MAX1838 查看數據表（PDF） - Maxim Integrated

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MAX1838

Dual USB Switch with Fault Blanking and Autoreset

Maxim Integrated 

Dual USB Switch with Fault Blanking

and Autoreset

during a load step. The MAX1838 is capable of driving

inductive loads, but avoid exceeding the device’s

absolute maximum ratings. Usually the load inductance

is relatively small, and the MAX1838’s input includes a

substantial bulk capacitance from an upstream regulator

as well as local bypass capacitors, limiting overshoot. If

severe ringing occurs due to large load inductance,

clamp the MAX1838’s output below 6V and above -0.3V.

Turn-On and Turn-Off Behavior

In absence of faults, the MAX1838’s internal switches

turn on and off slowly under the control of the ON input.

The slow charge-pump switch-drive minimizes load tran-

sients on the upstream power source. Under thermal

fault and undervoltage lockout, the power device turns

off rapidly to protect the switch and the power source.

Layout and Thermal Dissipation

To optimize the switch response to output short-circuit

conditions, keep all traces as short as possible to reduce

the effect of undesirable parasitic inductance. Place

input and output capacitors no more than 5mm from

device leads. IN, AUX, and OUT pins must be connect-

ed with short traces to the power bus. Wide power bus

planes provide superior switch heat dissipation. While

the switches are on, power dissipation is small and the

package temperature rise is minimal. Calculate the

power dissipation for this condition as follows:

P = (IOUT)2 RON

For the normal operating current (IOUT = 0.5A), the max-

imum on-resistance of the switch is 170mΩ, and the

power dissipation is:

P = (0.5A)2 x 0.170Ω = 43mW per switch

The worst-case power dissipation occurs when the

switch is in current limit and the output is greater than

1V. In this case, the power dissipated in each switch is

the voltage drop across the switch multiplied by the cur-

rent limit:

P = (ILIM) (VIN - VOUT)

For a 5V input and 1V output, the maximum power dissi-

pation per switch is:

P = (1.2A) (5.5V - 1V) = 5.4W

Since the package power dissipation is only 667mW, the

MAX1838 die temperature will exceed the thermal shut-

down threshold and the switch output shuts down until

the junction temperature cools by 10°C. The duty cycle

and period are strong functions of the ambient tempera-

ture and the PC board layout.

A short circuit at the output causes the power dissipated

across the switch and the junction temperature to

increase. If the fault condition persists, the thermal-over-

load-protection circuitry activates, and the output shuts

down until the junction temperature decreases by 10°C

(see Thermal Shutdown).

AUX

0.1µF

IN

0.1µF

AUXA

AUXB

100kΩ

FAULTB

FAULTA

100kΩ

INA

MAX1838 OUTB2

INB

OUTB1

OUTB

10µF

SEL

OUTA2

OUTA1

OUTA

ON

10µF

ADJB

ADJA

GND

67kΩ

67kΩ

Figure 2. Typical Application Circuit

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