SC4810EITSTRT datasheet(11/24 Pages) SEMTECH

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 2004 Semtech Corp.

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SC4810A/B/C/D/E

POWER MANAGEMENT

output inductance would affect the peak to peak value

of the output current, which would also influence the

output voltage ripple. The designer needs to take the

output inductance and output capacitance and the ESL

and ESR of the output capacitor into consideration during

the design.

For this application, one Panasonic power choke output

inductor was selected and three 6.3V, 100uF TDK

ceramic capacitors were adopted in the design.

Selection of the Current Sensing Resistor

The selection of the current sensing resistor is based on

the over-current protection triggering point. SC4810

employs a Hiccup mode over-current protection with an

overcurrent threshold of 600mV. A voltage signal above

600mV on the CS pin will trigger hiccup mode overcurrent

protection. Suppose the over-current protection setpoint

is set to be Iov. The threshold voltage of SC4810 is

Vthreshold. The turns ratio of the power transformer is

Ns/Np. The turns ratio of the current sensing transformer

is Ncs:1. Then the Rsense would be calculated as:

)

......(

Threshold

sense

Set Clock Frequency

The SC4810 uses a pair of resistors and capacitors to

generate a triangle signal as the clock signal, as illustrated

in Fig. 3.

RCT

VREF

VRCT

Fig. 3 Configuration for Clock Signal

The voltage waveform on the RCT pin is illustrated as in

Fig. 4.

Application Information (Cont.)

VRCT

Fig.4 Voltage Waveform on RCT Pin

VPK

As illustrated, the capacitor C is charged via the resistor

R from VREF. Whenever the voltage on the RCT pin

reaches 3V, the capacitor C will be discharged through

an internal FET shorted to ground. When the clock signal

circuit is connected as in Fig.3, the frequency of the clock

signal is defined, as in equation 2.

)

........(

)

(

REF





















−

•

−

REF is the reference voltage of the SC4810, 4V for

SC4810A/D and 5V for SC4810B/C/E.

In this application, to get 600kHz, C = 180pF,

(R + 1k) = 10k ohms and V

P-K = 3V.

Maximum Duty Ratio Limit

SC4810 features maximum duty ratio limitation for ex-

tra protection. The maximum duty ratio is determined by

the voltage on DMAX pin. As illustrated as in Fig. 5, V

DMAX

will be compared with V

RCT and DMAX is determined by

the comparison of the two signals.

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