MIC2267 Datasheet, PDF (12/19 Pages) Micrel Semiconductor – Input Current Limiting Synchronous Buck Regulator

Micrel Inc.
Application Information
FILTER
The USB 2.0 specification requires that current surges
during transitions should result in a droop at an
upstream USB port <330mV. In addition to the 10µF
allowable USB “device” input capacitance; there is also a
mandatory 120µF capacitor at each upstream Hub
VBUS terminal. This presents a challenge as there is an
amount of energy available within this hub capacitance
that can be tapped only if the droop can somehow be
limited remotely at the device end. Since the MIC2267
implements a charge limit system, this available energy
can be utilized by adjusting C_FILTER.
MIC2267
Effect of Changing FILTER Capacitor on V_OUT
C_OUT = 447µF, V_OUT = 3.3V, Load pulse width = 600µs:
Figure 2. USB Bus Powered Function Example
Where actual C_BUS is higher than 120µF, the FILTER
capacitor can be increased to allow greater current limit
delay times. This can help further reduce the value of
output bulk holdup capacitors (C_OUT) in applications that
require large, short term load pulses such as TDMA
wireless data modems.
The FILTER capacitor should be placed as close as
possible to the FILTER and VIN pins to set the input
current limit delay time. Recommended minimum value
is 100pF.
Reducing over current peak increases limit delay time in
accordance with the T_DELAY equation:
T_DELAY
≈
−ln_{⎜⎜⎝⎛}1
−
0.8
×
I_LIM
I_IN
⎟⎞
⎟⎠
×
50kΩ
×
C_FILTER
January 2011
12
M9999-011711-A