LTC1878 データシートの表示（PDF） - Linear Technology

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LTC1878

High Efficiency Monolithic Synchronous Step-Down Regulator

Linear Technology 

LTC1878

U

OPERATIO

Dropout Operation

When the input supply voltage decreases toward the

output voltage, the duty cycle increases toward the maxi-

mum on-time. Further reduction of the supply voltage

forces the main switch to remain on for more than one

cycle until it reaches 100% duty cycle. The output voltage

will then be determined by the input voltage minus the

voltage drop across the internal P-channel MOSFET and

the inductor.

Low Supply Operation

The LTC1878 is designed to operate down to an input

supply voltage of 2.65V although the maximum allowable

output current is reduced at this low voltage. Figure 1

shows the reduction in the maximum output current as a

function of input voltage for various output voltages.

Another important detail to remember is that at low input

supply voltages, the RDS(ON) of the P-channel switch

increases. Therefore, the user should calculate the power

dissipation when the LTC1878 is used at 100% duty cycle

with a low input voltage (see Thermal Considerations in

the Applications Information section).

Slope Compensation and Inductor Peak Current

Slope compensation provides stability in constant fre-

quency architectures by preventing subharmonic oscilla-

tions at high duty cycles. It is accomplished internally by

adding a compensating ramp to the inductor current

signal at duty cycles in excess of 40%. As a result, the

maximum inductor peak current is reduced for duty cycles

> 40%. This is shown in the decrease of the inductor peak

current as a function of duty cycle graph in Figure 2.

1200

L = 10µH

1000

800

600

VOUT = 1.5V

VOUT = 3.3V

VOUT = 2.5V

400

200

1100

VIN = 3.3V

1000

900

800

700

0

2.5 3.5 4.5 5.5 6.5 7.5

INPUT VOLTAGE (V)

1878 F01

Figure 1. Maximum Output Current vs Input Voltage

600

0

20

40

60

80 100

DUTY CYCLE (%)

1878 F02

Figure 2. Maximum Inductor Peak Current vs Duty Cycle

APPLICATIO S I FOR ATIO

The basic LTC1878 application circuit is shown on the first

page. External component selection is driven by the load

requirement and begins with the selection of L followed by

CIN and COUT.

Inductor Value Calculation

The inductor selection will depend on the operating fre-

quency of the LTC1878. The internal nominal frequency is

550kHz, but can be externally synchronized from 400kHz

to 700kHz.

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

of smaller inductor and capacitor values. However, oper-

ating at a higher frequency generally results in lower

efficiency because of increased internal gate charge losses.

The inductor value has a direct effect on ripple current. The

ripple current ∆IL decreases with higher inductance or

frequency and increases with higher VIN or VOUT.

8

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