Micrel, Inc.
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
Serial
Controller.
MIC2555’s
Serial
Controller
communicates with the OTG Controller as an I2C slave
using the SCL and SDA pins.
The Serial Controller includes the following functions:
•
•
•
•
•
•
Control registers
Status registers
Interrupt latches
Interrupt enable registers
Interrupt clear registers
Interrupt generator
Session
Valid
1.4V
V
BUS
Valid
MIC2555
V
BUS
Output
Enable
Cap-
Cap+
Cap++
V
BAT
Charge Pump
and Regulator
V
TRM
3.3V
2.3k
Charge
V
BUS
V
BUS
4.4V
VBUS Charge Pump and 5V regulator
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
VBUS. As VBAT increases, the maximum charge pump
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
VBUS, the charge pump can be shut down to conserve
power.
Session
End
0.5V
2.3k
Discharge
V
BUS
68k
VBUS Circuitry
VBUS Resistors and Switches
MIC2555 is able to:
•
•
•
•
charge up VBUS through a resistor
initiate SRP
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.
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.
December 2006
12
M9999-121406
MICREL [ MICREL SEMICONDUCTOR ]
