LTC1147-3.3 Datasheet, PDF(9/16 Page) Linear Technology – High Efficiency Step-Down Switching Regulator Controllers

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LTC1147-3.3 Datasheet, PDF (9/16 Pages) Linear Technology – High Efficiency Step-Down Switching Regulator Controllers

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LTC1147-3.3

LTC1147-5/LTC1147L

APPLICATIO S I FOR ATIO

used. For VIN > 8V, a standard threshold MOSFET (VGS(TH)

< 4V) may be used. If VIN is expected to drop below 8V,

a logic-level threshold MOSFET (VGS(TH) < 2.5V) is

strongly recommended. When a logic-level MOSFET is

used, the LTC1147 supply voltage must be less than the

absolute maximum VGS ratings for the MOSFET.

The maximum output current IMAX determines the RDS(ON)

requirement for the power MOSFET. When the LTC1147

series is operating in continuous mode, the simplifying

assumption can be made that either the MOSFET or

Schottky diode is always conducting the average load

current. The duty cycles for the MOSFET and diode are

given by:

P-Ch

Duty

Cycle

VOUT

VIN

Schottky

Diode

Duty

Cycle

(VIN

VOUT

VIN

VD)

From the duty cycle the required RDS(ON) for the MOSFET

can be derived:

P-Ch RDS(ON) =

(VIN)(PP)

(VOUT)(IMAX2)(1 + Î´P)

where PP is the allowable power dissipation and Î´P is the

temperature dependency of RDS(ON). PP will be deter-

mined by efficiency and/or thermal requirements (see

Efficiency Considerations). (1 + Î´) is generally given for a

MOSFET in the form of a normalized RDS(ON) vs tempera-

ture curve, but Î´ = 0.007/Â°C can be used as an approxima-

tion for low voltage MOSFETs.

Output Diode Selection (D1)

The Schottky diode D1 shown in Figure 1 only conducts

during the off-time. It is important to adequately specify

the diode peak current and average power dissipation so

as not to exceed the diode ratings.

The most stressful condition for the output diode is under

short circuit (VOUT = 0V). Under this condition the diode

must safely handle ISC(PK) at close to 100% duty cycle.

Under normal load conditions the average current con-

ducted by the diode is:

ID1

(VIN

VOUT

VIN

VD)

(ILOAD)

Remember to keep lead lengths short and observe proper

grounding (see Board Layout Checklist) to avoid ringing

and increased dissipation.

The forward voltage drop allowable in the diode is calcu-

lated from the maximum short-circuit current as:

ISC(PK)

where PD is the allowable power dissipation and will be

determined by efficiency and/or thermal requirements

(see Efficiency Considerations).

CIN and COUT Selection

In continuous mode, the source current of the P-channel

MOSFET is a square wave of duty cycle VOUT/VIN. To

prevent large voltage transients, a low ESR input capaci-

tor sized for the maximum RMS current must be used. The

maximum RMS capacitor current is given by:

CIN

Required

IRMS

IMAX

[VOUT(VIN â VOUT)]1/2

VIN

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT/2. This simple worst-case condition is com-

monly used for design because even significant devia-

tions do not offer much relief. Note that capacitor

manufacturerâs ripple current ratings are often based on

only 2000 hours of life. This makes it advisable to further

derate the capacitor, or to choose a capacitor rated at a

higher temperature than required. Several capacitors

may also be paralleled to meet size or height require-

ments in the design. Always consult the manufacturer if

there is any question. An additional 0.1ÂµF to 1ÂµF ceramic

decoupling capacitor is also required on VIN (Pin 1) for

high frequency decoupling.

The selection of COUT is driven by the required effective

series resistance (ESR). The ESR of COUT must be less

than twice the value of RSENSE for proper operation of the

LTC1147:

COUT Required ESR < 2RSENSE

sn1147 1147fds

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