IR3870MBF datasheet(13/20 Pages) IRF

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IR3870MBF

One can use equation 4 to find the required

inductance. The main advantage of small

inductance is increased inductor current slew

rate during a load transient, which leads to a

smaller output capacitance requirement as

discussed in the Output Capacitor Selection

section. The draw back of using smaller

inductances is increased switching power loss in

upper MOSFET, which reduces the system

efficiency and increases the thermal dissipation

as discussed in the Power Loss section.

Input Capacitor Selection

The main function of the input capacitor bank is

to provide the input ripple current and fast slew

rate current during the load current step up. The

input capacitor bank must have adequate ripple

current carrying capability to handle the total

RMS current. Figure 16 shows a typical input

current. Equation 5 shows the RMS input current.

The RMS input current contains the DC load

current and the inductor ripple current. As shown

in equation 4, the inductor ripple current is

unrelated to the load current. The maximum

RMS input current occurs at the maximum output

current. The maximum power dissipation in the

input capacitor equals the square of the

maximum RMS input current times the input

capacitor’s total ESR.

Figure 16. Typical Input Current Waveform.

Circuit Description

voltage is within regulation before four

consecutive soft-start cycles, PGOOD transitions

HIGH to reset the counter.

OVER VOLTAGE PROTECTION

The IR3870 monitors the voltage at the FB node.

If the FB voltage is above the over voltage

threshold, the gates are turned off and the

PGOOD signal is pulled low. Toggling VCC will

allow the next start up.

CHARGE PUMP

The purpose of the charge pump is to improve

the system efficiency. A combination of VCC, V5

and three external components are used to boost

PVCC up to VCPTH. PVCC drives the

synchronous MOSFET and reduces the RDSON

when compared to a regular 5V rail driver. The

lower FET RDSON reduces the conduction

power loss as discussed in the Power Loss

section. The charge pump is continuously

enabled for FCCM = HIGH. The charge pump

circuit is disabled when FCCM = LOW and the

output loading is less than half of inductor current

ripple. In this case, PVCC is two diode voltages

away from the V5 rail. Therefore, the power loss

for driver is reduced. The charge pump circuit

stops switching the CPO pin for PVCC above

VCPTH.

COMPONENT SELECTION

Selection of components for the converter is an

iterative process which involves meeting the

specifications and trade-offs between performance

and cost. The following sections will guide one

through the process.

Inductor Selection

Inductor selection involves meeting the steady

state output ripple requirement, minimizing the

switching loss of upper MOSFETs, meeting

transient response specifications and minimizing

the output capacitance. The output voltage

includes a DC voltage and a small AC ripple

component due to the low pass filter which has

incomplete attenuation of the switching harmonics.

Neglecting the inductance in series with the output

capacitor, the magnitude of the AC voltage ripple is

determined by the total inductor ripple current flow

through the total equivalent series resistance

(ESR) of the output capacitor bank.

()

(4)

ΔI

OUT

⋅

−

⋅

−

()

(5)

ΔI

OUT

IN_RMS

⎟

⎠

⎞

⎜

⎝

⎛

⋅

∫

The voltage rating of the input capacitor needs to be

greater than the maximum input voltage because of

high frequency ringing at the phase node. The typical

percentage is 25%.

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