ISL6353_14 Datasheet, PDF (22/30 Pages) Intersil Corporation – Multiphase PWM Regulator for VR12 DDR Memory Systems

ISL6353
and the other two phases inductor output pads. Equations 29
through 31 give the ISEN pin voltages:
V_ISEN1 = V_1p + V_2n + V_3n
(EQ. 29)
V_ISEN2 = V_1n + V_2p + V_3n
(EQ. 30)
V_ISEN3 = V_1n + V_2n + V_3p
(EQ. 31)
The ISL6353 will make V_ISEN1 = V_ISEN2 = V_ISEN3 as in
Equations 32 and 33:
V_1p + V_2n + V_3n = V_1n + V_2p + V_3n
(EQ. 32)
V_1n + V_2p + V_3n = V_1n + V_2n + V_3p
(EQ. 33)
Rewriting Equation 32 gives Equation 34:
V_1p – V_1n = V_2p – V_2n
and rewriting Equation 33 gives Equation 35:
V_2p – V_2n = V_3p – V_3n
Combining Equations 34 and 35 gives Equation 36:
V_1p – V_1n = V_2p – V_2n = V_3p – V_3n
Therefore:
R_dcr1 × I_L1 = R_dcr2 × I_L2 = R_dcr3 × I_L3
(EQ. 34)
(EQ. 35)
(EQ. 36)
(EQ. 37)
Current balancing (I_L1 = I_L2 = I_L3) is achieved when
R_dcr1 = R_dcr2 = R_dcr3. R_pcb1, R_pcb2 and R_pcb3 will not have any
effect.
Since the slave ripple capacitor voltages mimic the inductor
currents, the R³â„¢ modulator can naturally achieve excellent current
balancing during steady-state and dynamic operation. The inductor
currents follow the load current dynamic change, with the output
capacitors supplying the difference. The inductor currents can
track the load current well at low rep rate, but cannot keep up
when the rep rate gets into the hundred-kHz range, where it is out
of the control loop bandwidth. The controller achieves excellent
current balancing in all cases.
CCM Switching Frequency
The resistor connected between the COMP pin and the VW pin
sets the VW windows size, therefore setting the steady state
PWM switching frequency. When the ISL6353 is in continuous
conduction mode (CCM), the switching frequency is not
absolutely constant due to the nature of the R³ modulator. As
explained in the “Multiphase R3 Modulator” on page 11, the
effective switching frequency will increase during load step-up
and will decrease during load step-down to achieve fast transient
response. On the other hand, the switching frequency is relatively
constant at steady state. Equation 38 gives an estimate of the
frequency-setting resistor R_fset value. 20kΩ R_fset gives
approximately 300kHz switching frequency. Lower resistance
yields higher switching frequency.
R_fset(Ω)
=
1.293
⋅
10^–7
⋅
F_S
2
W
–
0.1445
⋅
F_S
W
+
52055
(EQ. 38)
Phase Count Configurations
The ISL6353 can be configured for 1, 2 or 3-phase operation.
For 2-phase configuration, tie the PWM3 pin to VDD. Phase 1 and
Phase 2 PWM pulses are 180° out-of-phase. Leave the ISEN3 pin
open for 2-phase configuration.
For 1-phase configuration, tie the PWM3 and ISEN2 pins to VDD.
In this configuration, only Phase 1 is active. The ISEN3, ISEN2,
ISEN1, and FB2 pins are not used because there is no need for
current balancing or the FB2 function.
Modes of Operation
TABLE 3. ISL6353 MODES OF OPERATION
CONFIGURATION
PS#
OPERATIONAL MODE
3-phase Configuration
PS0 3-phase CCM
PS1 2-phase CCM or
1-phase CCM
PS2 1-phase DE
PS3 1-phase DE
2-phase Configuration
PS0 2-phase CCM
PS1 1-phase CCM
PS2 1-phase DE
PS3 1-phase DE
1-phase Configuration
PS0 1-phase CCM
PS1 1-phase CCM
PS2 1-phase DE
PS3 1-phase DE
Table 3 shows the modes of operation for the various power states
programmed using the SetPS command through the SVID bus or
by changing the state of the PSI pin. Table 3 is used in conjunction
with the status of the PROG2 pin. Refer to Table 7 for the PROG2
programming options.
Dynamic Operation
The controller responds to VID changes by slewing to the new
voltage at a slew rate indicated in the SetVID command. There
are three SetVID slew rates SetVID_fast, SetVID_slew and
SetVID_decay.
The SetVID_fast command prompts the controller to enter CCM
and to actively drive the output voltage to the new VID value at a
minimum 10mV/µs slew rate.
The SetVID_slow command prompts the controller to enter CCM
and to actively drive the output voltage to the new VID value at a
minimum 2.5mV/µs slew rate.
The SetVID_decay command prompts the controller to enter DE
mode. The output voltage will decay down to the new VID value at
a slew rate determined by the load. If the voltage decay rate is
22
September 15, 2011
FN6897.0