LTM8083 datasheet(13/22 Pages) AD

13 / 22 page

LTM4693

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

The average current mode control used in the LTM4693

can be conceptualized as a voltage controlled current

source (VCCS), driving the output load formed primarily

by RLOAD and COUT, as shown in Figure 4.

The voltage error amplifier output (VC), provides a com-

mand input to the VCCS. As with peak current mode control,

the inner average current control loop effectively turns the

inductor into a current source over the frequency range of

interest, resulting in a frequency response from the power

stage that exhibits a single pole (–20dB/decade) roll-off.

The output capacitor (COUT) and load resistance (RLOAD)

form a dominant low frequency pole, where the effective

series resistance of the output capacitor and its capaci-

tance form a zero, usually at a high enough frequency to

be ignored, if ceramic capacitors are employed.

A potentially troublesome Right Half Plane Zero (RHPZ)

is also encountered if the converter is operated in boost

mode. The RHPZ causes an increase in gain, like a zero,

but a decrease in phase, like a pole. This can ultimately

limit the maximum converter bandwidth that can be

achieved with the LTM4693. The RHPZ is not present

when operating in buck mode.

–

COUT

VOUT

RLOAD

RESR

VCCS

VOLTAGE

ERROR

AMPLIFIER

gm = 110µA/V

gm VIN > VOUT: 10A/V

VIN > VOUT: (10A/V) • (VIN/VOUT)

4693 F04

0.9V

CHF

Figure 4. Simplified Representation of Average Current

Mode Control Loop Small Signal Model

The voltage amplifier’s frequency response is designed to

optimize the response for the overall loop. Measurement

of the power stage gain over line, load, component varia-

tion, and frequency is strongly recommended prior to

loop design. The design parameters for compensation

design will focus on the series resistor and capacitors

connected from VC to GND (RC, CC and CHF (Optional)).

Being a buck-boost converter, the target loop crossover

frequency for the compensation design will be dictated

by the highest boost ratio and load current as this will

result in the lowest RHPZ frequency. The general goal

is to set the crossover frequency and provide sufficient

phase boost using the external compensation network.

The LTpowerCAD design tool is available to download

online to perform loop compensation and transient

optimization.

Table 5 is provided for most application requirements.

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configuration

section of the data sheet are consistent with those param-

eters defined by JESD51-12 and are intended for use with

finite element analysis (FEA) software modeling tools that

leverage the outcome of thermal modeling, simulation,

and correlation to hardware evaluation performed on a

µModule package mounted to a hardware test board. The

motivation for providing these thermal coefficients can be

found in JESD51-12 (“Guidelines for Reporting and Using

Electronic Package Thermal Information”).

Many designers may opt to use laboratory equipment and

a test vehicle such as the demo board to anticipate the

µModule regulator’s thermal performance in their appli-

cation at various electrical and environmental operating

conditions to compliment any FEA activities. Without

FEA software, the thermal resistances reported in the

Pin Configuration section are, in-and-of themselves, not

relevant to providing guidance of thermal performance;

instead, the derating curves provided in the data sheet

can be used in a manner that yields insight and guidance

LTM8083 Datenblatt(PDF) 13 Page - Analog Devices

LTM8083 Datenblatt(HTML) 13 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

Datenblatt Download

Link URL