LTC1701(1999) 데이터 시트보기 (PDF) - Linear Technology

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LTC1701
(Rev.:1999)

1MHz Step-Down DC/DC Converter in SOT-23

Linear Technology 

LTC1701

APPLICATIO S I FOR ATIO

Soft-start can be implemented by ramping the voltage on

ITH/RUN during start-up as shown in Figure 3(c). As the

voltage on ITH/RUN ramps through its operating range the

internal peak current limit is also ramped at a proportional

linear rate.

During normal operation the voltage on the ITH/RUN pin

will vary from 1.25V to 2.25V depending on the load

current. Pulling the ITH/RUN pin below 0.8V puts the

LTC1701 into a low quiescent current shutdown mode

(IQ < 1µA). This pin can be driven directly from logic as

shown in Figures 3(a) and 3(b).

3.3V OR 5V

ITH/RUN

D1

ITH/RUN

CC

CC

RC

RC

(a)

(b)

ITH/RUN

R1 D1

CC

C1

RC

(c)

1701 F03

Figure 3. ITH/RUN Pin Interfacing

Efficiency Considerations

The percent efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efficiency and what change would

produce the most improvement. Percent efficiency can be

expressed as:

%Efficiency = 100% – (L1 + L2 + L3 + ...)

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

of input power.

Although all dissipative elements in the circuit produce

losses, 4 main sources usually account for most of the

losses in LTC1701 circuits: 1) LTC1701 VIN current,

2) switching losses, 3) I2R losses, 4) Schottky diode

losses.

1) The VIN current is the DC supply current given in the

electrical characteristics which excludes MOSFET driver

and control currents. VIN current results in a small (< 0.1%)

loss that increases with VIN, even at no load.

2) The switching current is the sum of the internal MOSFET

driver and control currents. The MOSFET driver current

results from switching the gate capacitance of the power

MOSFET. Each time a MOSFET gate is switched from low

to high to low again, a packet of charge dQ moves from VIN

to ground. The resulting dQ/dt is a current out of VIN that

is typically much larger than the control circuit current. In

continuous mode, IGATECHG = f • QP, where QP is the gate

charge of the internal MOSFET switch.

3) I2R Losses are predicted from the DC resistances of the

MOSFET and inductor. In continuous mode the average

output current flows through L, but is “chopped” between

the topside internal MOSFET and the Schottky diode. At

low supply voltages where the switch on-resistance is

higher and the switch is on for longer periods due to the

higher duty cycle, the switch losses will dominate. Using

a larger inductance helps minimize these switch losses. At

high supply voltages, these losses are proportional to the

load. I2R losses cause the efficiency to drop at high output

currents.

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

high currents and gets worse at low output voltages. The

diode loss is calculated by multiplying the forward voltage

drop times the diode duty cycle multiplied by the load

current.

Other “hidden” losses such as copper trace and internal

battery resistances can account for additional efficiency

degradations in portable systems. It is very important to

include these “system” level losses in the design of a

system. The internal battery and fuse resistance losses

can be minimized by making sure that CIN has adequate

charge storage and very low ESR at the switching fre-

quency. Other losses including Schottky conduction losses

during dead-time and inductor core losses generally ac-

count for less than 2% total additional loss.

8

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