. The continuous

load current (ILOAD) determines the thermal stress-

es and thus drives the selection of input capacitors,

MOSFETs, and other critical heat-contributing com-

ponents.

3) Switching Frequency. This choice determines the

basic trade-off between size and efficiency. The

optimal frequency is largely a function of maximum

input voltage due to MOSFET switching losses that

are proportional to frequency and VIN2.

4) Inductor Operating Point. This choice provides

trade-offs between size vs. efficiency. Low inductor

values cause large ripple currents, resulting in the

smallest size, but poor efficiency and high output

noise. The minimum practical inductor value is one

that causes the circuit to operate at the edge of criti-

cal conduction (where the inductor current just

touches zero with every cycle at maximum load).

Inductor values lower than this grant no further size-

reduction benefit.

The MAX8743’s pulse-skipping algorithm initiates

skip mode at the critical conduction point.

Therefore, the inductor operating point also deter-

mines the load-current value at which PFM/PWM

switchover occurs. The optimum point is usually

found between 20% and 50% of ripple current.

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