FN8808 datasheet(37/44 Pages) RENESAS

37 / 44 page

ISL78227

FN8808 Rev.6.02

Page 37 of 43

Feb 28, 2025

With an external PNP transistor as shown in Figure 63, the power

dissipation of the internal LDO can be moved from the ISL78227

to the external transistor. Set RS to 68Ω so that the LDO delivers

about 10mA when the external transistor begins to turn on. The

external circuit increases the minimum input voltage to

approximately 6.5V.

Application Information

There are several ways to define the external components and

parameters of boost regulators. This section shows one example

of how to decide the parameters of the external components

based on the typical application schematics as shown in Figure 4

on page 8. In the actual application, the parameters may need to

be adjusted and additional components may be needed for the

specific applications regarding noise, physical sizes, thermal,

testing, and/or other requirements.

Output Voltage Setting

The Output Voltage (VOUT) of the regulator can be programmed

by an external resistor divider connecting from VOUT to FB and FB

to GND as shown in Figure 4 on page 8. Use Equation 2 on

page 25 to calculate the desired VOUT, where VREF can be either

VREF_1.6V or VREF_TRK, whichever is lower. In the actual

application, the resistor value should be decided by considering

the quiescent current requirement and loop response. Typically,

between 4.7kΩ to 20kΩ is used for the RFB1.

Switching Frequency

Switching frequency is determined by requirements of transient

response time, solution size, EMC/EMI, power dissipation and

efficiency, ripple noise level, input, and output voltage range.

Higher frequency may improve the transient response and help

to reduce the solution size. However, this may increase the

switching losses and EMC/EMI concerns. Thus, a balance of

these parameters is needed when deciding the switching

frequency.

When the switching frequency fSW is decided, the frequency

setting resistor RFSYNC can be determined by Equation 6 on

page 27.

Input Inductor Selection

While the boost converter is operating in steady state Continuous

Conduction Mode (CCM), the output voltage is determined by

Equation 1 on page 24. With the required input and output voltage,

duty cycle D can be calculated by Equation 25:

where D is the on-duty of the boost low-side power transistor.

Under this CCM condition, the inductor peak-to-peak ripple

current of each phase can be calculated using Equation 26:

where T is the switching cycle 1/fSW and L is each phase

inductor’s inductance.

From the previous equations, the inductor value is determined

using Equation 27:

Use Equation 27 to calculate L, where values of VIN, VOUT and

IL(P-P) are based on the considerations described in following:

• One method is to select the minimum input voltage and the

maximum output voltage under long term operation as the

conditions to select the inductor. In this case, the inductor DC

current is the largest.

• The general rule to select the inductor is to have its ripple

current IL(P-P) around 30% to 50% of maximum DC current.

The individual maximum DC inductor current for the 2-phase

boost converter can be calculated using Equation 28, where

POUTmax is the maximum DC output power, and EFF is the

estimated efficiency:

Using Equation 27 with the two conditions listed above, a

reasonable starting point for the minimum inductor value can be

estimated from Equation 29, where K is typically selected as

30%.

Increasing the value of the inductor reduces the ripple current

and therefore, the ripple voltage. However, the large inductance

value may reduce the converter’s response time to a load

transient. This also reduces the current sense ramp signal and

may cause a noise sensitivity issue.

The peak current at maximum load condition must be lower than

the saturation current rating of the inductor with enough margin.

In the actual design, the largest peak current may be observed at

some transient conditions like the start-up or heavy load

transient. Therefore, the inductor’s size needs to be determined

with the consideration of these conditions. To avoid exceeding

the inductor’s saturation rating, OC1 peak current limiting (refer

to “Peak Current Cycle-by-Cycle Limiting (OC1)” on page 35) should

be selected below the inductor’s saturation current rating.

FIGURE 63. SUPPLEMENTING LDO CURRENT

VIN

PVCC

VIN

PVCC

ISL78227

(EQ. 25)

VIN

VOUT

----------------

–

IL P-P





D T VIN

----------

 

(EQ. 26)

VIN

VOUT

----------------

–













VIN

IL P-P



 f



---------------------------------



(EQ. 27)

ILmax

POUTmax

VINmin EFF 2



--------------------------------------------

(EQ. 28)

Lmin

VINmin

VOUTmax

---------------------------

–













VINmin

EFF 2



POUTmax K fSW

 

---------------------------------------------------



(EQ. 29)

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