MIC2527 Datasheet, PDF (8/12 Pages) Micrel Semiconductor – Quad USB Power Control Switch

MIC2527
Applications Information
Supply Filtering
A 0.1µF to 1µF bypass capacitor from IN to GND, located at
the MIC2527, is strongly recommended to control supply
transients. 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} = 8V) even for a short
duration.
V_IN
2.7V to 5.5V
0.1µF to 1µF
MIC2527
FLGA FLGB
ENA ENB
OUTA OUTB
GND
IN
IN
GND
OUTC OUTD
ENC END
FLGC FLGD
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 repect to GND.
Current Limit Induced Thermal Shutdown
Internal circuitry increases the output MOSFET on-resis-
tance until the series combination of the MOSFET on-resis-
tance and the load impedance limit current to typically 750mA.
The increase in power dissipation, in most cases, will cause
Micrel
the MIC2527 to go into thermal shutdown, disabling affected
channels. When this is undesirable, thermal shutdown can be
avoided by externally responding to the fault and disabling
the current limited channel before the shutdown temperature
is reached. The delay between the flag indication of a current
limit fault and thermal shutdown will vary with ambient tem-
perature, board layout, and load impedance, but is typically
several hundred milliseconds. The USB controller must there-
fore recognize a fault and disable the appropriate channel
within this time.
Soft Start
The MIC2527 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.
Transient Overcurrent Filter
The inrush current from the connection of a heavy capacitive
load may cause the fault flag to fall for 10µs to 200µs while the
switch is in a constant-current mode, charging the capaci-
tance. If needed, a simple 1ms RC low-pass filter in series
with the fault flag circuit will prevent erroneous overcurrent
reporting (see Figure 2). See Application Note 17 for sugges-
tions on layout and component selection to minimize tran-
sient effects.
USB Controller
OVERCURRENT
10k
10k
0.1µF
FLGA
FLGB
FLGC
FLGD
Figure 2. Transient Filter
MIC2527
8
May 7, 1998