7

Rev. 1/23/02

SP2026 +2.7V to +5.5V USB Power Control Switch

© Copyright 2002 Sipex Corporation

The SP2026 MOSFET switches will exhibit

very low resistance (100m

Ω) or voltage drop

until the current limit is reached. The fold back

current is the current that is delivered into a short

circuit at the output.

If the SP2026 MOSFET switch is enabled into a

heavy load or short-circuit, the switch will imme-

diately go into a constant-current mode, reducing

the output voltage. The respective fault flag will

pull low until the condition is removed.

When a heavy load is connected to the SP2026

switch output, a large transient current may flow

until the current limiting circuitry responds.

The SP2026 will provide a low resistance switch

(100m

Ω) between the input and output pins. This

low resistance will be maintained with increasing

current until the 1.25A limit is reached. If load

current exceeds this limit, the switches will in-

crease their resistance. The foldback current

(500mA minimum) is reached when there is a

short applied to either output. The 100m

Ω switch

resistance is guaranteed for all load currents, in-

creasing or decreasing, that are below 500mA.

Thermal Shutdown

Under nominal load conditions, the switch re-

sistances are very low and internal power dissi-

pation is low. Under short circuit conditions,

current is limited and internal power dissipation

is higher but not extreme. Under intermediate

load conditions, both the voltage across the

switch and the current through the switch are at

intermediate values and internal power dissipa-

tion is highest. In this last condition, the die

temperature will reach the thermal limit and the

switches in both channels will be shut off. As

the die subsequently cools, the switch will turn

on again. If the load is not removed, the device

will thermal cycle in this manner to protect itself

from damage.

The SP2026 has dual sensors to detect

overtemperature. If the die temperature reaches

140

°C and channel A is in current limit, output

switch A is disabled and flag A is asserted.

Unless channel B current limits, switch B re-

mains enabled and flag B is not asserted. If the

die temperature reaches 140

°C and channel B is

in current limit, output switch B is disabled and

flag B is asserted. Unless channel A current

limits, switch A remains enabled and flag A is

not asserted. If the die exceeds 160

°C, both

output switches are disabled and both flags are

asserted. A device is disabled in thermal shut-

down until the excessive current load is removed,

the appropriate enable pins are toggled, or the

temperature cools to 120

°C.

The delay between a current limit fault and

thermal shutdown will vary with ambient tem-

perature, board layout, and load impedance, but

is typically several hundred milliseconds. A

designer can command a USB controller to

recognize the fault and disable the appropriate

channel within this time.

TYPICAL APPLICATIONS

Bypass Capacitors

A 0.1

µFto1.0µFbypasscapacitorfromtheINpin

to the GND pin is recommended to control power

supply transients. Refer to Figure 4. Without a

bypass capacitor, an output short may cause suffi-

cient ringing and damage the device. Without a

bypass capacitor, excessive supply lead induc-

tance is also a concern.

Input or output transients must not exceed the

absolute maximum supply voltage of V

+6.0V even for a short duration to avoid risk of

damage to the device.

Soft Start Condition

The soft start feature of the SP2026 is imple-

mented by holding the output turn-on rise time

to 1ms. When off, the device has high imped-

ance MOSFET channels that slowly become

low impedance as the device powers on. This

prevents an inrush current from causing voltage

drops that result from charging a capacitive load

and can pull the USB voltage bus below speci-

fied levels. This satisfies the USB voltage droop

requirements for bus-powered applications.

Refer to the circuit in Figure 6.

The SP2026 can provide inrush current limiting

forapplicationswithlargeloadcapacitanceswhere

C

a configuration that will meet USB transient regu-

lation specifications with large load capacitances.

Enable Input

The ENA and ENB control pins must be driven

to a logic high or logic low for a clearly defined

signal input. Floating these control lines may

cause unpredictable operation.