LTC3124 datasheet(22/36 Pages) AD

22 / 36 page

LT3120

Rev. 0

For more information www.analog.com

PROG Programming

Acurrentsenseamplifiermonitorsthevoltagebetweenthe

ISP and ISN pins and outputs a proportional current from

the PROG pin. The voltage on the PROG pin is compared

to a temperature stable reference voltage of 0.795V, and

if the PROG pin voltage exceeds this reference value the

inductorcurrentisscaledbacktomaintainthedesiredlevel

of output current. The output current limit is configured

using a resistor connected between the PROG pin and

GND. A small capacitor may optionally be placed in parallel

with the resistor. The values for the RC components are

calculated using Equation 17.

RPROG =

0.795V

2mA V•RSEN •IOUT

CPROG =

3300nF

• Ω

RPROG

(17)

Compensation of the Buck-Boost Converter

TheLT3120incorporatesanaveragecurrentmodecontrol

architecture which consists of two control loops. Both the

inner average current mode control loop and outer control

loop require compensation to maintain stability. The inner

current mode control loop is internally compensated to

maintain wide bandwidth and good transient response.

For many applications, the inner current loop can be

treated like a voltage controlled current source (VCCS).

This current source is commanded by the voltage error

amplifier to regulate the output load formed primarily by

the load resistance (RLOAD) and output capacitor (COUT).

This simplified version is illustrated in Figure 10, show-

ing the key components that need to be considered when

compensating the converter.

With a full-scale command on VC, the LT3120 buck-boost

converter will generate an average inductor current of

9.5A. With a VC voltage range of 200mV to 900mV, the

resulting current gain for the inner average current loop

is 13.6A/V. Similar to peak current mode control, the inner

average current mode 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 the normally dominant low frequency pole

and the effective series resistance of the output capacitor

and its capacitance form a zero, usually at a high enough

frequency to be ignored. A potentially troublesome right

half plane zero (RHPZ) is also encountered if the LT3120

is operated in boost mode. The RHPZ causes an increase

in gain, like a zero, but a decrease in phase, like a pole.

APPLICATIONS INFORMATION

Figure 10. Simplified Representation of Control

Loop Components

0.795V

0.9V

gm = 13.6A/V

–

VOLTAGE

ERROR

AMP

VOLTAGE

CONTROLLED

CURRENT

SOURCE

GND

RTOP

RBOT

RCOSER

RLOAD

CP1

COUT

VOUT

CP2

3120 F10

Thiswillultimatelylimitthemaximumconverterbandwidth

that can be achieved with the LT3120. The RHPZ is not

present when operating in buck mode. The overall open

loop gain at DC is the product of the Equation 18 through

Equation 20.

Voltage Error Amplifier Gain:

GEA = gm •REA = 120µs • 5MΩ = 600

(18)

Voltage Divider Gain:

VFB

VOUT

0.795V

VOUT

(19)

Current Loop Transconductance:

GCS =

9.5A

0.7V

= 13.6A / V

(20)

It is important to note that GCS is the transconductance

gain from the control voltage VC to the inductor current

level, which equals the output current level in buck mode.

In boost mode, the output current level will be reduced

LTC3124 Datasheet(PDF) 22 Page - Analog Devices

LTC3124 Datasheet(HTML) 22 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 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL