LINEAR dn340f datasheet

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DC/DC Converter Drives Lumileds White LEDs from a Variety of Power Sources – Design Note 340 Keith Szolusha

Introduction RSENSE

LXHL-BW02

LEDs are usually driven with a constant DC current source VIN 3.3V TO 4.2V

in order to maintain constant luminosity. However, most

DC/DC converters are designed to deliver a constant

voltage by comparing a feedback voltage to an internal

LED VIN SW 10V reference via an internal error amplifier. The easiest way ON

SHDN SHDN ISN

to turn a simple DC/DC converter into a constant current 1µF ISP

6.3V LT1618 866k

source is to use a sense resistor to turn the output current

into a voltage and use it as the feedback. The problem with GND VC

this method is its reduced efficiency—700m A across a 124k

1.2V (typical reference voltage) drop produces an 840m W

power loss. One solution is to use an external op amp to D1: PHILIPS PMEG2010EA DN340 F01 L1: COOPER SD25

amplify the voltage of a low value resistor to the given reference voltage. This saves converter efficiency, but

Figure 1. The LT1618 Powers the LXHL-BW02 White LED

significantly increases the cost and complexity of a simple

from a Single Lithium-Ion Battery with 70% Efficiency

converter with the additional components and board space. for portable and battery-powered applications. The high

switching frequency allows the input and output capaci-

A superior solution is to use the LT®1618 constant cur-

tors and the inductor to be extremely small.

rent, constant voltage converter, which combines a tradi-

tional voltage feedback loop and a unique current feedback Although the LT1618 is conventionally used as a high loop to operate as a constant-voltage, constant-current frequency boost converter with the load being driven DC/DC converter. No external op amps are required for between VOUT and ground, the unique method, shown in

this extremely compact solution. The IADJ (current adjust- Figure 1, of tying the load from VOUT back to VIN allows it ment) pin provides the capability to dim the LED during to be used to drive the LXHL-BW02 1W white LED from a normal operation by varying the resistor setting or inject- lithium-ion battery input. Tying the load back to VIN allows ing a PWM signal. Access to both the positive and the forward voltage of the LED (the load voltage) to be negative inputs of the internal constant-current amplifier either above or below the input voltage as the battery allows the sense resistor to be placed anywhere in the voltage changes. This topology avoids the need for an converter’s output or input path and provides constant additional inductor as would be required in other buck- output or input current. Without access to both inputs, and-boost topologies such as SEPIC or flyback. either a ground referenced sense resistor or some addi-

The single inductor used here is extremely small and low

tional level-shifting transistors or operational amplifier

cost, matching the tiny all-ceramic capacitors and low-

would be required.

profile IC. Tying the load back to VIN increases the induc- Lithium-Ion Source (3.3V to 4.2V) tor current by summing both the input and output currents. Due to increased switch losses, the overall efficiency of

The LT1618EDD is a 1.4MHz constant-current, constant-

the solution is approximately 70% over the input voltage

voltage boost converter in a tiny 10-pin thermally

range. Nevertheless, at this efficiency, it is difficult to

enhanced DFN package. The monolithic (onboard) low side switch has a maximum peak current limit of 1.5A.

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