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LT1432CS8

5V High Efficiency Step-Down Switching Regulator Controller

Linear Technology 

LT1432

APPLICATI S I FOR ATIO

D1, L1 and C2 act as the conventional catch diode and

output filter of the buck converter. These components

should be selected carefully to maintain high efficiency

and acceptable output ripple. See other sections of this

data sheet for detailed discussions of these parts.

Current limiting is performed by R2. Sense voltage is only

60mV to maintain high efficiency. This also reduces the

value of the sense resistor enough to utilize a printed

circuit board trace as the sense resistor. The sense voltage

has a positive temperature coefficient of 0.33%/°C to

match the temperature coefficient of copper. See Current

Limiting section for details.

The basic regulator has three different operating modes,

defined by the mode pin drive. Normal operation occurs

when the mode pin is grounded. A low quiescent current

“burst” mode can be initiated by floating the mode pin.

Input supply current is typically 1.3mA in this mode, and

output ripple voltage is 100mVp-p. Pulling the mode pin

above 2.5V forces the entire regulator into micropower

shutdown where it typically draws less than 20µA. See

Mode Pin Drive for details.

Efficiency

Efficiency in normal mode is maximum at about 500mA

load current, where it exceeds 90%. At lower currents, the

operating supply current of the switching IC dominates

losses. The power loss due to this term is approximately

8mA × 5V, or 40mW. This is 4% of output power at a load

current of 200mA. At higher load currents, losses in the

switch, diode, and inductor series resistance begin to

increase as the square of current and quickly become the

dominant loss terms.

Loss in inductor series resistance;

P = RS (IOUT)2

Loss in switch on resistance;

( ) P = VOUT RSW (IOUT )2

VIN

Loss in switch driver current;

( ) P = IOUT VOUT 2

40VIN

Diode loss;

( )( ) P = VF VIN – VOUT IOUT

VIN

(Use VF vs IF graph on diode data sheet, assuming IF =

IOUT)

RS = Inductor series resistance

RSW = Switch resistance of LT1271, etc.

IF = Diode current

VF = Diode forward voltage at IF = IOUT

Inductor core loss depends on peak-to-peak ripple current

in the inductor, which is independent of load current for

any load current large enough to establish continuous

current in the inductor. Believe it or not, core loss is also

independent of the physical size of the core. It depends

only on core material, inductance value, and switching

frequency for fixed regulator operating conditions. In-

creasing inductance or switching frequency will reduce

core loss, because of the resultant decrease in ripple

current. For high efficiency, low loss cores such as ferrites

or Magnetics Inc. molypermalloy or KoolMµ are recom-

mended. The lower cost Type 52 powdered iron from

Phillips is acceptable only if larger inductance is used and

the increased size and slight loss in efficiency is accept-

able. In a typical buck converter using the LT1271 (60kHz)

with a 12V input, and a 50µH inductor, core loss with a

Type 52 powdered iron core is 203mW. A molypermalloy

core reduces this figure to 28mW. With a 1A output, this

translates to 4% and 0.56% core loss respectively – a big

difference in a high efficiency converter. For details on

inductor design and losses, see Application Note 44.

What are the benefits of using an active (synchronous)

switch to replace the catch diode? This is the trendy thing

to do, but calculations and actual breadboards show that

the improvement in efficiency is only a few percent at best.

This can be shown with the following simplified formulas:

( )( ) Diode Loss = VF VIN – VOUT IOUT

VIN

7

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