IR2157 データシートの表示（PDF） - International Rectifier

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IR2157

FULLY INTEGRATED BALLAST CONTROL IC

International Rectifier 

ADVANCED INFORMATION

IR2157

Description of Operation & Component Selection Tips

Supply Bypassing and PC Board Layout

Rules

Component selection and placement on the pc

board is extremely important when using power

control ICs VCC should be bypassed to COM as close

to the IC terminals as possible with a low ESR/ESL

capacitor, as shown in Figure 1 below.

pin 1

CBOOT (surface mount)

DBoot (surface mount)

CVCC (through hole)

CVCC (surface mount)

Figure 1: Supply bypassing PCB layout example

A rule of thumb for the value of this bypass capacitor

is to keep its minimum value at least 2500 times the

value of the total input capacitance (Ciss) of the

power transistors being driven. This decoupling

capacitor can be split between a higher valued

electrolytic type and a lower valued ceramic type

connected in parallel, although a good quality

electrolytic (e.g., 10mF) placed immediately adjacent

to the VCC and COM terminals will work well.

In a typical application circuit, the supply voltage to

the IC is normally derived by means of a high value

startup resistor (1/4W) from the rectified line voltage,

in combination with a charge pump from the output

of the half-bridge. With this type of supply

arrangement, the internal 15.6V zener clamp diode

from VCC to COM will determine the steady state IC

supply voltage.

Connecting the IC Ground (COM) to the

Power Ground

Both the low power control circuitry and low side

gate driver output stage grounds return to this pin

within the IC. The COM pin should be connected to

the bottom terminal of the current sense resistor in

the source of the low side power MOSFET using an

individual pc board trace, as shown in Figure 2. In

addition, the ground return path of the timing

components and VCC decoupling capacitor should

be connected directly to the IC COM pin, and not via

separate traces or jumpers to other ground traces on

the board.

IR2157 pin 1

timing

components

VBUS return

CVCC (surface mount)

CVCC (through hole)

RCS (through hole)

Figure 2: COM pin connection PCB layout example

These connection technique prevents high current

ground loops from interfering with sensitive timing

component operation, and allows the entire control

circuit to reject common-mode noise due to output

switching.

9

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