SC470ITSTRT datasheet(15/31 Pages) SEMTECH

15 / 31 page

 2005 Semtech Corp.

www.semtech.com

SC470

POWER MANAGEMENT

Application Information (Cont.)

Figure 4: Dynamic Voltage Switching Using Direct

Drive Method (V

OUT(HIGH)/VOUT(LOW) < 1.16 only)

Another option is to switch using an external discrete

MOSFET, as shown in Figure 5.

Figure 5: Dynamic Voltage Switching Using Indirect

Drive Method

The problem with the external MOSFET method is that

the drain-gate capacitance, c

DG, can cause the output

voltage to go even higher when the MOSFET is first turned

off (which should make the output voltage drop). This is

because the gate going low causes the drain to go low

momentarily due to c

DG, which in turn causes VFB to go

low, making the output rise. The extra R9 and C11 in the

gate drive for the MOSFET are there to slow down the

slew rate of the gate voltage, thus avoiding this problem.

Determining what circuit to use depends upon the ratio

between V

OUT(HIGH) and VOUT(LOW), since the goal is to avoid

inadvertently tripping the over-voltage protection.

If:

)

LOW

(

OUT

)

HIGH

(

OUT

This means that the ratio is less than the worst-case

OVP threshold (worst-case in this case is the lowest

threshold), then the direct drive (simplest) method may

be used. Of course the indirect drive method may also

be used if desired.

If:

)

LOW

(

OUT

)

HIGH

(

OUT

This means that the ratio is greater than the worst-case

OVP threshold, therefore we automatically need to slew

the rate of change, and the indirect drive method must

be used.

If using the indirect drive method, the goal is to slow

down the gate drive for the transition from V

OUT(HIGH) to

OUT(LOW), which is when the external MOSFET is turned

off. The pull-up resistor, pull-down resistor and gate

capacitor can be selected as follows:

1) V

GATE must be below the gate threshold voltage of the

MOSFET in order to ensure that it can be turned off (see

Figure 6):

2) The RC time constant of R9 and C11 should be at

least 4 times greater than the typical Over-Voltage Fault

Delay Time of 5µs to avoid V

OUT rising prior to falling.

3) V

PULLUP must be high enough to turn the MOSFET on.

Figure 6: Ensuring Q3 Will Turn Off

20k0

EN/PSV

TON

VOUT

VCCA

PGD

VSSA

PGND

VDDP

ILIM

BST

SC470

VOUT

0402

56p

R5 49k9

0402

Low = 1.0V

High = 1.2V

Open Drain Signal

pull-up

C11

20k0

EN/PSV

TON

VOUT

VCCA

PGD

VSSA

PGND

VDDP

ILIM

BST

SC470

VOUT

0402

56p

0402

R5 49k9

0402

Open Drain Signal

Low = 1.2V

High = 1.0V

pull-up

C11

VGATE

VPULLUP

()

)

(

PULLUP

•

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