ISL6398 Datasheet, PDF (32/57 Pages) Intersil Corporation – Programmable soft-start rate and DVID rate

ISL6398
R_AUTO  -¹---^.--²---^V-₆---₄--^---^N_---_D-^M----_C-^A----_R^X-----^-_--^R-_I--₁--^S----^E---^T--
(EQ. 31)
I_
N


I
1



1
+
K^-1--

N
–
1

(EQ. 32)
Equations 30 and 31 helps approximate the AUTO resistor, while
the trip point hysteresis can be programmed via PMBus D1[5:4].
Typically, the higher the inductor ripple current, the higher
percentage of hysteresis and k it requires. Following is an easy
way to estimate R_AUTO value:
1. Before AUTO trip point tuning, calibrate IMON current close to
zero with PMBus E4[4:0].
2. Disable AUTO mode via D4[2] or by tying AUTO pin GND
3. Obtain efficiency curve for 1- to 6-phase, programmed via
D0,with appropriate load (~25A/Phase. 0.5A step); and APA
disabled via D4h as needed.
4. Determine I1 from the above test result.
5. Short AUTO pin to ground with a current meter to measure the
IMON current (I_{IMON_OPTIMIZED_1_PHASE}) when VR is at I1 load.
6. Calculate R_AUTO as in Equation 31.
7. Solder down R_AUTO and enable AUTO mode.
8. Take efficiency curve and compare it with the 1-to 6-Phase
Efficiency Curves.
9. Tweak R_AUTO and D1 (k, I1, Hysteresis) as needed for optimal
Efficiency performance at targeted operating input and
output voltage as well as airflow.
10. Obtain efficiency curve for couple boards and tweak R_AUTO to
re-center overall efficiency of these boards.
PHASE
SHEDDING
ISL6398
CURRENT
THRESHOLD
SELECTION
AUTO
EXTERNAL CIRCUIT
R_AUTO
FIGURE 31. SIMPLIFIED AUTO-PHASE SHEDDING CIRCUIT
In addition, the SMBus, PMBus, or I²C gives flexibility to program
Auto K-factor (D1[3:2]), hysteresis (D1[5:4]), I1 (D1[7:6]), and
number of operating phases after soft-start and when auto mode
is disabled by D4[2]. However, all phases will be added back
when APA is triggered; and if APA is disabled, all phase are
added back only after reset or OCP retry.
The minimum of auto-phase shedding is defaulted by NPSI in
PSI1 mode and can also be programmed by the bus command
code D1h[1:0], as in Table 14. The phase dropping sequence is
summarized in Table 8.
TABLE 8. PHASE DROPPING SEQUENCE
PWM# TIED TO V_CC
N
(VCORE)
PHASE SEQUENCE
6
NONE
6-3-5-2-4-1
5
PWM6
5-4--2-3-1
4
PWM5
4-2-3-1
3
PWM4
3-2-1
2
PWM3
2-1
To ensure dropped phases have sufficient energy to turn on
high-side MOSET and sustain instant load apply after VR0 staying
in light load condition for a long time (hours to days), a
boot-refresh circuit turns on Low-side MOSFET of each dropped
phase to refresh the boot capacitor at a rate of slightly above
20kHz. The boot-fresh circuit is automatically turned off to boot
efficiency when DAC drops to 0.60V.
Resistor Reader (Patented)
Intersil has developed a high resolution ADC using a patented
technique with simple 1%, 100ppm/k or better temperature
coefficient resistor divider. The same type of resistors are
preferred so that it has similar change over-temperature. In
addition, the divider is compared to the internal divider off V_CC
and GND nodes and therefore must refer to VCC and GND pins,
not through any RC decoupling network.
ISL6398
EXTERNAL CIRCUIT
V_CC
REGISTER
TABLE
ADC
R_UP
R_DW
FIGURE 32. SIMPLIFIED RESISTOR DIVIDER ADC
NVM_BANK_BT pin is designed to allow the user to select
different NVM Banks (Pre-configured IC) and/or Boot Voltage
Level even without PMBus communications; while VRSEL_ADDR
is to select different operation mode (5mV or 10mV), indirectly
control Boot Voltage Level and DAC resolution, and PMBus
Addresses. The programmed values of resistor reader are stored
in DC and DD of PMBus.
As an example, Table 9 shows the R_UP and R_DW values of each
pin for a specific system design; DATA for corresponding registers
can be read out via PMBus commands (DC and DD). In addition,
some tie-high and tie-low options are available for easy
programming (save resistor dividers) and can also be used to
validate the VR operation during In-Circuit Test (ICT). For instance,
when the system boot voltage is required at 0V, the
NVM_BANK_BT pin can be set to different voltage level, prior to
power-up, to get a known boot voltage to check VR operation with
ICT. Resistor reader calculator is available, please contact Intersil
Application support at www.intersil.com/design.
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FN8575.1
August 13, 2015