LTC1149 查看數據表（PDF） - Linear Technology

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LTC1149

High Efficiency Synchronous Step-Down Switching Regulators

Linear Technology 

UU W U

APPLICATIO S I FOR ATIO

output ripple at low frequencies will be large enough to trip

the voltage comparator. This causes the Burst Mode

operation to be activated when the LTC1149 series would

normally be in continuous operation. The effect is most

pronounced with low values of RSENSE and can be

improved by operating at higher frequencies with lower

values of L. The output remains in regulation at all times.

Checking Transient Response

Switching regulators take several cycles to respond to a

step in DC (resistive) load current. When a load step

occurs, VOUT shifts by an amount equal to (∆ILOAD)(ESR),

where ESR is the effective series resistance of COUT.

∆ILOAD also begins to charge or discharge COUT until the

1000

800

L = 50µH

RSENSE = 0.02Ω

600

L = 25µH

RSENSE = 0.02Ω

400

L = 50µH

RSENSE = 0.05Ω

200

0

0

1

2

3

4

5

(VIN – VOUT) VOLTAGE (V)

1149 F04

Figure 4. Minimum Suggested COUT

LTC1149

LTC1149-3.3/LTC1149-5

regulator loop adapts to the current change and returns

VOUT to its steady state value. During this recovery time

VOUT can be monitored for overshoot or ringing which

would indicate a stability problem. The Pin 7 external

components shown in the Figure 1 circuit will prove

adequate compensation for most applications.

A second, more severe transient is caused by switching in

loads with large (>1µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in parallel

with COUT, causing a rapid drop in VOUT. No regulator can

deliver enough current to prevent this problem if the load

switch resistance is low and it is driven quickly. The only

solution is to limit the rise time of the switch drive so that

the load rise time is limited to approximately (25)(CLOAD).

Thus a 10µF capacitor would require a 250µs rise time,

limiting the charging current to about 200mA.

LTC1149 Adjustable Applications

When an output voltage other than 3.3V or 5V is required,

the LTC1149 adjustable version is used with an external

resistive divider from VOUT to VFB Pin 10. The regulated

voltage is determined:

) VOUT = 1.25

1+

R2

R1

In applications where VOUT is greater than the LTC1149

internally regulated VCC voltage, RSENSE must be moved to

1N4148

+

1µF

0.068µF

0V = NORMAL

>2V = SHUTDOWN

VIN

CAP

PGATE

VCC

VCC

PDRIVE

NGATE

LTC1149

SHDN2

VFB

3300pF

1k

ITH

CT

CT

200pF

SENSE +

SENSE –

GNDS

VIN

1N4148

0.047µF

IRF9Z34

IRFZ34

+ 150µF

50V

100µH

1N5819

100pF

1000pF

( ) VOUT =

1.25

1+

R2

R1

VALUES SHOWN FOR VOUT = 12V

R2

215k

1%

+

R1

25k

1%

150µF

16V

OS-CON

RSENSE

0.05Ω

VOUT LOAD

OUTPUT

GROUND

CONNECTION

1149 F05

Figure 5. High Efficiency Step-Down Regulator with VOUT > VCC

11

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