LTM4664 datasheet(71/138 Pages) AD

71 / 138 page

LTM4664A

Rev. 0

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CHECKING TRANSIENT RESPONSE

The regulator loop response can be checked by looking at

the load current transient response. Switching regulators

take several cycles to respond to a step in DC (resistive)

load current. When a load step occurs, VOUT shifts by an

amount equal to ΔILOAD(ESR), where ESR is the effective

series resistance of COUT. ΔILOAD also begins to charge or

discharge COUT generating the feedback error signal that

forces the regulator to adapt to the current change and

return VOUT to its steady-state value. During this recov-

ery time VOUT can be monitored for excessive overshoot

or ringing, which would indicate a stability problem. The

availability of the COMP pin not only allows optimization

of control loop behavior but also provides a DC-coupled

and AC-filtered closed-loop response test point. The DC

step, rise time and settling at this test point truly reflects

the closed-loop response. Assuming a predominantly

second order system, phase margin and/or damping fac-

tor can be estimated using the percentage of overshoot

seen at this pin. The bandwidth can also be estimated by

examining the rise time at the pin. The COMP_Cna exter-

nal capacitor shown in the Typical Application circuit will

provide an adequate starting point for most applications.

The programmable parameters that affect loop gain are

the voltage range, bit[1] of the MFR_PWM_CONFIG com-

mand, the current range bit[7] of the MFR_PWM_MODE

command, the gm of the PWM channel amplifier bits [7:5]

of MFR_PWM_COMP, and the internal RCOMP compensa-

tion resistor, bits[4:0] of MFR_PWM_COMP. Be sure to

establish these settings prior to compensation calculation.

The COMP_Cna series internal RCOMPandexternalCCOMP_Cna

filter sets the dominant pole-zero loop compensation. The

internal RCOMP value can be modified (from 0Ω to 62kΩ)

using bits[4:0] of the MFR_PWM_ COMP command.

Adjust the value of RCOMPtooptimizetransientresponseonce

the final PCB layout is done and the particular CCOMP_bn filter

capacitor and output capacitor type and value have been

determined. The output capacitors need to be selected

because the various types and values determine the loop

gain and phase. An output current pulse of 20% to 80%

of full-load current having a rise time of 1µs to 10µs will

produce output voltage and COMP pin waveforms that will

give a sense of the overall loop stability without break-

ing the feedback loop. Placing a power MOSFET with a

resistor to ground directly across the output capacitor and

driving the gate with an appropriate signal generator is

a practical way to produce to a load step. The MOSFET

+ RSERIES will produce output currents approximately

equal to VOUT/RSERIES. RSERIES values from 0.1Ω to 2Ω

are valid depending on the current limit settings and the

programmed output voltage. The initial output voltage step

resulting from the step change in output current may not

be within the bandwidth of the feedback loop, so this signal

cannot be used to determine phase margin. This is why

it is better to look at the COMP pin signal which is in the

feedback loop and is the filtered and compensated control

loop response. The gain of the loop will be increased by

increasing RCOMP and the bandwidth of the loop will be

increased by decreasing CCOMP_Cna. If RCOMP is increased

by the same factor that CCOMP is decreased, the zero fre-

quency will be kept the same, thereby keeping the phase

shift the same in the most critical frequency range of the

feedback loop. The gain of the loop will be proportional to

the transconductance of the error amplifier which is set

using bits[7:5] of the MFR_PWM_COMP command. The

output voltage settling behavior is related to the stability

of the closed-loop system and will demonstrate the actual

overall supply performance. A second, more severe tran-

sient is caused by switching in loads with large (>1µF)

supply bypass capacitors. The discharged bypass capac-

itors are effectively put in parallel with COUT, causing a

rapid drop in VOUT. No regulator can alter its delivery of

current quickly enough to prevent this sudden step change

in output voltage if the load switch resistance is low and

Figure 33. RCOMP Adjust

INCREASE RCOMP

FREQUENCY

4664A F33

GAIN

TYPE II COMPENSATION

DUAL 25A/30A PSM APPLICATIONS INFORMATION

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