LTM4682 datasheet(63/130 Pages) AD

63 / 130 page

LTM4682

Rev. A

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APPLICATIONS INFORMATION

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 ∆ILOAD • ESR, where ESR is the effective

series resistance of COUT. ∆ILOAD 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 recovery

time, VOUT can be monitored for excessive overshoot or

ringing, indicating 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 reflect the closed-loop

response. Assuming a predominantly second-order sys-

tem, phase margin and/or damping factor can be esti-

mated using the percentage of overshoot seen at this

pin. The bandwidth can also be estimated by examining

the rise time at the pin. The COMPna external capaci-

tor shown in the Typical Application circuit section 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_MODE 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 before compensation calculation.

The COMPna series internal RCOMPn and external CCOMPna

filter sets the dominant pole-zero loop compensation. The

internal RCOMPn value can be modified (from 0Ω to 62k)

using bits[4:0] of the MFR_PWM_COMP command. Adjust

the value of RCOMPn to optimize transient response once

the final PCB layout is done and the particular CCOMPbn

filter capacitor and output capacitor type and value have

been determined. The output capacitors must 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 the ground directly across the output capacitor

and driving the gate with an appropriate signal generator

is a practical way to produce 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 the 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 CCOMPna. 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, gm, which is

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

The output voltage settling behavior is related to the sta-

bility of the closed-loop system and will demonstrate the

actual overall supply performance. A second, more severe

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

Figure 29. RCOMP Adjust

INCREASE RCOMPn

FREQUENCY

4682 F29

GAIN

TYPE II COMPENSATION

Adjusting the RCOMP will change the pole and zero loca-

tion, as shown in Figure 29. It is recommended that the

user determines the appropriate value for the gm and

RCOMPn using the LTpowerCAD tool.

LTM4682 Datasheet(PDF) 63 Page - Analog Devices

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