ISL62883 Datasheet, PDF(22/39 Page) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5 Mobile CPUs and GPUs

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ISL62883 Datasheet, PDF (22/39 Pages) Intersil Corporation – Multiphase PWM Regulator for IMVP-6.5 Mobile CPUs and GPUs

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ISL62883, ISL62883B

ISUM+

Resistor Current-Sensing Network

Phase1 Phase2 Phase3

Rntcs

Cn.1

Rntc

OPTIONAL

Cn.2 Vcn

Ri ISUM-

Rip Cip

OPTIONAL

FIGURE 19. OPTIONAL CIRCUITS FOR RING BACK

REDUCTION

Figure 18 shows the output voltage ring back problem

during load transient response. The load current io has a

fast step change, but the inductor current iL cannot

accurately follow. Instead, iL responds in first order

system fashion due to the nature of current loop. The

ESR and ESL effect of the output capacitors makes the

output voltage Vo dip quickly upon load current change.

However, the controller regulates Vo according to the

droop current idroop, which is a real-time representation

of iL; therefore it pulls Vo back to the level dictated by iL,

causing the ring back problem. This phenomenon is not

observed when the output capacitor have very low ESR

and ESL, such as all ceramic capacitors.

Figure 19 shows two optional circuits for reduction of the

ring back. Rip and Cip form an R-C branch in parallel with

Ri, providing a lower impedance path than Ri at the

beginning of io change. Rip and Cip do not have any

effect at steady state. Through proper selection of Rip

and Cip values, idroop can resemble io rather than iL, and

Vo will not ring back. The recommended value for Rip

is100Î©. Cip should be determined through tuning the

load transient response waveforms on an actual board.

The recommended range for Cip is 100pF~2000pF.

Cn is the capacitor used to match the inductor time

constant. It usually takes the parallel of two (or more)

capacitors to get the desired value. Figure 19 shows that

two capacitors Cn.1 and Cn.2 are in parallel. Resistor Rn is

an optional component to reduce the Vo ring back. At

steady state, Cn.1+Cn.2 provides the desired Cn

capacitance. At the beginning of io change, the effective

capacitance is less because Rn increases the impedance

of the Cn.1 branch. As explained in Figure 16, Vo tends to

dip when Cn is too small, and this effect will reduce the

Vo ring back. This effect is more pronounced when Cn.1 is

much larger than Cn.2. It is also more pronounced when

Rn is bigger. However, the presence of Rn increases the

ripple of the Vn signal if Cn.2 is too small. It is

recommended to keep Cn.2 greater than 2200pF. Rn

value usually is a few ohms. Cn.1, Cn.2 and Rn values

should be determined through tuning the load transient

response waveforms on an actual board.

DCR

Rsen Rsen Rsen

Rsum

ISUM+

Vcn

Ri ISUM-

FIGURE 20. RESISTOR CURRENT-SENSING NETWORK

Figure 20 shows the resistor current-sensing network for

a 3-phase solution. Each inductor has a series

current-sensing resistor Rsen. Rsum and Ro are

connected to the Rsen pads to accurately capture the

inductor current information. The Rsum and Ro resistors

are connected to capacitor Cn. Rsum and Cn form a a

filter for noise attenuation. Equations 25 thru 27 give

VCn(s) expression:

VCn(s)

R-----s---e---n--

(

)

(EQ. 25)

ARsen(s)

-----------1-----------

1 + -Ï----s-s--n---s-

ÏRsen

-------------1---------------

R-----s---u---m---

(EQ. 26)

(EQ. 27)

Transfer function ARsen(s) always has unity gain at DC.

Current-sensing resistor Rsen value will not have

significant variation over temperature, so there is no

need for the NTC network.

Cn = 5600pF.

Overcurrent Protection

Refer to Equation 1 and Figures 9, 14 and 20; resistor Ri

sets the droop current Idroop. Table 3 shows the internal

OCP threshold. It is recommended to design Idroop

without using the Rcomp resistor.

For example, the OCP threshold is 60ÂµA for 3-phase

solution. We will design Idroop to be 38.8ÂµA at full load,

so the OCP trip level is 1.55 times of the full load current.

For inductor DCR sensing, Equation 28 gives the DC

relationship of Vcn(s) and Io(s).

Substitution of Equation 28 into Equation 1 gives

Equation 29:

FN6891.2

February 25, 2010

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