LTC1773EMS データシートの表示（PDF） - Linear Technology

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LTC1773EMS

Synchronous Step-Down DC/DC Controller

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OPERATIO (Refer to Functional Diagram)

and forces the main switch to stay off for the same number

of cycles. Increasing the output load current slightly,

above the minimum required for discontinuous conduc-

tion mode, allows constant frequency PWM.

Frequency synchronization is inhibited when the feedback

voltage, VFB, is below 0.6V. This prevents the external

clock from interfering with the frequency foldback for

short-circuit protection.

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 IR

voltage drop across the external P-channel MOSFET,

sense resistor, and the inductor.

Undervoltage Lockout

A precision undervoltage lockout shuts down the LTC1773

when VIN drops below 2.5V, making it ideal for single

lithium-ion battery applications. In shutdown, the LTC1773

draws only several microamperes, which is low enough to

prevent deep discharge and possible damage to the lithium-

ion battery that’s nearing its end of charge. A 150mV

hysteresis ensures reliable operation with noisy supplies.

LTC1773

Low Supply Operation

The LTC1773 is designed to operate down to a 2.65V

supply voltage. For proper operation at this low input

voltage, sub-logic level MOSFETs are required. When the

value of the output voltage is very close to the input

voltage, the converter is running at high duty cycles or in

dropout where the main switch is on continuously. See

Efficiency Considerations in the Applications Information

section.

Slope Compensation and Inductor Peak Current

Slope compensation provides stability by preventing

subharmonic oscillations. It works by internally adding a

ramp to the inductor current signal at duty cycles in excess

of 30%. This causes the internal current comparator to trip

earlier. The ITH clamp level is also reached earlier than

conditions in which the duty cycle is below 30%. As a

result, the maximum inductor peak current is lower for

VOUT/VIN > 0.3 than when VOUT/VIN < 0.3.

To compensate for this loss in maximum inductor peak

current during high duty cycles, the LTC1773 uses a

patent pending scheme that raises the ITH clamp level

(proportional to the amount of slope compensation) when

duty cycle is above 30%.

APPLICATIONS INFORMATION

The basic LTC1773 application circuit is shown in

Figure 1. External component selection is driven by the

load requirement and begins with the selection of RSENSE.

Once RSENSE is known, L can be chosen, followed by the

external power MOSFETs. Finally, CIN and COUT are se-

lected.

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current.

The LTC1773 current comparator has a maximum thresh-

old of 100mV/RSENSE. The current comparator threshold

sets the peak of the inductor current, yielding a maximum

average output current IMAX equal to the peak value less

half the peak-to-peak ripple current ∆IL.

Allowing a margin for variations in the LTC1773 and

external component values yields:

RSENSE = 70mV/IMAX

Inductor Value Calculation

The inductor selection will depend on the operating fre-

quency of the LTC1773. The internal preset frequency is

550kHz, but can be externally synchronized up to 750kHz.

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