MVPG15x/MVPG16
Datasheet
3.1
Regulation and Startup
The step-down switching regulator uses Pulse Width Modulation (PWM) and Pulse Frequency
Modulation (PFM) modes to regulate the output voltage using digital control. The mode of operation
depends on the level of output current and the output voltage.
In steady states, the step-down switching regulator monitors the current flowing through the inductor
to determine if the regulator is handling heavy or light load applications. For heavy load applications,
the step-down regulator operates in the PWM mode (B and C) to minimize the ripple current for
optimum efficiency and to minimize the ripple output voltage. The step-down regulator operates in
the PFM and Discontinuous Conduction Mode (DCM) (A and D) to limit the switching actions for
optimum efficiency in light load applications. In this mode, the average output voltage is slightly
higher than the average output voltage for heavy transient load applications.
Figure 5: Output Voltage Window
PFM Mode
PWM Mode
PFM Mode
A
B
C
D
Typical VOUT
3.1.1
Digital Soft Start
During startup, the MVPG15x/MVPG16 provides a soft start function. Soft start reduces surge
currents from the input voltage and provides well-controlled output voltage rise characteristics. The
rate of the output voltage startup is limited by the value of the output capacitor and the internal
current limit circuitry. This combination forces the output voltage to ramp up slowly, providing a soft
start characteristic.
During soft start, the MVPG15x/MVPG16 feeds a constant current to the output capacitor in several
steps. Figure 6 shows the inductor current waveform during startup. The current limit is ramped up in
seven steps beginning at approximately 40% of the current limit rating and ending at 100% at 25 µs
per step. The buck regulator behaves like a current source during this time as the output ramps up
slowly.
Figure 7 shows that the rise time for a MVPG15x/MVPG16 increases from 20 µs at for a 0.8V output
to 70 µs for a 3.3V output with a 20 mA load. From Figure 8, the rise time can be estimated by
assuming an average charging current of 0.75A. Rise time with a 3.3V output is calculated using the
following equation.
Cout • Vout
------------------------------
RiseTime =
I
22μF • 3.3V
------------------------------
=
= 96.8μs
0.75A
Doc. No. MV-S102809-00 Rev. G
Page 24
Copyright © 2008 Marvell
April 14, 2008, 2.00
Document Classification: Proprietary
MARVELL [ MARVELL TECHNOLOGY GROUP LTD. ]
