FSD200B Ver la hoja de datos (PDF) - Fairchild Semiconductor

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FSD200B

Green Mode Fairchild Power Switch (FPS™)

Fairchild Semiconductor 

FSD210B, FSD200B

4. Protection Circuit : The FSD200B/210B have 2 self-

protection functions : over load protection (OLP) and

thermal shutdown (TSD). Because these protection circuits

are fully integrated inside the IC without external compo-

nents, the reliability is improved without increasing cost.

Once a fault condition occurs, switching is terminated and

the Sense FET remains off. This causes Vcc to fall. When

Vcc reaches the UVLO stop voltage VSTOP (6.7V-

FSD210B, 6V-FSD200B), 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 VSTART (8.7V-FSD210B, 7V-FSD200B), the

device 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 elimi-

nated.

VFB

4.5V

Over Load Protection

3V

t12= CFB×(V(t2)-V(t1)) / IDELAY

t1

t2 t

t12

=

C FB

V (t2 ) − V (t1 ) ;

I DELAY

I DELAY = 5µ A, V (t1 ) = 3V , V (t2 ) = 4.5V

Figure 10. Over Load Protection (OLP)

OSC

Vfb

4

Cfb

5uA 400uA

+

-

R

OLP

RESET

4.5V TSD

SQ

R

GATE

DRIVER

SQ

R

A/R

FSD2xxB

OLP, TSD

Protection Block

4.2 Thermal Shutdown (TSD) : The Sense FET and the

control IC are integrated, making it easier for the control IC

to detect the temperature of the Sense FET. When the

temperature exceeds approximately 145°C, thermal

shutdown is activated.

Figure 9. Protection Block

4.1 Over Load Protection (OLP) : Overload is defined as

the load current exceeding a pre-set level due to an

unexpected event. In this situation, the protection circuit

should be activated in order to protect the SMPS. However,

even when the SMPS is operating normally, the over load

protection (OLP) circuit can be activated during the load

transition. In order to avoid this undesired operation, the

OLP circuit is designed to be activated after a specified time

to determine whether it is a transient situation or an overload

situation. In conjunction with the Ipk current limit pin (if

used) the current mode feedback path would limit the current

in the Sense FET when the maximum PWM duty cycle is

attained. If the output consumes more than this maximum

power, the output voltage (Vo) decreases below its rating

voltage. This reduces the current through the opto-coupler

LED, which also reduces the opto-coupler transistor current,

thus increasing the feedback voltage (VFB). If VFB exceeds

3V, the feedback input diode is blocked and the 5uA current

source (IDELAY) starts to charge Cfb slowly up to Vcc. In

this condition, VFB increases until it reaches 4.5V, when the

switching operation is terminated as shown in Figure 10. The

shutdown delay time is the time required to charge Cfb from

3V to 4.5V with 5uA current source.

5. Soft Start : FSD200B/210B has an internal soft start

circuit that gradually increases current through the Sense

FET as shown in Figure 11. The soft start time is 3msec in

FSD200B/210B.

I(A)

3ms

0.2A

0.25A

0.3A

t

Figure 11. Internal Soft Start

10

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