Micrel, Inc.
MIC2555
VBUS
Output
Enable
Circuits still operating:
Cap-
Cap+
Cap++
ID detect and D- receiver continue to function. This
includes the ID comparators, ID pull-up circuits, and
D- data receiver.
Charge Pump
and Regulator
VBAT
Serial Controller Block
The Serial Controller manages MIC2555 operations.
Turning ON/OFF features, changing operating modes,
setting and selecting interrupts are all handled by the
2.3k
VTRM
3.3V
VBUS
Charge
VBUS
Serial
Controller.
MIC2555’s
Serial
Controller
communicates with the OTG Controller as an I2C slave
using the SCL and SDA pins.
VBUS
Valid
4.4V
The Serial Controller includes the following functions:
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•
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Control registers
Session
Valid
Status registers
1.4V
0.5V
Interrupt latches
Interrupt enable registers
Interrupt clear registers
Interrupt generator
Session
End
VBUS Charge Pump and 5V regulator
2.3k
68k
The charge pump draws power from VBAT and boosts the
voltage to the requisite 5V to power VBUS. This
subsystem is actually a combination of a charge pump
circuit and a control loop that gates the charge pump’s
oscillator. If the output voltage is below 5V, then the
oscillator is ON; otherwise, the oscillator is gated OFF.
The charge pump’s maximum output is controlled by the
magnitude of VBAT. When VBAT is at 3.0V, the charge
pump is designed to support loads of at least 8 mA on
Discharge
VBUS
VBUS Circuitry
VBUS Resistors and Switches
MIC2555 is able to:
VBUS. As VBAT increases, the maximum charge pump
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•
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•
•
charge up VBUS through a resistor
initiate SRP
output current also increases. For proper operation the
charge pump circuit requires 2 capacitors; one for the
voltage doubler, connected between C- and C+, and a
reservoir/filter capacitor between C++ and ground. The
charge pump’s nominal operating frequency is 200 kHz,
which is set by an on-chip oscillator. A special feature of
MIC2555 is that an external oscillator can drive the
charge pump as well, allowing the designer to shift
radiated noise away from sensitive frequencies when
necessary. Also, when 5V power is not required from
pull down VBUS through a resistor to ground
discharge VBUS before initiating SRP
switch VBUS power from the charge pump ON/OFF
Dedicated bits in the control registers control all of these
functions. Because these bits act independently, it is
possible to have VBUS both charging and discharging at
the same time. This situation will not harm the MIC2555.
VBUS, the charge pump can be shut down to conserve
power.
To prevent system leakage currents from biasing VBUS to
a voltage that would mimic a session valid condition, the
MIC2555 maintains a 68K resistor between VBUS and
ground to insure that at no time will VBUS assume a
floating condition.
M9999-060805
(408) 955-1690
June 2005
12
MICREL [ MICREL SEMICONDUCTOR ]
