LTC1626 데이터 시트보기 (PDF) - Linear Technology

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LTC1626

Low Voltage, High Efficiency Step-Down DC/DC Converter

Linear Technology 

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OPERATIO

The nominal off-time of the LTC1626 is set by an external

timing capacitor connected between the CT pin and ground.

The operating frequency is then determined by the off-

time and the difference between VIN and VOUT.

The output voltage is set by an external divider returned to

the VFB pin. A voltage comparator V and a gain block G

compare the divided output voltage with a reference

voltage of 1.25V.

To optimize efficiency, the LTC1626 automatically switches

between continuous and Burst Mode operation. The volt-

age comparator is the primary control element when the

device is in Burst Mode operation, while the gain block

controls the output voltage in continuous mode.

When the load is heavy, the LTC1626 is in continuous

operation. During the switch “ON” time, current compara-

tor C monitors the voltage between the SENSE + and

SENSE – pins connected across an external shunt in series

with the inductor. When the voltage across the shunt

reaches the comparator’s threshold value, its output sig-

nal changes state, resetting the flip-flop and turning the

internal P-channel MOSFET off. The timing capacitor

connected to the CT pin is now allowed to discharge at a

rate determined by the off-time controller.

When the voltage on the timing capacitor has discharged

past VTH1, comparator T trips, sets the flip-flop and causes

the switch to turn on. Also, the timing capacitor is

recharged. The inductor current will again ramp up until

the current comparator C trips. The cycle then repeats.

When the load current increases, the output voltage

LTC1626

decreases slightly. This causes the output of the gain stage

(Pin 6) to increase the current comparator threshold, thus

tracking the load current.

When the load is relatively light, the LTC1626 automati-

cally switches to Burst Mode operation. The current loop

is interrupted when the output voltage reaches the desired

regulated value. The hysteretic voltage comparator V trips

when VOUT is above the desired output voltage, turning off

the switch and causing the timing capacitor to discharge.

This capacitor discharges past VTH1 until its voltage drops

below VTH2. Comparator S then trips and a sleep signal is

generated. The circuit now enters into sleep mode with the

power MOSFET turned off. In sleep mode, the LTC1626 is

in standby and the load current is supplied by the output

capacitor. All unused circuitry is shut off, reducing quies-

cent current from 1.9mA to 165µA. When the output

capacitor discharges by the amount of the hysteresis of

the comparator V, the P-channel switch turns on again and

the process repeats itself. During Burst Mode operation,

the peak inductor’s current is set at 25mV/RSENSE.

To avoid the operation of the current loop interfering with

Burst Mode operation, a built-in offset VOS is incorporated

in the gain stage. This prevents the current from increas-

ing until the output voltage has dropped below a minimum

threshold.

In dropout, the P-channel MOSFET is turned on continu-

ously (100% duty cycle) providing low dropout operation

with VOUT ≅ VIN.

APPLICATIONS INFORMATION

The basic LTC1626 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, CT and L can be chosen. Next, the

Schottky diode D1 is selected followed by CIN and COUT.

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current.

With the current comparator monitoring the voltage devel-

oped across RSENSE, the threshold of the comparator

determines the peak inductor current. Depending upon

the load current condition, the threshold of the compara-

tor lies between 25mV/RSENSE and 150mV/RSENSE. The

maximum output current of the LTC1626 is:

IOUT(MAX) = 150mV/RSENSE – IRIPPLE/2 (A)

Where IRIPPLE is the peak-to-peak inductor ripple current.

At a relatively light load, the LTC1626 is in Burst Mode

operation. In this mode, the peak current is set at 25mV/

RSENSE. To fully benefit from Burst Mode operation, the

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