LTC1771 データシートの表示（PDF） - Linear Technology

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LTC1771

10µA Quiescent Current High Efficiency Step-Down DC/DC Controller

Linear Technology 

LTC1771

APPLICATIO S I FOR ATIO

continuous mode, IGATECHG = fQP where QP is the gate

charge of the internal switch. Both the DC bias and gate

charge losses are proportional to VIN and thus their

effects will be more pronounced at higher supply

voltages.

3. I2R losses are predicted from the internal switch, induc-

tor and current sense resistor. In continuous mode the

average output current flows through L but is “chopped”

between the P-channel MOSFET in series with RSENSE

and the output diode. The MOSFET RDS(ON) plus RSENSE

multiplied by the duty cycle can be summed with the

resistance of L to obtain I2R losses.

4. The catch diode loss is proportional to the forward drop

as the diode conducts current during the off-time and is

more pronounced at high supply voltages where the

off-time is long. However, as discussed in the Catch

Diode section, diodes with lower forward drops often

have higher leakage currents, so although efficiency is

improved, the no-load supply current will increase. The

diode loss is calculated by multiplying the forward

voltage drop times the diode duty cycle multiplied by

the load current.

Other losses including CIN and COUT ESR dissipative

losses, and inductor core losses, generally account for

less than 2% total additional loss.

Output Voltage Programming

The output voltage is programmed with an external divider

from VOUT to VFB (Pin 1) as shown in Figure 2. The

regulated voltage is determined by:

VOUT = 1.231+ RR21

VOUT

R2

CFF

5pF

VFB

LTC1771

R1

GND

1771 F02

Figure 2. LTC1771 Adjustable Configuaration

To minimize no-load supply current, resistor values in the

megohm range should be used. The increase in supply

current due to the feedback resistors can be calculated

from:

∆IVIN

=

VOUT

R1+ R2





VOUT

VIN





A 5pF feedforward capacitor across R2 is recommended

to minimize output voltage ripple in Burst Mode operation.

Run/Soft-Start Function

The RUN/SS pin is a dual purpose pin that provides the

soft- start function and a means to shut down the LTC1771.

Soft-start reduces the input surge current from VIN by

gradually increasing the internal current limit. Power

supply sequencing can also be accomplished using

this pin.

An internal 1µA current source charges up an external

capacitor CSS. When the voltage on the RUN/SS reaches

1V, the LTC1771 begins operating. As the voltage on the

RUN/SS continues to ramp from 1V to 2.2V, the internal

current limit is also ramped at a proportional linear rate.

The current limits begins near 40% maximum load at

VRUN/SS = 1V and ends at maximum load at VRUN/SS =

2.2V. The output current thus ramps up slowly, reducing

the starting surge current required from the input power

supply. If the RUN/SS has been pulled all the way to

ground, there will be a delay before the current limit starts

increasing and is given by:

tDELAY ≈ CSS/ICHG

where ICHG ≅ 1µA. Pulling the RUN/SS pin below 0.5V

puts the LTC1771 into a low quiescent current shutdown

(IQ < 2µA).

Foldback Current Limiting

As described in the Catch Diode Selection, the worst-case

dissipation for diode occurs with a short-circuit output,

when the diode conducts the current limit value almost

continuously. In most applications this will not cause

excessive heating, even for extended fault intervals. How-

ever, when heat sinking is at a premium or higher forward

voltage drop diodes are being used, foldback current

10

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