FSDM0465RB データシートの表示（PDF） - Fairchild Semiconductor

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FSDM0465RB

Green Mode Fairchild Power Switch (FPS™)

Fairchild Semiconductor 

FSDM0465RB

Functional Description

1. Startup: In previous generations of Fairchild Power

Switches (FPSTM) the Vcc pin had an external start-up

resistor to the DC input voltage line. In this generation the

startup resistor is replaced by an internal high voltage current

source. At startup, an internal high voltage current source

supplies the internal bias and charges the external capacitor

(Ca) that is connected to the Vcc pin as illustrated in Figure

4. When Vcc reaches 12V, the FSDM0465RB begins

switching and the internal high voltage current source is

disabled. Then, the FSDM0465RB continues its normal

switching operation and the power is supplied from the

auxiliary transformer winding unless Vcc goes below the

stop voltage of 8V.

VDC

Ca

2.1 Pulse-by-Pulse Current Limit: Because current mode

control is employed, the peak current through the SenseFET

is limited by the inverting input of the PWM comparator

(Vfb*) as shown in Figure 5. Assuming that the 0.9mA

current source flows only through the internal resistor (2.5R

+R= 2.8 kΩ), the cathode voltage of diode D2 is about 2.5V.

Since D1 is blocked when the feedback voltage (Vfb)

exceeds 2.5V, the maximum voltage of the cathode of D2 is

clamped at this voltage, thus clamping Vfb*. Therefore, the

peak value of the current through the SenseFET is limited.

2.2 Leading Edge Blanking (LEB): At the instant the

internal SenseFET is turned on, there usually exists a high

current spike through the SenseFET, caused by primary-side

capacitance and secondary-side rectifier reverse recovery.

Excessive voltage across the Rsense resistor would lead to

incorrect feedback operation in the current mode PWM

control. To counter this effect, the FSDM0465RB employs

an LEB circuit. This circuit inhibits the PWM comparator for

a short time (TLEB) after the, SenseFET is turned on.

Vcc

3

8V/12V

VVccccGgoooodd

6 Vstr

ICH

Vref

Internal

Bias

Vcc Vref

Idelay

IFB

Vo

Vfb

4

H11A817A

CB

OSC

D1 D2

2.5R

KA431

+

Vfb* R

-

SenseFET

GGaattee

Ddriivveer r

VSD

OLP

Rsense

Figure 4. Internal Startup Circuit

2. Feedback Control: FSDM0465RB employs current

mode control, as shown in Figure 5. An opto-coupler (such

as the H11A817A) and shunt regulator (such as the KA431)

are typically used to implement the feedback network.

Comparing the feedback voltage with the voltage across the

Rsense resistor plus an offset voltage makes it possible to

control the switching duty cycle. When the reference pin

voltage of the KA431 exceeds the internal reference voltage

of 2.5V, the H11A817A LED current increases, thus

decreasing the feedback voltage and reducing the duty cycle.

This event typically happens when the input voltage is

increased or the output load is decreased.

Figure 5. Pulse Width Modulation (PWM) Circuit

3. Protection Circuit: The FSDM0465RB has several self

protective functions such as over load protection (OLP), over

voltage protection (OVP), and thermal shutdown (TSD).

Because these protection circuits are fully integrated into the

IC without external components, the reliability can be

improved without increasing cost. Once the fault condition

occurs, switching is terminated and the SenseFET remains

off. This causes Vcc to fall. When Vcc reaches the UVLO

stop voltage, 8V, the protection is reset and the internal high

voltage current source charges the Vcc capacitor via the Vstr

pin. When Vcc reaches the UVLO start voltage,12V, the

FSDM0465RB resumes its normal operation. In this manner,

the auto-restart can alternately enable and disable the

switching of the power Sense FET until the fault condition is

eliminated (see Figure 6).

10

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