Atmel ATA6264 - Manual (Page 35)

35
4929B–AUTO–01/07
ATA6264 [Preliminary]
12. VCORE Power Supply
The voltage of the VCORE regulator is generated out of the K30 voltage using a step-down reg-
ulator as long as the K30 voltage is available. During times when K30 is not present
(power-down or stand-alone time), the VCORE regulator is supplied out of VEVZ. Depending on
the initial programming, the supply switch signal is derived from the CORESWAP comparator or
the EVZEN comparator. The VCORE voltage can be programmed via the serial interface to 3
different voltage values during initial programming. In the case of short spikes, a logic circuit dis-
ables multiple-pulse operation during one oscillating period. The regulator uses both current and
voltage feedback. In the following cases, the output transistor of the regulator is switched off at
once and may be switched on again with the beginning of the next clock period:
1. If the current through the transistor exceeds the output current limit value, the transistor
is switched off immediately.
2. If overvoltage is detected at the pin VCORE, the transistor is switched off immediately.
3. If the feedback voltage at the pin VCORE is missing (disconnected pin), the regulator is
switched off.
Figure 12-1. Functional Principle of the VCORE Regulator
In order to trim the compensation of the regulation loop and to improve the behavior at load
changes, pin COMCOO has to be connected to COMCOI via a compensation network. Because
of the fact that current-mode-controlled converters exhibit sub-harmonic oscillations when oper-
ating at duty cycles larger than 50%, a slope compensation (which adds an artificial ramp to the
comparator) is implemented. If the regulator input voltage at pin EVZ or pin K30 is too low, the
regulator switches to a duty cycle of 100% (Permanent-on mode). Backward feeding of EVZ and
K30 is avoided. In order to ensure the gate voltage for the output transistors of the regulator, the
driver stages are supplied by the charge pump (pin CP).
VCORE
VCORE
-
+
-
+
EVZ
SVCORE
K30
Logic and
driver
Slope
compensation
Comp.
OSC
Error
amp.
Bandgap
reference
Overvoltage
Control-
signal
K30/EVZ
Overcurrent
Q
R
S
Current
measurement
and leading edge
blanking
SPI
OTP
+
Slope
compensation
Current
measurement
and leading edge
blanking
COMCOI
COMCOO
CP