LT1936H 查看數據表（PDF） - Linear Technology

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LT1936H

1.4A, 500kHz Step-Down Switching Regulator

Linear Technology 

LT1936

APPLICATIONS INFORMATION

the SHDN pin, the SW pin current will drop to essentially

zero. However, if the VIN pin is grounded while the output

is held high, then parasitic diodes inside the LT1936 can

pull large currents from the output through the SW pin

and the VIN pin. Figure 6 shows a circuit that will run only

when the input voltage is present and that protects against

a shorted or reversed input.

D4

MBRS140

VIN

VIN

BOOST

LT1936

SHDN

SW

VC

COMP GND FB

VOUT

BACKUP

1936 F06

Figure 6. Diode D4 Prevents a Shorted Input from Discharging

a Backup Battery Tied to the Output; It Also Protects the Circuit

from a Reversed Input. The LT1936 Runs Only When the Input

is Present

PCB Layout

For proper operation and minimum EMI, care must be

taken during printed circuit board layout. Figure 7 shows

the recommended component placement with trace,

ground plane and via locations. Note that large, switched

currents flow in the LT1936’s VIN and SW pins, the catch

diode (D1) and the input capacitor (C2). The loop formed

by these components should be as small as possible. These

components, along with the inductor and output capacitor,

should be placed on the same side of the circuit board,

and their connections should be made on that layer. Place

a local, unbroken ground plane below these components.

The SW and BOOST nodes should be as small as possible.

Finally, keep the FB and VC nodes small so that the ground

traces will shield them from the SW and BOOST nodes.

The Exposed Pad on the bottom of the package must be

soldered to ground so that the pad acts as a heat sink. To

keep thermal resistance low, extend the ground plane as

much as possible, and add thermal vias under and near

the LT1936 to additional ground planes within the circuit

board and on the bottom side.

IN

MINIMIZE

LT1936

C2

C2, D1 LOOP

D2

GND

R4

C3

D1

R2

R1

L1

C1

OUT

VIAS

GND

1936 F07

Figure 7. A Good PCB Layout Ensures Low EMI Operation

High Temperature Considerations

The die temperature of the LT1936 must be lower than the

maximum rating of 125°C (150°C for the H grade). This is

generally not a concern unless the ambient temperature

is above 85°C. For higher temperatures, care should be

taken in the layout of the circuit to ensure good heat sink-

ing of the LT1936. The maximum load current should be

derated as the ambient temperature approaches 125°C

(150°C for the H grade).

The die temperature is calculated by multiplying the LT1936

power dissipation by the thermal resistance from junction

to ambient. Power dissipation within the LT1936 can be

estimated by calculating the total power loss from an

efficiency measurement and subtracting the catch diode

loss. The resulting temperature rise at full load is nearly

independent of input voltage. Thermal resistance depends

on the layout of the circuit board, but values from 40°C/W

to 60°C/W are typical.

Die temperature rise was measured on a 4-layer, 5cm ×

6.5cm circuit board in still air at a load current of 1.4A.

For 12V input to 3.3V output the die temperature elevation

above ambient was 26°C; for 24V in to 3.3V out the rise

was 31°C; for 12V in to 5V the rise was 31°C and for 24V

in to 5V the rise was 34°C.

1936fd

14

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