MIC2526
Micrel
Soft Start
The MIC2526 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 ca-
pacitances (C
BULK
> 10uF). The MIC2526 will provide inrush
current limiting for these applications.
Transient Overcurrent Filter
When the MIC2526 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 thresh-
old, and the flag will then be deasserted.
Applications Information
Supply Filtering
A 0.1µF to 1µF bypass capacitor from IN to GND, located at
the device, is strongly recommended to control supply tran-
sients. Without a bypass capacitor, an output short may
cause sufficient ringing on the input (from supply lead induc-
tance) 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.
3.0V to 5.5V
MIC2526
ENA
FLGA
FLGB
ENB
OUTA
IN
GND
OUTB
0.1µF to 1µF
Figure 1. Supply Bypassing
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.
USB
Controller
MIC2526-xBM
V
BUS
1
2
3
ENA
FLGA
FLGB
ENB
OUTA
IN
GND
OUTB
8
7
6
5
0.1µF
C
BULK
USB Host
4.7
µF
Capacitive
Load
4
GND
Cable
Bus Powered Hub
Cable
Downstream USB Device
Figure 2. Soft Start (Single Channel)
USB
Controller
MIC2526-2BM
V
BUS
4.7
µF
USB Hub
1
2
3
4
ENA
FLGA
FLGB
ENB
OUTA
IN
GND
OUTB
8
7
6
5
USB
Function
C
BULK
USB
Function
C
BULK
GND
Cable
USB Peripheral
Figure 3. Inrush Current-Limit Application
February 2000
9
MIC2526
MICREL [ MICREL SEMICONDUCTOR ]
