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ADP3207 Datasheet(PDF) 14 Page - ON Semiconductor

Part No. ADP3207
Description  CPU Synchronous Buck Controller
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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ADP3207 Datasheet(HTML) 14 Page - ON Semiconductor

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ADP3207
Rev. 1 | Page 14 of 29 | www.onsemi.com
mask is set by an internal timer to be about 100 μs. In
conditions where a larger than 200 mV voltage drop occurs
during deeper sleep entry or slow deeper sleep exit, the
duration of PWRGD masking is extended by an internal logic
circuit.
POWER-UP SEQUENCE AND SOFT START
The power-on ramp-up time of the output voltage is set with a
capacitor tied from the SS pin to GND. The capacitance on the
SS pin also determines the current-limit latch-off time as
explained in the Soft Transient section. The whole power-up
sequence, including soft start, is illustrated in Figure 9.
In VCC UVLO or in shutdown, the SS pin is held at zero
potential. When VCC ramps above the upper UVLO threshold
and EN is asserted high, the ADP3207 enables internal bias and
starts a reset cycle that lasts about 50 μs to 60 μs. Next, when
initial reset is over, the chip detects the number of phases set by
the user, and gives a go signal to ramp up the SS voltage. During
soft start, the external SS capacitor is charged by an internal
8 μA current source. The VCORE voltage follows the ramping SS
voltage up to the VBOOT voltage level, which is determined by a
burnt-in VID code (the 1.2 V code by IMVP-6 specification).
While VCORE is being regulated at VBOOT voltage, the SS capacitor
continues to rise. When the SS pin voltage reaches 1.7 V, the
ADP3207 asserts the CLKEN signal low, given that the VCORE
voltage is within the power-good window of VBOOT. The
ADP3207 reads the VID codes provided by the CPU on VID0
to VID6 input pins. The VCORE voltage changes from VBOOT to
the VID voltage by a well controlled soft transition, as
introduced in the Soft Transient section. Meanwhile, the SS pin
voltage is quickly charged up to a clamp voltage of 2.9 V.
The PWRGD signal is not asserted until there is a tCPU_PWRGD
delay of about 3 ms to 10 ms as specified by the IMVP-6. The
power-good delay can be programmed by the capacitor
connected from PGDELAY to GND. Before the CLKEN signal
is asserted low, PGDELAY is reset to zero. After the assertion of
the CLKEN signal, an internal source current of 2 μA starts
charging up the external capacitor on the PGDELAY pin.
Assuming the VCORE voltage is settled within the power-good
window defined by the VID DAC voltage, the PWRGD signal is
asserted high when the PGDELAY voltage reaches the 2.9 V
power-good delay termination threshold.
If either EN is taken low or VCC drops below the lower VCC
UVLO threshold, then both the SS capacitor and PGDELAY
capacitor are reset to ground to be ready for another soft-start cycle.
VCC
EN
SS
CLKEN
PWRGD
VCORE
tCPU_PWRGD
VVID
VBOOT
1.2V
1.7V
2.9V
Figure 9. Power-Up Sequence
SOFT TRANSIENT
The ADP3207 provides a soft transient function to reduce inrush
current during various transitions, including the entrance/exit of
deeper sleep and the transition from VBOOT to VID voltage.
Reducing the inrush current helps decrease the acoustic noise
generated by the MLCC input capacitors and inductors.
The soft transient feature is implemented with an STSET buffer
amplifier that outputs constant sink or source current on the
STSET pin where an external capacitor is connected. The
capacitor is used to program the slew rate of VCORE voltage
during any VID voltage transient. During steady-state
operation, both the reference input of the voltage error
amplifier and the STSET amplifier are connected to the VID
DAC output. Consequently, the STSET voltage is a buffered
version of VID DAC output. When system signals trigger a soft
transition, the reference input of the voltage error amplifier
switches from the DAC output to the STSET output, while the
input of the STSET amplifier remains connected to the DAC.
The STSET buffer input sees the almost instantaneous VID
voltage change and tries to track it. Tracking is not
instantaneous because the buffer slew rate is limited by the
source/sink current capability of the STSET output. Therefore,
VCORE voltage follows the VID DAC output voltage change with
a controlled slew rate. When the transient period is complete,
the reference input of the voltage amplifier switches back to the
VID DAC output to ensure higher accuracy.


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