SC4503 datasheet(9/22 Pages) SEMTECH

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SC4503

POWER MANAGEMENT

Applications Information (Cont.)

Duty Cycle

The duty cycle D of a boost converter in continuous-conduc-

tion mode (CCM) is:

−

(3)

where V

CESAT is the switch saturation voltage and VD is volt-

age drop across the rectifying diode.

Maximum Output Current

In a boost switching regulator the inductor is connected

to the input. The inductor DC 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 (see Figure 4). The output current available from

a boost converter therefore depends on the converter oper-

ating duty cycle. The power switch current in the SC4503 is

internally limited to at least 1.4A. This is also the maximum

peak inductor or the peak 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:

()

−

(4)

Switch Current

Diode Current

OFF

(1-D)T

Inductor

Current

OUT

Switch Current

Diode Current

OFF

(1-D)T

Inductor

Current

OUT

Figure 4. Current Waveforms in a Boost Converter

Switch Current

Diode Current

OFF

(1-D)T

Inductor

Current

OUT

Switch Current

Diode Current

OFF

(1-D)T

Inductor

Current

OUT

Figure 4. Current Waveforms in a Boost Converter

where I

LIM is the switch current limit.

It is worth noting that I

OUTMAX is directly proportional to the

ratio

and that switching losses are neglected in its

derivation. Equation (4) therefore over-estimates the

maximum output current, however it is a useful ﬁrst-order

approximation.

Using V

CESAT = 0.3V, VD = 0.5V and ILIM =1.4A in (3) and (4),

the maximum output current for three V

IN and VOUT combi-

nations are tabulated (Table 1).

Maximum Duty-Cycle Limitation

The power transistor in the SC4503 is turned off every

switching period for 80ns. This minimum off time limits the

maximum duty cycle of the regulator. A boost converter with

high

ratio requires long switch on time and high duty

cycle. If the required duty cycle is higher than the attain-

able maximum, then the converter will operate in dropout.

(Dropout is a condition in which the regulator cannot attain

its set output voltage below current limit.)

Note: dropout can occur when operating at low input volt-

ages (<3V) and with off times approaching 100ns. Shorten

the PCB trace between the power source and the device

input pin, as line drop may be a signiﬁcant percentage of

the input voltage. A regulator in dropout may appear as

if it is in current limit. The cycle-by-cycle current limit of

the SC4503 is duty-cycle and input voltage invariant and

should be at least 1.4A. If the converter output is below

its set value and switch current limit is not reached (1.4A),

then the converter is likely in dropout.

Example

: Determine the highest attainable output voltage

when boosting from a single Li-ion cell.

Equation (3) can be re-arranged as:

Table 1. Calculated Maximum Output Currents

IN (V)

OUT (V)

OUT (A)

3.3

0.754

0.34

3.3

0.423

0.80

0.615

0.53

IN (V)

OUT (V)

OUT (A)

3.3

0.754

0.34

3.3

0.423

0.80

0.615

0.53

Table 1. Calculated Maximum Output Currents

IN (V)

OUT (V)

OUT (A)

3.3

0.754

0.34

3.3

0.423

0.80

0.615

0.53

IN (V)

OUT (V)

OUT (A)

3.3

0.754

0.34

3.3

0.423

0.80

0.615

0.53

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