SC4530 datasheet(9/23 Pages) SEMTECH

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Applications Information

SC4530

Setting the Output Voltage

Figure 3. R

1 and R2 Set the Output Voltage

The SC4530 output voltage is programmed using a

resistive divider (Figure 3) with its center tap tied to the

FB pin. For a given R

2, R can be determined:

232

OUT

()

The percentage error due to the input bias current of the

error amplifier is:

232

)

(

100

OUT

(2)

Example: Determine the output voltage error caused by

the amplifier input bias current in a 5V output converter.

Assuming R

2 = 200kW and using Equations () and (2),

619

232

200

232

)

619

200

(

100

OUT

Using large R

and R2 helps in maintaining light-load

efficiency, since the current drawn by the feedback

resistive divider is not delivered to the converter output.

The simple calculation above shows that relatively large

and R2 can be used without introducing more error

than that resulting from the tolerance of the standard %

resistors.

Maximum Duty Cycle Limitation

The SC4530 is a non-synchronous, step-down switching

regulator. Its duty cycle in continuous-conduction

20nA

SC4530

VOUT

20nA

SC4530

VOUT

mode (CCM) primarily depends on the input and output

voltages:

CESAT

OUT

(3)

where V

CESAT = 0.25V is the switch saturation voltage and

D = 0.6V is the forward voltage drop of the freewheeling

diode.

Whenever the power switch is turned off, it is kept off for

at least 530ns. Moreover, the control circuit prevents the

power transistor from turning on for more than 3.5ms.

The inductor current pulls the SW node low as the power

switch turns off, allowing the inductor current to charge

the bootstrap capacitor. The maximum on-time ensures

that the bootstrap capacitor gets replenished after a

long switch-on interval. The minimum off-time, together

with the maximum on-time, put an upper limit on the

achievable duty cycle (≈ 0.96). From Equation (3), the

minimum V

IN to avoid dropout is:

CESAT

OUT

)

MIN

(

(4)

If V

IN falls below this minimum, then the regulator will not

be able to attain its set output voltage regardless load.

Using Equation (4), the input supply voltage must be at

least 5.5V in order to generate a 5V output.

Inductor Selection

The SC4530 uses a hysteretic current-mode control

topology. The peak-to-peak inductor ripple current, ∆I

is theoretically constant. However, propagation delays

of the current comparators (IPK and IVLY in Figure 2), as

well as the error amplifier (EA) output ripples, will cause

the actual inductor ripple current to vary depending on

the input voltage and the duty cycle. The inductor should

be chosen so that the valley current comparator, not the

minimum off-time, determines the switch turn-on instant.

To simplify inductance calculation, we will assume that

∆I

L is constant and equal to 50mA. Furthermore, we will

use .5 times the typical t

OFF(MIN), to allow for tolerance and

temperature variation.

)

MIN

(

OFF

OUT

MIN

(5)

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