11

LTC1147-3.3

LTC1147-5/LTC1147L

sn1147 1147fds

where L1, L2, etc., are the individual losses as a percent-

age of input power. (For high efficiency circuits only small

errors are incurred by expressing losses as a percentage

of output power.)

Although all dissipative elements in the circuit produce

losses, four main sources usually account for most of the

losses in LTC1147 circuits: 1) LTC1147 DC bias current,

2) MOSFET gate charge current, 3) I

voltage drop of the Schottky diode.

1. The DC supply current is the current which flows into

the LTC1147 series DC supply current is 160

µA for no

load, and increases proportionally with load up to a

constant 1.6mA after the LTC1147 series has entered

continuous mode. Because the DC bias current is

generally less than 1% for load currents over 30mA.

However, at very low load currents the DC bias current

accounts for nearly all of the loss.

2. MOSFET gate charge current results from switching

the gate capacitance of the power MOSFET. Each time

a MOSFET gate is switched from low to high to low

which is typically much larger than the DC supply

typical gate charge for a 0.135

Ω P-channel power

100kHz continuous operation for a 2% to 3% typical

Note that the gate charge loss increases directly with

both input voltage and operating frequency. This is the

principal reason why the highest efficiency circuits

operate at moderate frequencies. Furthermore, it ar-

gues against using a larger MOSFET than necessary to

control I

well as money!

3. I

tances of the MOSFET, inductor and current shunt. In

continuous mode the average output current flows

multiplied by the P-channel duty cycle can be summed

to 10% as the output current increases from 0.5A to

output currents.

4. The Schottky diode is a major source of power loss at

high currents and gets worse at high input voltages.

The diode loss is calculated by multiplying the forward

voltage drop times the Schottky diode duty cycle

multiplied by the load current. For example, assuming

a duty cycle of 50% with a Schottky diode forward

voltage drop of 0.4V, the loss increases from 0.5% to

8% as the load current increases from 0.5A to 2A.

Figure 5 shows how the efficiency losses in a typical

LTC1147 series regulator end up being apportioned.

The gate charge loss is responsible for the majority of

the efficiency lost in the midcurrent region. If Burst

Mode operation was not employed at low currents,

the gate charge loss alone would cause efficiency to

drop to unacceptable levels. With Burst Mode opera-

tion, the DC supply current represents the lone (and

unavoidable) loss component which continues to

become a higher percentage as output current is

diode loss dominate at high load currents.

Figure 5. Efficiency Loss

OUTPUT CURRENT (A)

0.01

90

95

1

LTC1147 • F05

85

80

0.03

0.1

0.3

3

100

GATE CHARGE

SCHOTTKY

DIODE

APPLICATIO S I FOR ATIO