CM2026
D
UAL
USB P
OWER
C
ONTROL
S
WITCH
APPLICATIONS INFORMATION
Supply Filtering
A 0.1μF to 0.1μF bypass capacitor from IN to GND, located
at the device, is strongly recommended to control supply
transients. Without a bypass capacitor, an output short may
cause sufficient ringing on the input (from supply lead
inductance) to damage internal control circuitry.
Input or output transients must not exceed the absolute
maximum supply voltage (V
IN max
= 6V) even for a short
duration.
Transient Overcurrent Filter
When the CM2026 is enabled, large values of capacitance at
the output of the device will cause inrush current to exceed the
short circuit current-limit threshold of the device and assert the
flag. The duration of this time will depend on the size of the
output capacitance. Refer to the "Functional Characteristics"
turn-on and turnoff behaviors for details. During the
capacitance charging time, the device enters into
constant-current mode. As the capacitance is charged, the
current decreases below the short circuit current-limit
threshold old, and the flag will then be deserted.
In USB applications, it is required that output bulk capacitance
is utilized to support hot-plug events. When the CM2026 is
enabled, the flag may go active for about 1ms due to inrush
current exceeding the current-limit setpoint. Additionally,
during hot-plug events, inrush currents may also cause the
flag to go active for 30uS. Since these conditions are not valid
overcurrent faults, the USB controller must ignore the flag
during these events. To prevent this erroneous overcurrent
reporting, a 1ms RC filter as shown in Figure 4 may be
programmed into the USB logic controller, eliminating the
need for the RC filter.
Enable Input
EN must be driven logic high or logic low for a clearly defined
input. Floating the input may cause unpredictable operation.
EN should not be allowed to go negative with respect to GND.
Soft Start
The CM2026 presents a high impedance when off, and slowly
becomes a low impedance as it turns on. This reduces inrush
current and related voltage drop that results from charging a
capacitive load, satisfying the USB voltage droop
requirements for bus-powered applications as shown in Figure
2.
The soft start circuit shown in Figure 3 can be utilized to meet
USB transient regulation specifications with large load
capacitances (C
BULK
> 10uF). The CM2026 will provide inrush
current limiting for these applications.
2002/05/29
Rev. 2
Champion Microelectronic Corporation
Page 5
CHAMP [ CHAMPION MICROELECTRONIC CORP. ]
