SC4530 datasheet(12/23 Pages) SEMTECH

12 / 23 page

SC4530

Applications Information (Continued)

The C

3 voltage increases with each subsequent switching

cycle, as does the bootstrapped voltage at the BST pin.

After a number of switching cycles, C

3 will be fully charged

to a voltage approximately equal to that applied to the

anode of D

2. The minimum BST to SW voltage required

to fully saturate the power transistor is shown in the

Typical Characteristics (pages 6-7). This difference voltage

must be at least .72V at room temperature. This is also

specified in the Electrical Characteristics (pages 3-4) as

the Minimum Bootstrap Voltage. The minimum required

C3 increases as temperature decreases. The bootstrap

circuit reaches equilibrium when the base charge drawn

from C

3 during transistor on-time is equal to the charge

replenished during the off interval.

Figure 4 summarizes various ways of bootstrapping the

SC4530. In Figure 4(a) the BIAS pin is connected to the

converter output. The bootstrap charge is obtained

from the output of the step-down converter. The input-

referred charge is reduced by the step-down ratio. This is

the most efficient configuration and it also results in the

least voltage stress at the BST pin. The maximum BST pin

voltage is about V

IN + VOUT. If the output voltage is between

2.5V and 3V, then a 0.33-0.47mF bootstrap capacitor may

be needed to reduce droop. In most other cases, a 0.22mF

ceramic capacitor is adequate.

Figure 4(b) shows the SC4530 can also be bootstrapped

from the input. This way it is not as efficient as the

configuration shown in Figure 4(a). However this may

be only option if the output voltage is less than 2.5V and

there is no other supply with voltage higher than 2.5V.

Voltage stress at the BST pin can be somewhat higher

than 2V

IN. The BST pin voltage should not exceed its

absolute maximum rating of 42V. Figure 4(c) shows how

to bootstrap the SC4530 from an independent power

supply V

S with its voltage > 2.5V.

To demonstrate the effect of an under-sized bootstrap

capacitor, C

3 (Figure , page ) is deliberately reduced to

0nF. TheBIASpinistiedtoanexternalpowersupplysimilar

to Figure 4(c). By adjusting the external supply voltage

(d)

(e)

(c)

VS > 2.5V

SC4530

BST

GND

BIAS

0.1µF

VOUT

VIN

Max VBST ˜ VIN + VS

SC4530

BST

GND

BIAS

VOUT

VIN

VBST ˜ 2VIN - VZ

VIN – VZ > 2.5V

(a)

SC4530

BST

GND

VIN

BIAS

Max VBST ˜ VIN + VOUT

VOUT > 2.5V

(b)

SC4530

BST

GND

BIAS

VIN

Max VBST ˜ 2VIN

VOUT < 2.5V

VOUT > VZ + 2.5V

SC4530

BST

GND

VIN

BIAS

- VZ +

VBST ˜ VIN + VOUT -VZ

0.1µF

Figure 4. Methods of Bootstrapping the SC4530

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