CS5332

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13

Ramp Size and Current Sensing

Because the current ramp is used for both the PWM ramp

and to sense current, the inductor and sense resistor values

will be constrained. A small ramp will provide a quick

transient response by minimizing the difference over which

the COMP pin must travel between light and heavy loads,

required to prevent pulse skipping and minimize pulse width

jitter. For resistive current sensing, the combination of the

inductor and sense resistor values must be chosen to provide

a large enough steady state ramp. For large inductor values

the sense resistor value must also be increased.

For inductive current sensing the RC network must meet

and DC components of the current the signal. Again the

a large enough steady state ramp with a compensated current

be required. But unlike resistive sensing, with inductive

sensing small adjustments can be made easily with the

values of R and C to increase the ramp size if needed.

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 R × C. It will

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

decay with the time constant of R × C. If this error is

excessive it will effect transient response, adaptive

positioning and current limit. During transients, the COMP

pin will be required to overshoot along with the current

will also overshoot during transients and possibly slow the

response. 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 13 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,

signal compensation the value of R1 should be 31 kΩ. Due

to the faster than ideal RC time constant there is an

overshoot of 50% and the overshoot decays with a 200 μs

threshold must be set more than 50% above the full load

current to avoid triggering hiccup mode during a large

output load step.

Figure 13. Inductive Sensing waveform during a

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

Current Limit

Two levels of overcurrent protection are provided. Any

more than the Single Phase Pulse by Pulse Current Limit, the

PWM comparator for that phase is turned off. This provides

fast peak current protection for individual phases. The

outputs of all the currents are also summed and filtered to

compare an averaged current signal to the voltage on the

the Soft Start capacitor is discharged by a 7.5 μA source until

the COMP pin reaches 0.2 V. Then Soft−Start begins. The

converter will continue to operate in this mode until the fault

condition is corrected.

Overvoltage Protection

Overvoltage protection (OVP) is provided as a result of

with synchronous rectifiers. The control loop responds to an

overvoltage condition within 400 ns, causing the top

MOSFET’s to shut off, and the synchronous MOSFET’s 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