LTC1159CS datasheet(9/20 Pages) LINER

9 / 20 page

LTC1159

LTC1159-3.3/LTC1159-5

inductor ripple current and consequent output voltage

ripple which can cause Burst Mode operation to be falsely

triggered in the LTC1159. Do not allow the core to saturate!

Molypermalloy (from Magnetics, Inc.) is a low loss core

material for toroids, but it is more expensive than ferrite.

A reasonable compromise from the same manufacturer is

Kool M

µ. Toroids are very space efficient, especially when

you can use several layers of wire. Because they generally

lack a bobbin, mounting is more difficult. However, new

surface mount designs available from Coiltronics do not

increase the height significantly.

Power MOSFET Selection

Two external power MOSFETs must be selected for use

with the LTC1159: a P-channel MOSFET for the main

switch and an N-channel MOSFET for the synchronous

switch.

The peak-to-peak drive levels are set by the VCC voltage on

the LTC1159. This voltage is typically 4.5V during start-up

and 5V to 7V during normal operation (see EXTVCC Pin

Connection). Consequently,

logic-level threshold

MOSFETs must be used in most LTC1159 family applica-

tions. The only exception is applications in which EXTVCC

is powered from an external supply greater than 8V, in

which standard threshold MOSFETs (VGS(TH) < 4V) may be

used. Pay close attention to the BVDSS specification for the

MOSFETs as well; many of the logic-level MOSFETs are

limited to 30V.

Selection criteria for the power MOSFETs include the “ON”

resistance RDS(ON), reverse transfer capacitance CRSS,

input voltage and maximum output current. When the

LTC1159 is operating in continuous mode, the duty cycle

for the P-channel MOSFET is given by:

P-Ch Duty Cycle =

VOUT

VIN

N-Ch Duty Cycle =

VIN – VOUT

VIN

The MOSFET dissipations at maximum output current are

given by:

N-Ch PD =

VIN – VOUT

VIN

(IMAX)2 (1 + ∂N) RDS(ON)

P-Ch PD =

VOUT

VIN

(IMAX)2 (1 + ∂P) RDS(ON) +

k(VIN)2 (IMAX) (CRSS) (f)

where

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

is a constant inversely related to the gate drive current.

Both MOSFETs have I2R losses while the P-channel

equation includes an additional term for transition losses,

which are highest at high input 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 actually provides higher effi-

ciency. The N-channel MOSFET losses are the greatest at

high input voltage or during a short circuit when the

N-channel duty cycle is nearly 100%.

The term (1 +

∂)isgenerallygivenforaMOSFETintheform

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

∂ = 0.007/°C can be used as an approximation for low

voltage MOSFETs. CRSS is usually specified in the MOSFET

electrical characteristics. The constant k = 5 can be used for

the LTC1159 to estimate the relative contributions of the

two terms in the P-channel dissipation equation.

The Schottky diode D1 shown in Figure 1 only conducts

during the dead time between the conduction of the two

power MOSFETs. D1 prevents the body diode of the

N-channel MOSFET from turning on and storing charge

during the dead time, which could cost as much as 1% in

efficiency (although there are no other harmful effects if

D1 is omitted). Therefore, D1 should be selected for a

forward voltage of less than 0.6V when conducting IMAX.

APPLICATIO S I FOR ATIO

LTC1159CS Datasheet(PDF) 9 Page - Linear Technology

LTC1159CS Datasheet(HTML) 9 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL