SC1404 Ver la hoja de datos (PDF) - Semtech Corporation

Número de pieza

componentes Descripción

Fabricante

SC1404

Mobile Multi-Output PWM Controller with Virtual Current Sense™

Semtech Corporation 

SC1404

POWER MANAGEMENT

Startup Sequence Chart

SEQ

ON3

ON5

REF

LOW

LOW

REF

LOW

HIGH

REF

HIGH

LOW

REF

HIGH

HIGH

GND

LOW

X

GND

HIGH

HIGH/LOW

VL

LOW

X

VL

HIGH

HIGH/LOW

RESET

Follows 3.3V SMPS.

Low.

Follows 3.3V SMPS.

Follows 3.3V SMPS.

Low.

High after both outputs are

in regulation.

Low.

High after both outputs are

in regulation.

DESCRIPTION

Independant start control mode. Both SMPSs off.

5V SMPS ON, 3.3V SMPS OFF.

3.3V SMPS ON, 5V SMPS OFF.

Both SMPSs on.

Both SMPSs off.

5V starts when ON3 goes high. If ON5 = HIGH,

3V is on. If ON5 = LOW, 3V is off.

Both SMPSs off.

3V starts when ON3 goes high. If ON5 = HIGH,

5V is on. If ON5 = LOW, 5V is off.

Applications Information

Reference Circuit Design

The schematic for the reference circuit is shown on page 20. The

reference circuit is configured as follows:

Switching Regulator 1

Switching Regulator 2

VL Regulator

12V regulator

Input voltage

Vout1 = 3.3V @ 6A

Vout2 = 5.0V @ 6A

Vout3 = 5.0V @ 30mA

Vout4 = 12.0V @ 200mA

Vin = 7 to 21V

Designing the Output Filter

Before calculating the filter inductance and capacitance, an

acceptable inductor ripple current is determined. Ripple current

is usually set at 10% to 20% of the maximum load. However,

increasing the ripple current allows for a smaller inductor and will

also quicken the output transient response. In this example, we

set the ripple current to be 25% of maximum load.

∆IO = 25 % × 6 A = 1.5A

The inductance is found from ripple current, frequency, input

voltage, and output voltage. Minimum required inductance is

found at maximum Vin, where ripple current is the greatest.

L min

=

Vo

×

(1 − Vo / Vin

F × ∆ Io

)

=

6.18 uH

To specify the output capacitance, the allowable output ripple

voltage must be determined. Output ripple is often specified at

1% of the output voltage. For the 3.3V output, we selected a

maximum ripple voltage of 33mVp-p. The maximum allowable

ESR would then be:

ESR MAX = ∆VO / ∆IO = 33mV /1.5A = 22mΩ

Panasonic SP Polymer Aluminum capacitors are a good choice.

For this design, use one 180uF, 4V device, with ESR of 15mΩ.

The output capacitor must support the inductor RMS ripple current.

To check the actual ripple versus the capacitor’s RMS rating:

I = RMS_ actual ∆IO = 1.5A = 0.43A

12 12

This is much less than the capacitor’s ripple rating of 3.3A.

Choosing the Main Switching mosfet

The IRF7143 is used in the reference design. Before choosing the

main (high-side) mosfet, we need to check three parameters: volt-

age, power, and current rating.

The maximum drain to source voltage of the mosfet is mainly

determined by the switcher topology. With a buck topology,

VDS _ MAX = VIN _ MAX = 21V

For the reference design, the Coiltronics DR127-6R8 is used. This

is acceptable for the 3V output, which uses a simple inductor. The

5V inductor must have a 12V winding with a turns ratio of 2.2:1.

For the 5V inductor, the TTI-8215 from Transpower Technologies

is used, which has 5V inductance of 6.4uH.

The IRF7413 is a 30V device, which allows for 70% derating at

21V operation.

The mosfet power dissipation has three components: conduction

losses, switching losses, and gate drive losses. The conduction

loss is determined using the RMS mosfet current; the equation is

 2004 Semtech Corp.

11

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