MAX1946ETA(2003) Ver la hoja de datos (PDF) - Maxim Integrated

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MAX1946ETA
(Rev.:2003)

Single USB Switch with Autoreset and Fault Blanking in Tiny QFN

Maxim Integrated 

Single USB Switch with Autoreset and

Fault Blanking in Tiny QFN

INPUT

2.7V TO 5.5V

0.1µF

OFF

ON

IN

IN

FAULT

ON MAX1946 OUT

SEL

OUT

GND

100kΩ

1µF

*USB APPLICATIONS MAY REQUIRE

ADDITIONAL BULK CAPACITANCE

Figure 2. Typical Application Circuit

Output Capacitor

Bypass OUT to GND with a 1µF ceramic capacitor for

local decoupling. Additional bulk capacitance (up to

470µF) reduces output-voltage transients under dynamic

load conditions. Using output capacitors greater than

470µF can assert FAULT if the current limit cannot

charge the output capacitor within the 20ms fault-

blanking period. In addition to bulk capacitance, small-

value (0.1µF or greater) ceramic capacitors improve

the output’s resilience to electrostatic discharge (ESD).

Driving Inductive Loads

A wide variety of devices (mice, keyboards, cameras, and

printers) typically connect to the USB port with cables,

which can add an inductive component to the load. This

inductance causes the output voltage at the USB port to

oscillate during a load step. The MAX1946 drives induc-

tive loads, but avoid exceeding the device’s absolute

maximum ratings. The load inductance is usually relatively

small, and the MAX1946 input typically includes substan-

tial 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

MAX1946 output below +6V and above -0.3V.

Turn-On and Turn-Off Behavior

When turned on, the MAX1946 output ramps up over

1.2ms to eliminate load transients on the upstream

power source. When turned off, the output ramps down

for 3ms. Under fault conditions, the output of the

MAX1946 turns off rapidly to provide maximum safety

for the upstream power source and downstream

devices. Internal blocks shut down to minimize supply

current when the switch is off.

Layout and Thermal Dissipation

Keep all traces as short as possible to reduce the effect

of undesirable parasitic inductance, and to optimize the

switch response time to output short-circuit conditions.

Place input and output capacitors no more than 5mm

from device leads. Connect IN and OUT to the power

bus with short traces. The exposed pad on the QFN

package underside must be soldered to the PC board in

order to realize the rated package dissipation.

An active switch dissipates little power with minimal

change in package temperature. Calculate the power

dissipation for this condition as follows:

P = (IOUT)2 ✕ RON

At the normal operating current (IOUT = 0.5A) and the

maximum on-resistance of the switch (90mΩ), the

power dissipation is:

P = (0.5A)2 ✕ 0.09Ω = 22.5mW

The worst-case power dissipation occurs when the out-

put current is just below the current-limit threshold

(1.2A max) with an output voltage just greater than 1V.

In this case, the power dissipated is the voltage drop

across the switch multiplied by the current limit:

P = ILIM ✕ (VIN - VOUT)

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

sipation per switch is:

P = 1.2A ✕ (5V - 1V) = 4.8W

Since the package power dissipation is 1951mW for the

8-pin thin QFN, the MAX1946 die temperature exceeds

the +160°C thermal-shutdown threshold. The switch

output shuts down until the junction temperature cools

by 15°C. The duty cycle and period are strong func-

tions of the ambient temperature and the PC board lay-

out (see the Thermal Shutdown section).

If the output current exceeds the current-limit threshold,

or the output voltage is pulled below the short-circuit

detect threshold, the MAX1946 enters a fault state after

20ms, at which point autoreset mode is enabled and

25mA is sourced by the output. For a 5V input, OUT

short-circuited to GND, and autoreset mode active, the

power dissipation is as follows:

P = 0.025A ✕ 5V = 0.125W

Chip Information

TRANSISTOR COUNT: 2004

PROCESS: BiCMOS

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