34716 datasheet(20/25 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34716

TYPICAL APPLICATIONS

SELECTING INDUCTOR

Inductor calculation process is the same for both

Channels. The equation is the following:

If channel 1 will be serving as power supply for channel 2,

it is necessary to locate the LC poles at different frequencies

in order to ensure that the input impedance of the second

converter is always higher than the output impedance of the

first converter, and thus, ensure system stability. This can be

achieved by selecting different values for L1 and L2 slightly

higher than the calculated value.

SELECTING THE OUTPUT FILTER CAPACITOR

For the output capacitor, the following considerations are

most important and not the actual Farad value: the physical

size, the ESR of the capacitor, and the voltage rating.

Calculate the minimum output capacitor using the

following formula:

Transient Response percentage:

TR_%

Maximum Transient Voltage:

TR_V_dip = VOUT*TR_%

Maximum Current Step:

Inductor Current Rise Time:

The following formula will be helpful to find the maximum

allowed ESR.

The effects of the ESR is often neglected by the design-

ers and may present a hidden danger to the ultimate supply

stability. Poor quality capacitors have widely disparate ESR

value, which can make the closed loop response inconsis-

tent.

BOOTSTRAP CAPACITOR

The bootstrap capacitor is needed to supply the gate

voltage for the high side MOSFET. This N-Channel MOSFET

needs a voltage difference between its gate and source to be

able to turn on. The high side MOSFET source is the SW

node, so it is not at ground and it is floating and shifting in

voltage. We cannot just apply a voltage directly to the gate of

the high side that is referenced to ground. We need a voltage

referenced to the SW node. This is why the bootstrap

capacitor is needed. This capacitor charges during the high-

side off time. Since the low side will be on during that time,

the SW node and the bottom of the bootstrap capacitor will be

connected to ground, and the top of the capacitor will be

connected to a voltage source. The capacitor will charge up

to that voltage source (for example 5V). Now when the low

side MOSFET switches off and the high side MOSFET

switches on, the SW nodes rise to VIN, and the voltage on the

boot pin will be VCAP + VIN. The gate of the high side will have

VCAP across it and it will be able to stay enhanced. A 0.1µF

capacitor is a good value for this bootstrap element.

TYPE III COMPENSATION NETWORK

Power supplies are desired to offer accurate and tight

regulation output voltages. A high DC gain is required to

accomplish this, but with high gain comes the possibility of

instability. The purpose of adding compensation to the

internal error amplifier is to counteract some of the gains and

phases contained in the control-to-output transfer function

that could jeopardized the stability of the power supply. The

Type III compensation network used for 34716 is comprised

of two poles (one integrator and one high frequency, to

cancel the zero generated from the ESR of the output

capacitor) and two zeros to cancel the two poles generated

from the LC filter as shown in Figure 10.

Maximum Off Time Percentage

Switching Period

Drain – to – Source

Resistance of FET

Winding Resistance of Inductor

Output Current Ripple

Rds

)

(

out

MAX

Rds

∆

∗

))

)

(

max

Vin

out

MAX

−

r _

OUT

∆

dip

rise

OUT

Fsw

Vout

Vin

step

Iout

max

)

min

(

−

∆

step

rise

OUT

∆

min)

(

max

Fsw

ESR

OUT

−

∆

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