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LTM8042/LTM8042-1

80421fa

APPLICATIONS INFORMATION

Which mode to use depends upon the operating condi-

tions. Boost is generally selected when the voltage across

the LED string is always higher than the input voltage. Buck

mode is the dual of boost, used when the voltage across

the LED string is always lower than VIN. Finally, buck-boost

mode is used when the VIN can vary both above and below

the voltage across the LED string.

The land grid array of the LTM8042/LTM8042-1 is designed

to conveniently accommodate all three operating modes.

Please refer to the PCB Layout section for suggested

examples of how to lay out each operating mode.

Dimming Control

There are two methods to control the current source for

dimming using the LTM8042/LTM8042-1. One method

uses the PWM pin to modulate the current source between

zero and full current to achieve a precisely programmed

average current. To make this method of current control

more accurate, the switch demand current is internally

stored during the quiescent phase (PWM low). This feature

minimizes recovery time when the PWM signal returns

high. When using PWM dimming, use a P-channel MOSFET

disconnect switch in the LED current path (see Figure 1)

to prevent the output capacitor from discharging during

the PWM off-time. Enable this function by pulling TGEN

above 1.5V.

Figure 1. The LTM8042/LTM8042-1

Can Control a P-Channel PWM Switch

The minimum PWM on or off-time will depend on the

choice of operating frequency through the RT input pin or

the SYNC pin. When using the SYNC function, the SYNC

and PWM signals must have the rising edges aligned to

achieve the optimized high PWM dimming ratio. For best

current accuracy, the minimum PWM low or high time

should be at least six switching cycles (3μs for ƒSW =

2MHz). The maximum PWM period is determined by the

system. The maximum PWM dimming ratio (PWMRATIO)

can be calculated from the maximum PWM period (TMAX)

and the minimum PWM pulse width (TMIN) as follows:

PWMRATIO =

TMAX

TMIN

(1)

A set of values that give a 3000:1 dimming ratio, for ex-

ample, would be a switching frequency of ƒSW = 2MHz,

TMAX = 9ms and TMIN = 3μs. Equation (1) becomes:

PWMRATIO = 9ms/3μs = 3000:1

The second method of dimming control uses the CTL pin

to linearly adjust the current sense threshold during the

PWM high state. When the CTL pin voltage is less than

1V, the LED current is:

ILED = VCTL

When VCTL is higher than 1.1V, the LED current is clamped

to 1A.

The LED current programming feature can be used in

conjunction with the PWM to possibly increase the total

dimming range by an additional factor of ten.

80421 TA01a

LTM8042

GND

CTL

VCC

BSTIN/BKLED–

RUN

TGEN

PWM

SYNC

VIN

LED+

BSTOUT/BKIN

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