LT3154AVPBF datasheet(15/32 Pages) AD

15 / 32 page

LT3154

Rev. 0

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OPERATION

generates an output, VC, used by the current mode con‑

trol loop to command the appropriate inductor current

level. To ensure stability, external frequency compensa‑

tion components (RC, CC and CHF) must be installed

between VC and GND. The procedure for determining

these components is provided in the Applications Section

of this data sheet.

VC is internally connected to the non‑inverting input of

a second amplifier, referred to in Figure 2 as I_AMP. The

inverting input of the average current amplifier is con‑

nected to the inductor current sense resistor RCS with

a 200mV offset. I_AMP contains an internal averaging

filter and frequency compensation network to stabilize

operation of the internal current loop. The average cur‑

rent amplifier’s output (ICOMP) provides the cycle‑by‑cycle

duty cycle command into the buck‑boost PWM circuitry.

The non‑inverting reference level input to the average

current amplifier is VC and the feedback or inverting input

is driven from the inductor current sensing circuitry. The

inductor current sensing circuitry alternately measures

the current through switches A and B. The output of the

sensing circuitry produces a voltage across resistor RCS

thatresemblestheinductorcurrentwaveformtransformed

to a voltage. If there is an increase in the power converter

load on VOUT, the instantaneous level of VOUT will drop

slightly, which will increase the voltage level on VC by

the inverting action of the voltage error amplifier. When

the increase on VC first occurs, the output of the current

averaging amplifier, ICOMP, will increase momentarily to

command a larger duty cycle. This duty cycle increase will

result in a higher inductor current level, ultimately raising

the average voltage across RCS. Once the average value of

the voltage on RCS is equivalent to the VC level, the voltage

onICOMPwillrevertverycloselytoitspreviouslevelintothe

PWM and force the correct duty cycle to maintain voltage

regulation at this new higher inductor current level. The

average current amplifier is configured, so in steady state,

theaveragevalueofthevoltageappliedtoitsinvertinginput

(voltage across RCS) will be equivalent to the voltage on

its non‑inverting input VC. As a result, the average value

of the inductor current is controlled in order to maintain

voltage regulation. The entire current amplifier and PWM

can be simplified as a voltage controlled current source,

with the driving voltage coming from VC.

ThevoltageerroramplifiermonitorsVOUTthroughavoltage

divider and makes adjustments to the current command

as necessary to maintain regulation. The voltage error

amplifier therefore controls the outer voltage regulation

loop. The average current amplifier makes adjustments

to the inductor current as directed by the voltage Error

Amplifier output via VC and is commonly referred to as

the inner current loop amplifier. The average current mode

control technique is similar to peak current mode control

except that the average current amplifier controls aver‑

age current instead of the peak current. This difference

eliminates the peak to average current error inherent to

peak current mode control, while maintaining most of

the advantages inherent to peak current mode control.

Control loop compensation techniques are detailed in the

Applications section of this data sheet.

Inductor Current Sense and Maximum Output Current

As part of the current control loop, the LT3154 has cur‑

rent sense circuitry that measures the inductor current

of the buck‑boost converter as shown in Figure 2. This

circuitry measures the current through switches A and B

separately and produces proportional output currents that

are summed at the current sense resistor RCS. Sensed A

and B switch currents form a voltage replica of the induc‑

tor current at RCS, which is used by the average current

amplifier as described in the Quick Reference section.

The voltage amplifier output, VC, is internally clamped to a

nominal value of 0.9V. Since the average inductor current

is proportional to VC, the 0.9V clamp sets the maximum

average inductor current that can be programmed by the

inner current loop. Taking into account the current sense

amplifier’s gain and the value of RCS, the maximum aver‑

age inductor current is 7A typical.

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