LTC1174 查看數據表（PDF） - Linear Technology

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LTC1174

High Efficiency Step-Down and Inverting DC/DC Converter

Linear Technology 

LTC1174

LTC1174-3.3/LTC1174-5

APPLICATIO S I FOR ATIO

Catch Diode Selection

The catch diode carries load current during the off-time. The

average diode current is therefore dependent on the

P-channel switch duty cycle. At high input voltages the

diode conducts most of the time. As VIN approaches VOUT

the diode conducts only a small fraction of the time. The

most stressful condition for the diode is when the output is

short-circuited. Under this condition the diode must safely

handle IPEAK at close to 100% duty cycle. A fast switching diode

must also be used to optimize efficiency. Schottky diodes are

a good choice for low forward drop and fast switching times.

Most LTC1174 circuits will be well served by either a 1N5818,

a MBRS140T3 or a MBR0520L Schottky diode.

Short-Circuit Protection

The LTC1174 is protected from output short by its internal

current limit. Depending on the condition of IPGM pin, the

limit is either set to 340mA or 600mA. In addition, the off-

time of the switch is increased to allow the inductor’s

current to decay far enough to prevent any current build-up

(see Figure 2).

IPGM = VIN

IPGM = 0

GND

L = 100µH

VIN = 13.5V

20µs/DIV

1174 F02

Figure 2. Inductor's Current with Output Shorted

Low-Battery Detector

The low-battery indicator senses the input voltage through

an external resistive divider. This divided voltage connects

to the “–” input of a voltage comparator (Pin 3) which is

compared with a 1.25V reference voltage. With the current

going into Pin 3 being negligible, the following expression

is used for setting the trip limit:

VLBTRIP = 1.25⎛⎝⎜1+ RR34⎞⎠⎟

When the LTC1174 is shut down, the low-battery detector

is inactive.

VIN

R4

LTC1174

3

–

+

R3

1.25V

REFERENCE

1174 F03

Figure 3. Low-Battery Comparator

LTC1174 Adjustable/Low Noise Applications

The LTC1174 develops a 1.25V reference voltage between

the feedback (Pin 1) terminal and ground (see Figure 4). By

selecting resistor R1, a constant current is caused to flow

through R1 and R2 to set the overall output voltage. The

regulated output voltage is determined by:

VOUT = 1.25⎛⎝⎜1 + RR21⎞⎠⎟

For most applications, a 30k resistor is suggested for R1.

To prevent stray pickup, a 100pF capacitor is suggested

across R1 located close to the LTC1174. Alternatively, a

capacitor across R2 can be used to increase the switching

frequency for low noise operation.

Inverting Applications

The LTC1174 can easily be set up for a negative output

voltage. If – 5V is desired, the LTC1174-5 is ideal for this

application as it requires the least components. Figure 5

shows the schematic for this application. Note that the

1174fe

9

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