CS5308GDWR28 datasheet(14/31 Pages) ONSEMI

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CS5308

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Current Sense Amplifier (CSA) input mismatch and the

value of the current sense component will determine the

accuracy of the current sharing between phases. The worst

case Current Sense Amplifier input mismatch is ±5.0 mV

and will typically be within 3.0 mV. The difference in peak

currents between phases will be the CSA input mismatch

divided by the current sense resistance. If all current sense

components are of equal resistance a 3.0 mV mismatch with

a 2.0 m

W sense resistance will produce a 1.5 A difference in

current between phases.

External Ramp Size and Current Sensing

The internal ramp allows flexibility of current sense time

constant. Typically, the current sense RCSnCCSn time

constant (n = 1 or 2) should be equal to or slower than the

inductor’s time constant. If RC is chosen to be smaller

(faster) than L/RL, the AC or transient portion of the current

sensing signal will be scaled larger than the DC portion. This

will provide a larger steady state ramp, but circuit

performance will be affected and must be evaluated

carefully. The current signal will overshoot during transients

and settle at the rate determined by RCSn • CCSn. It will

eventually settle to the correct DC level, but the error will

decay with the time constant of RCSn • CCSn. If this error is

excessive it will effect transient response, adaptive

positioning and current limit. During a positive current

transient, the COMP pin will be required to overshoot in

response to the current signal in order to maintain the output

voltage. Similarly, the VDRP signal will overshoot which

will produce too much transient droop in the output voltage.

Single phase overcurrent will trip earlier than it would if

compensated correctly and hiccup mode current limit will

have a lower threshold for fast rise step loads than for slowly

rising output currents.

The waveforms in Figure 12 show a simulation of the

current sense signal and the actual inductor current during

a positive step in load current with values of L = 500 nH,

RL = 1.6 mW, RCSn = 20 k and CCSn = 0.01 mF. For ideal

current signal compensation the value of RCSn should be 31

W. Due to the faster than ideal RC time constant there is an

overshoot of 50% and the overshoot decays with a 200

time constant. With this compensation the ILIM pin threshold

must be set more than 50% above the full load current to

avoid triggering hiccup mode during a large output load

step.

Figure 12. Inductive Sensing Waveform During a

Load Step with Fast RC Time Constant (50 ms/div)

Current Limit

Two levels of over−current protection are provided. First,

if the voltage on the Current Sense pins (either CS1 or CS2)

exceeds CSREF by more than a fixed threshold (Single Pulse

Current Limit), the PWM comparator is turned off. This

provides fast peak current protection for individual phases.

Second, the individual phase currents are summed and

low−pass filtered to compare an averaged current signal to

a user adjustable voltage on the ILIM pin. If the ILIM voltage

is exceeded, the fault latch trips and the Soft Start capacitor

is discharged until the COMP pin reaches 0.27 V. Then Soft

Start begins. The converter will continue to operate in a low

current hiccup mode until the fault condition is corrected.

Overvoltage Protection

Overvoltage protection (OVP) is provided as a result of

the normal operation of the Enhanced V2 control topology

with synchronous rectifiers. The control loop responds to an

overvoltage condition within 400 ns, causing the top

MOSFET to shut OFF and the synchronous (lower)

MOSFET to turn ON. This results in a “crowbar” action to

clamp the output voltage and prevent damage to the load.

The regulator will remain in this state until the overvoltage

condition ceases or the input voltage is pulled low.

Transient Response and Adaptive Positioning

For applications with fast transient currents the output

filter is frequently sized larger than ripple currents require in

order to reduce voltage excursions during load transients.

Adaptive voltage positioning can reduce peak−peak output

voltage deviations during load transients and allow for a

smaller output filter. The output voltage can be set higher

than nominal at light loads to reduce output voltage sag

when the load current is applied. Similarly, the output

voltage can be set lower than nominal during heavy loads to

reduce overshoot when the load current is removed. For low

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