NCP5331 datasheet(29/38 Pages) ONSEMI

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NCP5331

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RS1

CS1

IMAX/2

GVDRP

RS2

CS2

IMAX/2

GVDRP

CSREF

COMP

Error

Amp

VID Setting

IBIASVFB

RDRP

RF1

VDRP = VID +

IMAX • RL • GVDRP

VFB = VID

VCORE

IDRP

IFBK

VCORE = VID - (IDRP - IBIASVFB) w RF1

Figure 34. AVP Circuitry at Full-Load

IDRP = IMAX • RL • GVDRP/RDRP

IFBK = IDRP - IBIASVFB

+ -

Figure 35. VDRP Tuning, RC Time Too Long

NOTE:

The RC time constant of the current sense network is

too long (slow); VDRP and VCORE respond too slowly.

Figure 36. VDRP tuning, RC Time Too Short

NOTE:

The RC time constant of the current sense network is

too short (fast); VDRP and VCORE both overshoot.

7. Current Sensing

For inductive current sensing, choose the current sense

network (RSx, CSx) to satisfy

RSx

@ CSx + Lo (RL ) RPCB)

(32)

For resistive current sensing, choose the current sense

network (RSx, CSx) to satisfy

RSx

@ CSx + Lo (Rsense)

(33)

This will provide an adequate starting point for RSx and

CSx. After the converter is constructed, the value of RSx

(and/or LSx) should be fine-tuned in the lab by observing

the VDRP signal during a step change in load current. Tune

the RSx

⋅ CSx network to provide a “square-wave” at the

VDRP output pin with maximum rise time and minimal

overshoot as shown in Figures 34 - 36.

Figure 37. VDRP Tuning, RC Time Optimal

NOTE:

The RC time constant of the current sense network is

optimal; VDRP and VCORE respond to the load current

quickly without overshooting.

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