LM1771_15 datasheet(9/26 Pages) TI

部品番号	LM1771

部品情報	LM1771 Low-Voltage Synchronous Buck Controller with Precision Enable and No External Compensation

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LM1771

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SNVS446C – JUNE 2006 – REVISED APRIL 2013

To maintain a set frequency in an application,

α is always held constant by varying TON inversely with VIN. The

three versions of the LM1771 are identified by the on times at a VIN of 3.3V for consistency. For clarification see

the table below:

Product ID

TON @ 3.3V

α (V µs)

LM1771S

0.5µs

1.65

LM1771T

1.0µs

3.3

LM1771U

2.0µs

6.6

The variation of TON versus VIN can also be expressed graphically. These graphs can be found in the typical

curves section of the datasheet.

With

α being a constant regardless of the version of the LM1771 used, Equation 6 shows that the only

dependent variable remaining is VOUT. Since VOUT will be a constant in any application, the frequency will also

remain constant. The switching frequency at which the application runs depends upon the VOUT desired and the

LM1771 version chosen. For any VOUT, three frequency options (LM1771 versions) can be selected. This can be

seen in the table below. The recommended frequency range of operation is 100kHz to 1000kHz.

Timing Options (1)

VOUT

500ns

1000ns

2000ns

0.8

485

242

121

1

606

303

152

1.2

727

364

182

1.5

909

455

227

1.8

1091

545

273

2.5

1515

758

379

3.3

2000

1000

500

(1)

Switching Frequency (kHz) of LM1771 based on output voltage and

timing option.

Short-Circuit Protection

The LM1771 has an internal short circuit comparator that constantly monitors the feedback node (except during

soft-start). If the feedback voltage drops below 0.55V (equivalent to the output voltage dropping below 68% of

nominal), the comparator will trip causing the part to latch off. The LM1771 will not resume switching until the

input voltage is taken below the UVLO threshold and then brought back into its normal operating range, or the

part is disabled then re-enabled through the enable pin. The purpose of this function is to prevent a severe short

circuit from causing damage to the application. Due to the fast transient response of the LM1771 a severe short

on the output causing the feedback to drop would only occur if the load applied had an effective resistance that

approaches the PMOS RDS(ON).

Precision Enable

The LM1771 features a precision enable circuit. If the voltage on the EN pin is 1.2V or greater, the part is

enabled and switching will occur. If the enable voltage falls below 1.2V, the part will be placed into a shutdown

state and the drivers will be tri-stated. This allows the LM1771 to be easily sequenced using a resistive divider

from the output of another regulator, or the working input voltage range of the LM1771 to be set using a resistive

divider on VIN. There is no internal pull-up connected to the EN pin, so an external signal is required to initiate

switching. It should be noted that when power is first applied to the LM1771, there is a slight delay before the

enable comparator is functional. During this delay, typically on the order of 400 µs, the part will be disabled

regardless of the voltage on the EN pin. The falling enable threshold features 50 mV of hysteresis

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