SC4501MSETRT datasheet(9/21 Pages) SEMTECH

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POWER MANAGEMENT

SC4501

Duty Cycle

The duty cycle D of a boost converter is:

OUT

CESAT

OUT

−

(3)

where V

CESAT is the switch saturation voltage and VD is

voltage drop across the rectifying diode.

Maximum Output Current

In a boost switching regulator the inductor is connected

to the input. The DC inductor current is the input current.

When the power switch is turned on, the inductor current

flows into the switch. When the power switch is off, the

inductor current flows through the rectifying diode to the

output. The output current is the average diode current.

The diode current waveform is trapezoidal with pulse width

(1 – D)T (Figure 5). The output current available from a

boost converter therefore depends on the converter

operating duty cycle. The power switch current in the

SC4501 is internally limited to 2A. This is also the maximum

inductor or the input current. By estimating the conduction

losses in both the switch and the diode, an expression of

the maximum available output current of a boost converter

can be derived:

(

)













−

CESAT

OUT

LIM

OUTMAX

(4)

where I

LIM is the switch current limit.

It is worth noting that I

OUTMAX is directly proportional to the

ratio

OUT

. Equation (4) over-estimates the maximum

output current at high frequencies (>1MHz) since

switching losses are neglected in its derivation.

Nevertheless it is a useful first-order approximation.

Using V

CESAT = 0.3V, VD = 0.5V and ILIM = 2A in (3) and (4),

the maximum output currents for three V

IN and VOUT

combinations are shown in Table 1.

IN ( V )

OUT ( V )

OUTMAX ( A )

2.5

0.820

0.35

3.3

0.423

1.14

0.615

0.76

Considerations for High Frequency Operation

The operating duty cycle of a boost converter decreases as

IN approaches VOUT. The PWM modulating ramp in a

current-mode switching regulator is the sensed current ramp

of the control switch. This current ramp is absent unless

the switch is turned on. The intersection of this ramp with

the output of the voltage feedback error amplifier

determines the switch pulse width. The propagation delay

time required to immediately turn off the switch after it

is turned on is the minimum switch on time. Regulator

closed-loop measurement shows that the SC4501 has

a minimum on time of about 150ns at room temperature.

The power switch in the SC4501 is either not turned on

at all or for at least 150ns. If the required switch on time

is shorter than the minimum on time, the regulator will

either skip cycles or it will start to jitter.

Example: Determine the maximum operating frequency

of a Li-ion cell to 5V converter using the SC4501.

Assuming that V

D=0.5V, VCESAT=0.3V and VIN=2.6 - 4.2V,

the minimum duty ratio can be found using (3).

MIN

−

Application Information

Figure 5. Current Waveforms in a Boost Regulator

Inductor Current

Switch Current

Diode Current

OFF

IOUT

IIN

(1-D)T

Table 1. Calculated Maximum Output Current [ Equation (4)]

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