IR3521
Page 15 V3.03
IR3521 THEORY OF OPERATION
Block Diagram
The Block diagram of the IR3521 is shown in Figure 8. The following discussions are applicable to either output
plane unless otherwise specified.
Serial VID Control
The two Serial VID Interface (SVID) pins SVC and SVD are used to program the Boot VID voltage upon assertion of
ENABLE while PWROK is de-asserted. See Table 1 for the 2-bit Boot VID codes. Both VDAC1 and VDAC2
voltages will be programmed to the Boot VID code until PWROK is asserted. The Boot VID code is stored by the
IR3521 to be utilized again if PWROK is de-asserted.
Serial VID communication from the processor is enabled after the PWROK is asserted. Addresses and data are
serially transmitted in 8-bit words. The IR3521 has three fixed addresses to control VDAC1, VDAC2, or both
VDAC1 and VDAC2 (See Table 6 for addresses). The first data bit of the SVID data word represents the PSI_L bit
and if pulled low will force all phase ICs, connected to the PSI_L pin, in to a power-saving mode. The remaining
data bits SVID[6:0] select the desired VDACx regulation voltage as defined in Table 3. SVID[6:0] are the inputs to
the Digital-to-Analog Converter (DAC) which then provides an analog reference voltage to the transconductance
type buffer amplifier. This VDACx buffer provides a system reference on the VDACx pin. The VDACx voltage along
with error amplifier and remote sense differential amplifier input offsets are post-package trimmed to provide a
0.5% system set-point accuracy, as measured in Figures 3A and 3B. VDACx slew rates are programmable by
properly selecting external series RC compensation networks located between the VDACx and the LGND pins. The
VDACx source and sink currents are derived off the external oscillator frequency setting resistor, R
ROSC
. The
programmable slew rate enables the IR3521 to smoothly transition the regulated output voltage throughout VID
transitions. This results in power supply input and output capacitor inrush currents along with output voltage
overshoot to be well controlled.
The two Serial VID Interface (SVID) pins SVC and SVD can also program the VFIX VID voltage upon assertion of
ENABLE while PWROK is equal to VCCL. See Table 2 for the 2-bit VFIX VID codes. Both VDAC1 and VDAC2
voltages will be programmed to the VFIX code.
The SVC and SVD pins require external pull-up biasing and should not be floated.
Output 1 (VDD) Adaptive Voltage Positioning
The IR3521 provides Adaptive Voltage Positioning (AVP) on the output1 plane only. AVP helps reduces the peak
to peak output voltage excursions during load transients and reduces load power dissipation at heavy load. The
circuitry related to the voltage positioning is shown in Figure 9. Resistor R
FB1
is connected between the error
amplifiers inverting input pin FB1 and the remote sense differential amplifier output, VOUT1. An internal current sink
on the FB1 pin along with R
FB1
provides programmability of a fixed offset voltage above the VDAC1 voltage. The
offset voltage generated across R
FB1
forces the converter’s output voltage higher to maintain a balance at the error
amplifiers inputs. The FB1 sink current is derived by the external resistor R
ROSC
that programs the oscillator
frequency.
The VDRP1 pin voltage is a buffered reproduction of the IIN1 pin which is connected to the current share bus
ISHARE. The voltage on ISHARE represents the system average inductor current information. At each phase IC,
an RC network across the inductor provides current information which is gained up 32.5X and then added to the
VDAC
X
voltage. This phase current information is provided on the ISHARE bus via a 3K resistor in the phase ICs.
