LTC1624CS datasheet(8/28 Pages) LINER

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LTC1624

Step-Down Converter: Power MOSFET Selection

One external N-channel power MOSFET must be selected

for use with the LTC1624 for the top (main) switch.

The peak-to-peak gate drive levels are set by the INTVCC

voltage. This voltage is typically 5V. Consequently, logic

level threshold MOSFETs must be used in most LTC1624

applications. If low input voltage operation is expected

(VIN < 5V) sublogic level threshold MOSFETs should be

used. Pay close attention to the BVDSS specification for the

MOSFETs as well; many of the logic level MOSFETs are

limited to 30V or less.

Selection criteria for the power MOSFET include the “ON”

resistance RDS(ON), reverse transfer capacitance CRSS,

input voltage and maximum output current. When the

LTC1624 is operating in continuous mode the duty cycle

for the top MOSFET is given by:

Main

Switch Duty Cycle =

VOUT +

The MOSFET power dissipation at maximum output

current is given by:

MAIN

OUT

MAX

RSS

() +

()

() ( )( )( )

()

185

δ RDS ON

where

δ is the temperature dependency of RDS(ON) and k

is a constant inversely related to the gate drive current.

MOSFETs have I2R losses, plus the PMAIN equation

includes an additional term for transition losses that are

highest at high output voltages. For VIN < 20V the high

current efficiency generally improves with larger MOSFETs,

while for VIN > 20V the transition losses rapidly increase to

the point that the use of a higher RDS(ON) device with lower

CRSS actual provides higher efficiency. The diode losses

are greatest at high input voltage or during a short circuit

when the diode duty cycle is nearly 100%.

The term (1+

δ)isgenerallygivenforaMOSFETintheform

of a normalized RDS(ON) vs Temperature curve, but

δ = 0.005/°C can be used as an approximation for low

voltage MOSFETs. CRSS is usually specified in the MOSFET

APPLICATIONS INFORMATION

characteristics. The constant k = 2.5 can be used to

estimate the contributions of the two terms in the PMAIN

dissipation equation.

Step-Down Converter: Output Diode Selection (D1)

The Schottky diode D1 shown in Figure 1 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 simply:

DIODE AVG

LOAD AVG

OUT

()













−

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:

SC AVG

≈









()

where PD is the allowable diode power dissipation and will

be determined by efficiency and/or thermal requirements

(see Efficiency Considerations).

Step-Down Converter: CIN and COUT Selection

In continuous mode the source current of the top

N-channel MOSFET is a square wave of approximate duty

cycle VOUT/VIN. To prevent large voltage transients, a low

ESR input capacitor sized for the maximum RMS current

must be used. The maximum RMS capacitor current is

given by:

MAX

OUT

Required IRMS ≈

−

()

[]12/

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

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

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