LTM8049 datasheet(12/22 Pages) AD

12 / 22 page

LTM4693

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

duty cycle command into the buck-boost PWM circuitry.

The inductor current sensing circuitry alternately mea-

sures the current through the power switches. The output

of the sensing circuitry produces a voltage across resis-

tor RCS that resembles the inductor current waveform

transformed 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 on ICOMP will revert very closely to

its previous level into the 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, the average value of the

voltage applied to its inverting input (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 volt-

age coming from VC. The voltage error amplifier moni-

tors VOUT through a voltage divider and makes adjust-

ments 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 cur-

rent amplifier controls average 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 cur-

rent mode control. The inner loop compensation compo-

nents are fixed internally on the LTM4693 to simplify the

loop design and provide the highest possible bandwidth

over a wide operating range. However, the compensation

of the voltage loop is external for the LTM4693 which

allows the overall loop characteristics to be customized

depending on the programmed output voltage, oscillator

frequency, output capacitance and equivalent ESR of the

output capacitors.

Figure 3. Average Current Mode Control Loop

ICOMP

VOUT

CHF

4693 F03

RCS

SW2

PWM

DRIVE

LOGIC

SWITCHES

PWM RAMPS/

OSCILLATOR

SW1

VAMP

IAMP

INDUCTOR

CURRENT SENSE

CLAMP

0.9V

INTERNAL

CURRENT

AMPLIFIER

LTM8049 Datasheet(PDF) 12 Page - Analog Devices

LTM8049 Datasheet(HTML) 12 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Datasheet Download

Link URL