SC4502 datasheet(8/19 Pages) SEMTECH

8 / 19 page

 2005 Semtech Corp.

POWER MANAGEMENT

SC4502/SC4502H

www.semtech.com

OUT

CESAT

OUT

−

(3)

where V

CESAT is the switch saturation voltage and VD is the

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 through the switch. When the power switch is off,

the inductor current flows through the rectifying diode to

the output. The maximum output current is the average

diode current. The diode current waveform is trapezoidal

with pulse width (1 – D)T (Figure 4). The output current

available from a boost converter therefore depends on

the converter operating duty cycle. The power switch

current in the SC4502/SC4502H 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 rectifying diode, an expression of the maximum

available output current of a boost converter can be

derived as follows:

()













−

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 of

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 = 1.4A in (3) and

(4), the maximum output currents for three V

IN and VOUT

combinations are shown in Table 1.

)

(

)

(

Considerations for High Frequency Operation

The operating duty cycle of a boost converter decreases

as V

IN approaches VOUT. The PWM modulating ramp in a

current-mode switching regulator is the sensed current

signal. 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 SC4502/SC4502H has a

minimum on time of about 150ns at room temperature.

The power switch in the SC4502/SC4502H is either not

turned on at all or on 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 SC4502.

Assuming that V

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

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

0.25

0.5

0.3

0.5

4.2

MIN

−

Figure 4. 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)]

Applications Information

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