CS5305GDWR28 datasheet(17/33 Pages) ONSEMI

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CS5305

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will decay with the time constant (Rcsx)(Ccsx). The VDRP

voltage will also overshoot and response will be slowed,

since the current signal is a component of that voltage. The

single phase current limit will trip earlier since the current

signal appears larger than it should be, and the module

current limit will have a lower threshold for fast transients

than it will for slow transients. Additional external

components in the droop circuit and in the error amp

compensation will correct this condition. Details are provided

in the data sheet section on choosing external components.

Adaptive Voltage Positioning

Adaptive voltage positioning is a technique used to reduce

peak−to−peak output deviations during output current

transients. The output voltage is set higher than nominal at

light loads to reduce output voltage sag when load current is

suddenly increased. Similarly, output voltage is set lower

than nominal at heavy loads to reduce overshoot when load

current suddenly decreases. The CS5305 implements

adaptive voltage positioning by placing a resistor divider

between VDRP and VOUT. The center tap of the divider

connects to VFB. These resistors, along with two or three

other external components, implement a lossless droop

voltage function.

Past implementations of adaptive voltage positioning

used a droop resistor. This resistor was placed in series

between the regulation point of the output voltage and the

load. Increasing the current to the load caused the voltage at

the load to droop below the regulation point. The amount of

droop was equal to the change in current multiplied by the

droop resistor value. This method was acceptable for low

values of output current, where the droop resistor provided

a minimal change in voltage without dissipating a great deal

of power. Higher output current levels and tighter droop

voltage requirements in today’s microprocessors have

rendered this droop resistor technique unusable. The

lossless technique solves these problems.

The AVP function addresses DC and slow transient output

voltage positioning. Response during the first few hundreds

of nanoseconds of a transient are addressed primarily by the

power stage output impedance, and the ESR and ESL of the

output filter. The ramp size and the error amplifier

compensation control the transition between these two

regions. If ramp size is too large or the error amp is too slow,

there will be a long transition to the final voltage after a

transient. This will be most apparent if the output

capacitance is low.

Figure 28 shows how adaptive positioning works. The

waveform labeled “normal” shows output voltage for a

converter without adaptive voltage positioning. The voltage

sags when current steps up, returns to its nominal value and

then overshoots when the current load is decreased. Using

a slow adaptive positioning circuit can actually worsen

performance. The slow adaptive positioning waveform

above shows the output voltage sag, but the voltage recovers

to its initial value before the adaptive positioning circuit

becomes active. When the load decreases, the overshoot

causes the output voltage to exceed the upper limit. The fast

adaptive positioning waveform shows how AVP can reduce

transient voltage requirements by about one half compared

to a “normal” converter.

Adaptive Positioning

Normal

Fast

Slow

Limits

Figure 28. Adaptive Positioning

Current Limit

The CS5305 features two separate current limit circuits.

First, the per−phase current limit terminates topside switch

conduction in a phase if the voltage between any CSx pin

and CSREF exceeds a typical value of 90 mV. This provides

fast peak current protection for individual phases. In

addition, the output current signals for all three phases are

summed and filtered to provide an average module current

signal. This signal is compared to a voltage that is

user−programmable. If this voltage is exceeded, the fault

latch is set and the COMP capacitor is discharged by a 5 μA

current sink until the COMP voltage falls below 0.2 V. The

soft−start cycle begins when this threshold is reached, and the

converter will operate in hiccup−mode until the overcurrent

condition is cleared.

Error Amplifier

The CS5305 uses the Enhanced V2 control method to

offer the fastest and most accurate regulation available. One

of the features of this control method is ease of error

amplifier compensation. A single capacitor placed from the

COMP pin to ground is sufficient to adequately stabilize the

error amplifier.

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