LT3745EUJ Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LT3745EUJ

16-Channel 50mA LED Driver with Buck Controller

Linear Technology 

LT3745

Applications Information

Inductor Selection

The critical parameters for selection of an inductor are

inductance value, DC or RMS current, saturation current,

and DCR resistance. For a given input and output voltage,

the inductor value and switching frequency will determine

the peak-to-peak ripple current, ∆IL. The ∆IL value usually

ranges from 20% to 50% of the maximum output load

current, IOUT(MAX). Lower values of ∆IL require larger and

more costly inductors; higher values of ∆IL increase the

peak currents and the inductor core loss. An inductor

current ripple of 30% to 40% offers a good compromise

between inductor performance and inductor size and cost.

However, for high duty cycle applications, a ∆IL value of

~20% should be used to prevent sub-harmonic oscillation

due to insufficient slope compensation.

The largest inductor ripple current occurs at the highest

VIN. To guarantee that the ripple current stays below the

specified maximum, the inductor value should be chosen

according to the following equation:

L ≥ VOUT + VD • VIN(MAX) – VOUT

VIN(MAX) + VD

fSW • ∆I L

The inductor DC or RMS current rating must be greater

than the maximum output load current IOUT(MAX) and its

saturation current should be higher than the maximum

inductor current IL(MAX). To achieve high efficiency, the

DCR resistance should be less than 0.1Ω, and the core

material should be intended for high frequency applications.

Power MOSFET Selection

Important parameters for the external P-channel MOSFET

M1 include drain-to-source breakdown voltage (V(BR)DSS),

maximum continuous drain current (ID(MAX)), maximum

gate-to-source voltage (VGS(MAX)), total gate charge (QG),

drain-to-source on resistance (RDS(ON)), reverse transfer

capacitance (CRSS). The MOSFET V(BR)DSS specification

should exceed the maximum voltage across the source to

the drain of the MOSFET, which is VIN(MAX) plus VD. The

ID(MAX) should exceed the peak inductor current, IL(MAX).

Since the gate driver circuit is supplied by the internal

6.8V VIN referenced regulator, the VGS(MAX) rating should

be at least 10V.

Each switching cycle the MOSFET is switched off and on, a

packet of gate charge QG is transferred from the VIN pin to

the GATE pin, and then from the GATE pin to the CAP pin.

The resulting dQG/dt is a current that must be supplied to

the CCAP capacitor by the internal regulator. The maximum

22mA current capability of the internal regulator limits the

maximum QG(MAX) it can deliver to:

Q G(MAX)

=

22mA

fSW

Therefore, the QG at VGS = 6.8V from the MOSFET data

sheet should be less than QG(MAX).

For maximum efficiency, both RDS(ON) and CRSS should

be minimized. Lower RDS(ON) means less conduction loss

while lower CRSS reduces transition loss. Unfortunately,

RDS(ON) is inversely related to CRSS. Thus balancing the

conduction loss with the transition loss is a good criterion

in selecting a MOSFET. For applications with higher VIN

voltages (≥24V) a lower CRSS is more important than a

low RDS(ON).

Catch Diode Selection

The catch diode D1 carries load current during the switch

off-time. Important parameters for the catch diode includes

peak repetitive reverse voltage (VRRM), forward voltage

(VF), and maximum average forward current (IF(AV)). The

diode VRRM specification should exceed the maximum

reverse voltage across it, i.e., VIN(MAX). A fast switching

Schottky diode with lower VF should be used to yield lower

power loss and higher efficiency.

In continuous conduction mode, the average current

conducted by the catch diode is calculated as:

ID(AVG) = IOUT • (1 – D)

The worst-case condition for the diode is when VOUT is

shorted to ground with maximum VIN and maximum IOUT

at present. In this case, the diode must safely conduct

the maximum load current almost 100% of the time. To

improve efficiency and to provide adequate margin for

short circuit operation, a Schottky diode rated to at least

the maximum output current is recommended.

3745f

21

Share Link: