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LT3950

Rev. 0

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Maximum Switch and Load Current

An important system parameter is the current limit. This

can prevent damage to the switch and external compo-

nents by limiting the maximum instantaneous current

conducting through the power switch. In a well-designed

system, there will be margin between the maximum switch

current to drive the LED load and the switch current limit.

The LT3950 offers a current limit that does not change

with duty ratio and also has sufficient slope compensa-

tion so that reaching the current limit during line and load

transients does not result in subharmonic oscillations.

A good rule of thumb for setting maximum LED current

for boost and buck-boost power stages appears below.

This equation assumes that the inductor selection used

limits current ripple to around 30% of average current. For

more information on inductor selection, see the External

Component Selection section.

ILED,MAX(30%ripple) ≈1.4A •

VIN

VISP

In the boost and buck-boost mode 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 following

result for buck mode current limit.

ILED,MAX(BUCK) = 1.4A

For more information about power stage topologies,

review the example application circuits included below.

Loop Compensation

Loop compensation normally will take the form of an RC

network connected between the VC pin and ground. A

single capacitor can fulfill stability requirements if PCB

area is extremely limited. The addition of a series resis-

tor, however, will increase response speed and can also

recover phase margin. A schematic diagram of the typical

compensation scheme is illustrated below.

APPLICATIONS INFORMATION

Figure 6.

LT3950

3950 F06

Typical Compensation

For many cases, a 1nF capacitor and 10kΩ resistor will

suffice. This is a good place to start for all applications. If

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

loop remains unstable, the information below can help.

First, try reducing or eliminating the compensation resis-

tor, RC, especially if transient response is ringing or under-

damped. 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 fre-

quency) 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

increasing the output capacitor or decreasing the inductor

to separate the load pole and right half plane zero.

EXTERNAL COMPONENT SELECTION

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. Even though X7R has

less variation with temperature and DC bias voltage than

many other materials, the effect of capacitance derating

with voltage stress must be considered. It is generally a

good rule of thumb to pick capacitors with a voltage rating

about 60% higher than the application demands.

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, buck-boost mode, and SEPIC

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