ISL6277HRZ datasheet(31/37 Pages) RENESAS

31 / 37 page

ISL6277

FN8270 Rev 1.00

Page 31 of 37

Mar 8, 2012

Load Line Slope

See Figure 14 for load-line implementation.

For inductor DCR sensing, substitution of Equation 27 into

Equation 2 gives the load-line slope expression:

For resistor sensing, substitution of Equation 31 into Equation 2

gives the load line slope expression:

Substitution of Equation 28 and rewriting Equation 34, or

substitution of Equation 32 and rewriting Equation 35, gives the

same result as in Equation 36:

One can use the full-load condition to calculate Rdroop. For

example, given Iomax = 65A, Idroopmax = 45µA and LL = 2.1mΩ,

Equation 36 gives Rdroop = 3.03kΩ.

It is recommended to start with the Rdroop value calculated by

Equation 36 and fine-tune it on the actual board to get accurate

load-line slope. One should record the output voltage readings at

no load and at full load for load-line slope calculation. Reading

the output voltage at lighter load instead of full load will increase

the measurement error.

Compensator

Figure 23 shows the desired load transient response waveforms.

Figure 29 shows the equivalent circuit of a voltage regulator (VR)

with the droop function. A VR is equivalent to a voltage source

(= VID) and output impedance Zout(s). If Zout(s) is equal to the

load-line slope LL, i.e., a constant output impedance, then in the

entire frequency range, Vo will have a square response when Io

has a square change.

Intersil provides a Microsoft Excel-based spreadsheet to help

design the compensator and the current sensing network so that

VR achieves constant output impedance as a stable system.

A VR with active droop function is a dual-loop system consisting of

a voltage loop and a droop loop, which is a current loop. However,

neither loop alone is sufficient to describe the entire system. The

spreadsheet shows two loop gain transfer functions, T1(s) and

T2(s), that describe the entire system. Figure 30 conceptually

shows T1(s) measurement set-up, and Figure 31 conceptually

shows T2(s) measurement set-up. The VR senses the inductor

current, multiplies it by a gain of the load-line slope, adds it on top

of the sensed output voltage, and then feeds it to the

compensator. T1 is measured after the summing node, and T2 is

measured in the voltage loop before the summing node. The

spreadsheet gives both T1(s) and T2(s) plots. However, only T2(s)

can actually be measured on an ISL6277 regulator.

T1(s) is the total loop gain of the voltage loop and the droop loop.

It always has a higher crossover frequency than T2(s), therefore

has a higher impact on system stability.

T2(s) is the voltage loop gain with closed droop loop, thus having

a higher impact on output voltage response.

Design the compensator to get stable T1(s) and T2(s) with sufficient

phase margin and an output impedance equal to or smaller than

the load-line slope.

Current Balancing

Refer to Figures 15 through 22 for information on current

balancing. The ISL6277 achieves current balancing through

matching the ISEN pin voltages. Risen and Cisen form filters to

Vdroop

-------------------

Rdroop

-------------------

Rntcnet

Rsum

---------------

------------------------------------------ DCR

-------------



(EQ. 34)

Vdroop

-------------------

Rsen Rdroop



NRi



---------------------------------------

(EQ. 35)

Rdroop

Idroop

---------------- LL



(EQ. 36)

FIGURE 29. VOLTAGE REGULATOR EQUIVALENT CIRCUIT

VID

Zout(s) = LL

LOAD

FIGURE 30. LOOP GAIN T1(s) MEASUREMENT SET-UP

COUT

VIN

GATE

DRIVER

COMP

MOD.

LOAD LINE SLOPE

VID

CHANNEL B

CHANNEL A

EXCITATION OUTPUT

ISOLATION

TRANSFORMER

LOOP GAIN =

CHANNEL B

CHANNEL A

NETWORK

ANALYZER



FIGURE 31. LOOP GAIN T2(s) MEASUREMENT SET-UP

VIN

GATE

DRIVER

COMP

MOD.

LOADLINESLOPE

VID

CHANNEL B

CHANNEL A

EXCITATION OUTPUT

ISOLATION

TRANSFORMER

LOOP GAIN =

CHANNEL B

CHANNEL A

NETWORK

ANALYZER



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