MAX6469UT285BD3(2003) Просмотр технического описания (PDF) - Maxim Integrated

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MAX6469UT285BD3
(Rev.:2003)

300mA LDO Linear Regulators with Internal Microprocessor Reset Circuit

Maxim Integrated 

300mA LDO Linear Regulators with Internal

Microprocessor Reset Circuit

remotely sense the output voltage of the device. Using

FB with an external NPN transistor, the current drive

capability can be increased according to the following

equation (Figure 2):

IOUT(TOTAL) = IOUT ✕ (β+1)

The external NPN pass transistor must meet specifica-

tions for current gain, power dissipation, and collector

current. The beta influences the maximum output cur-

rent the circuit can deliver. The largest guaranteed out-

put current is given by ILOAD (max) = 300mA × beta

(min). The transistor’s rated power dissipation must

exceed the actual power dissipated in the transistor.

The power dissipated (PD) equals the maximum load

current (ILOAD (max)) times the maximum input-to-out-

put voltage differential: PD = ILOAD (max) × (VIN (max) -

VOUT). The rated transistor collector current must

exceed the maximum load current.

Reverse Leakage Protection

Reverse OUT to IN Current

An internal circuit monitors the MAX6469–MAX6484

input and output voltages. When the output voltage is

greater than the input voltage, the internal IN-to-OUT

pass transistor and parasitic diode turn off. An external

voltage applied to OUT does not reverse charge a bat-

tery or power source applied to IN (the leakage path

from OUT to IN is 0.01µA typ). When the output voltage

exceeds the input voltage, OUT powers the device and

shutdown must be logic high (greater than 0.7 ✕ VOUT).

RESET asserts until IN exceeds OUT and OUT is above

the specified VTHOUT threshold (based on the selected

or adjusted regulator OUT nominal voltage).

Reverse OUT to Ground Current

The MAX6469–MAX6484 maintain a low OUT-to-GND

reverse-current flow when the IN power source is

removed. When IN floats (input battery removed) and

SHDN is pulled up to VOUT (by an external diode), the

OUT-to-GND current through the LDO is 40µA (typ). The

regulator output can be held up with an external super

capacitor or backup battery at OUT until the IN battery is

replaced. The RESET output is asserted while the IN bat-

tery is removed to place the system in a low-power

mode. Volatile memory content is maintained until the

super capacitor or battery voltage drops below RAM

standby specifications. RESET deasserts when the IN

battery has been replaced and OUT exceeds the

desired reset threshold. For nonrechargeable backup

battery applications, place a reverse diode between

OUT and the backup battery (to prevent battery charg-

ing). The external diode does not affect the regulator’s

dropout voltage because it is not between the LDO out-

put and the processor/memory Vcc supply. The diode

can be replaced with a current-limiting resistor for

rechargeable backup battery applications.

Current Limit

The MAX6469–MAX6484 include an internal current-

limit circuit that monitors and controls the pass transis-

tor’s gate voltage, limiting the output current to 450mA

(min). The output can be shorted to ground indefinitely

without damaging the part.

Thermal Shutdown

When the junction temperature (TJ) exceeds +180°C

(typ), the thermal sensor signals the shutdown logic,

turning off the pass transistor and allowing the IC to

REMOVABLE

LITHIUM ION OR

3-CELL

ALKALINE

IN

OUT

MAX6469–

3.3µF

MAX6484

3.0V

LITHIUM

µP

MEMORY

SHDN

5.0V

IN

OUT

MAX6475/MAX6476

MAX6483/MAX6484

FB

GND RESET

1A TOTAL CURRENT

VCC = 3.3V

330Ω

3.3µF

0.1µF RPULLUP

µP

REMOVABLE

LITHIUM ION OR

3-CELL

ALKALINE

OUT

IN

MAX6469–

MAX6484

3.3µF

µP

MEMORY

SUPERCAP

SHDN

Figure 2. High-Current, External Transistor Application

Figure 3. Battery Backup

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