LTC1574CS(RevA) 查看數據表（PDF） - Linear Technology

零件编号

产品描述 (功能)

生产厂家

LTC1574CS
(Rev.:RevA)

High Efficiency Step-Down DC/DC Converters with Internal Schottky Diode

Linear Technology 

LTC1574

LTC1574-3.3/LTC1574-5

APPLICATIO S I FOR ATIO

inductor L1 is chosen for its excellent self-shielding prop-

erties. Open magnetic structures such as drum and rod

cores are to be avoided since they inject high flux levels

into their surroundings. This can become a major source

of noise in any converter circuit.

Design Example

As a design example, assume VIN = 9V (nominal),

VOUT = 5V and IOUT = 350mA maximum. The LTC1574-5

is used for this application with IPGM (Pin 6) connected to

VIN. The minimum value of L is determined by assuming

the LTC1574-5 is operating in continuous mode.

IPEAK

AVG CURRENT = IOUT

IV

= IPEAK + IV

2

= 350mA

TIME

1574 • F06

Figure 6. Continuous Inductor Current

With IOUT = 350mA and IPEAK = 0.6A (IPGM = VIN), IV = 0.1A.

The peak-to-peak ripple inductor current, IRIPPLE, is 0.5A

and is also equal to:

( ) IRIPPLE

=

4

•

10−6





VOUT +

L

VD 



AP-P

Solving for L in the above equation and with VD = 0.5V,

L = 44µH. The next higher standard value of L is 50µH

(example: Coiltronics CTX50-4). The operating frequency,

ignoring voltage across diode VD is:

f

≈

2.5

•

105



1

−

VOUT

VIN





= 111kHz

With the value of L determined, the requirements for CIN

and COUT are calculated. For CIN, its RMS current rating

should be at least:

[ ( )]1/2

IOUT VOUT VIN − VOUT

IRMS =

VIN

= 174mA

( ) ARMS

For COUT, the RMS current rating should be at least:

( ) IRMS

≈

IPEAK

2

A RMS

= 300mA

Absolute Maximum Ratings and Latchup Prevention

The absolute maximum ratings specify that SW

(Pins 3, 14) can never exceed VIN (Pin 5) by more than

0.3V. Normally this situation should never occur. It could,

however, if the output is held up while the supply is pulled

down. A condition where this could potentially occur is

when a battery is supplying power to an LTC1574 regula-

tor and also to one or more loads in parallel with the the

regulator’s VIN. If the battery is disconnected while the

LTC1574 regulator is supplying a light load and one of the

parallel circuits is a heavy load, the input capacitor of the

LTC1574 regulator could be pulled down faster than the

output capacitor, causing the absolute maximum ratings

to be exceeded. The result is often a latchup which can be

destructive if VIN is reapplied. Battery disconnect is pos-

sible as a result of mechanical stress, bad battery contacts

or use of a lithium-ion battery with a built-in internal

disconnect. The user needs to assess his/her application

to determine whether this situation could occur. If so,

additional protection is necessary.

Prevention against latchup can be accomplished by

simply connecting a Schottky diode across the SW and

VIN pins as shown in Figure 7. The diode will normally be

reverse biased unless VIN is pulled below VOUT at which

time the diode will clamp the (VOUT – VIN) potential to less

than the 0.6V required for latchup. Note that a low

leakage Schottky should be used to minimize the effect

LATCHUP

PROTECTION

SCHOTTKY

VIN SW

LTC1574

VOUT

+

1574 F07

Figure 7. Preventing Absolute Maximum

Ratings from Being Exceeded

6

Share Link: