LT3745IUJ#TRPBF【PDF 21/28 页】IC LED DVR 16CH 50MA 40QFN

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LT3745

APPLICATIONS INFORMATION

V OUT + V D V IN(MAX) – V OUT

Q G (MAX) =

InductorSelection

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, ?I L . The ?I L value usually

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

current, I OUT(MAX) . Lower values of ?I L require larger and

more costly inductors; higher values of ?I L 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 ?I L 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

V IN . To guarantee that the ripple current stays below the

specified maximum, the inductor value should be chosen

according to the following equation:

L ≥ ?

V IN(MAX) + V D f SW ? ? I L

The inductor DC or RMS current rating must be greater

than the maximum output load current I OUT(MAX) and its

saturation current should be higher than the maximum

inductor current I L(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 (I D(MAX) ), maximum

gate-to-source voltage (V GS(MAX) ), total gate charge (Q G ),

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

capacitance (C RSS ). The MOSFET V (BR)DSS specification

should exceed the maximum voltage across the source to

the drain of the MOSFET, which is V IN(MAX) plus V D . The

I D(MAX) should exceed the peak inductor current, I L(MAX) .

Since the gate driver circuit is supplied by the internal

6.8V V IN referenced regulator, the V GS(MAX) rating should

be at least 10V.

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

packet of gate charge Q G is transferred from the V IN pin to

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

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

the C CAP capacitor by the internal regulator. The maximum

22mA current capability of the internal regulator limits the

maximum Q G(MAX) it can deliver to:

22mA

f SW

Therefore, the Q G at V GS = 6.8V from the MOSFET data

sheet should be less than Q G(MAX) .

For maximum efficiency, both R DS(ON) and C RSS should

be minimized. Lower R DS(ON) means less conduction loss

while lower C RSS reduces transition loss. Unfortunately,

R DS(ON) is inversely related to C RSS . Thus balancing the

conduction loss with the transition loss is a good criterion

in selecting a MOSFET. For applications with higher V IN

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

low R DS(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 (V RRM ), forward voltage

(V F ), and maximum average forward current (I F(AV) ). The

diode V RRM specification should exceed the maximum

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

Schottky diode with lower V F 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:

I D(AVG) = I OUT ? (1 – D)

The worst-case condition for the diode is when V OUT is

shorted to ground with maximum V IN and maximum I OUT

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

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