LT3756 datasheet(18/31 Pages) AD

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LT8355-1

Rev. A

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APPLICATIONS INFORMATION

In the boost and buck-boost topologies, average switch

current is related to average LED current by the ratio of

VIN to VISP. In the buck topology, average LED current

approximately equals average inductor current. For this

reason, the buck topology provides the highest possible

LED current capability. The peak instantaneous switch

current is thus the average LED current plus half of the

peak-to-peak ripple current. This leads to the result in

Equation 7 for buck mode current limit.

ILED,MAX(BUCK)=

100mV

RSENSE

–

VLED(VIN – VLED)

2 • VIN •L • fSW

(7)

For more information about power stage topologies,

review the Typical Applications section. Since LT8355-1

offers a switch current limit that does not significantly

change with duty ratio, the switch current limit can also

limit the input current. Like the load current, the average

input current and average switch current are not always

the same. However, while the switch is on, the same

instantaneous current flows in the switch and inductor.

Because the inductor connects directly to the input, the

peak input current, which is also the peak inductor cur-

rent, cannot exceed the programmed switch current limit.

Note that average input and load current will be strictly

less than maximum peak switch current.

Loop Compensation

To ensure stable operation of the control loop realized by

LT8355-1 it is necessary to limit the loop bandwidth. To

do this, connect an RC network between the VC pin and

ground. A single capacitor can fulfill stability requirements

if PCB area is extremely limited. The addition of a series

resistor, however, will increase response speed and can

also recover phase margin. A schematic diagram of the

typical compensation scheme is illustrated in Figure 6. For

many cases, a 1nF capacitor and 47k resistor will suffice.

These are good values to start with for all applications.

Figure 6. LT8355-1 Compensation Network

If settling time is unacceptable, ringing is too large, or

the loop remains unstable the following steps can help.

First, try reducing to 10k or eliminating the compensa-

tion resistor, RC, especially if transient response is ringing

or underdamped. Reducing the compensation resistor

will cause longer settling time and larger deviation from

load-steps such as PWM dimming. Next, increase the

size of the compensation capacitor, CC. This will reduce

the frequency of the dominant (low frequency) pole and

thereby the unity gain frequency. It will usually be possible

to stabilize the loop given a big enough compensation

capacitor. Increasing the compensation capacitor slows

down the transient response to line and load activity. If

the compensation capacitor cannot change by a small

amount to achieve stability, consider instead decreasing

the output capacitor or decreasing the inductor to sepa-

rate the load pole and right half plane zero (for boost and

SEPIC topologies).

Input and Output Capacitor Selection

The input and output capacitors supply the transient cur-

rent for the power stage and should be placed and cho-

sen according to the transient current requirements. An

X7R type ceramic capacitor is usually a good choice for

both input and output capacitor. Ceramic capacitors have

a large voltage coefficient (i.e. lower capacitance when

biased near their rated Voltage). It is generally a good rule

of thumb to choose a capacitance 60% higher to com-

pensate for the voltage coefficient. Consult the capacitor

manufacturer to ensure capacitance is sufficient at the

operating voltage.

The switching frequency, output current, inductor ripple

current, and tolerable input voltage ripple are key param-

eters to consider when determining the value of the input

capacitor. Typically, boost and SEPIC converters require a

lower value input capacitor than buck mode converters.

Use Equation 8 to estimate the value of the input capacitor.

If the inductor is selected according to the directions in

the Inductor Selection section, the input capacitance value

can be calculated.

CIN(BOOST) =

20mV

8 •

ΔVIN •RSENSE • fSW

(8)

LT8355-1

8355-1 F06

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