FN8808 datasheet(31/44 Pages) RENESAS

31 / 44 page

ISL78227

FN8808 Rev.6.02

Page 31 of 43

Feb 28, 2025

CURRENT SENSE FOR INDIVIDUAL PHASE - ISENX

The RC network between the RSENx and ISENxP/N pins as shown

in Figure 60 is the recommended configuration. The ISENxP pin

should be connected to the positive potential of the RSEN_CHx

through resistor RSETx, where in Figure 60, RSETx is composed by

RSETxA plus RSETxB. RSET is used to set the current sense gain

externally.

Because there is an 112µA bias current sinking to each of the

ISENxP and ISENxN pins, RBIASx with the same value to RSETx

should be placed between the ISENxN pin to the low potential of

the RSENx, where in Figure 60, RBIASx is composed by RBIASxA

plus RBIASxB.

It is recommended to have RSETxA = RBIASxA and

RSETxB = RBIASxB, and insert a capacitor (CISENx) between them

as shown in Figure 60. This will form a symmetric noise filter for

the small current sense signals. The differential filtering time

constant equals to (RSETxA+RBIASxA)*CISENx. This time constant

is typically selected in range of tens of ns depending on the

actual noise levels.

CSA generates the sensed current signal ISENx by forcing ISENxP

voltage to be equal to ISENxN voltage. Because RSETx is equal to

RBIASx, the voltage drop across RSETx and RBIASx incurred by the

fixed 112µA bias current cancels each other. Therefore, the

resulting current at CSA output ISENx is proportional to each

phase inductor current, ILx. ISENx per phase can be derived in

Equation 11, where ILx is the per-phase current flowing through

RSENx.

RSENx is normally selected with smallest resistance to minimize

the power loss on it. With RSENx selected, RSETx is selected by

the desired cycle-by-cycle peak current limiting level OC1 (refer to

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

AVERAGE CURRENT SENSE FOR TWO PHASES - IMON

The IMON pin serves to monitor the total average input current of

the 2-phase boost. As shown in Figure 3 on page 7, the individual

current sense signals (ISENx) are divided by eight and summed

together. A 17µA offset current is added to form a current source

output at the IMON pin with the value calculated as shown in

Equation 12.

Assume RSEN1 = RSEN2, RSET1 = RSET2, and IIN = IL1 + IL2,

which is the total boost input average current:

As shown in Figure 4 on page 8, a resistor RIMON is placed

between the IMON pin and ground, which turns the current sense

output from the IMON pin to a voltage VIMON. A capacitor CIMON

should be used in parallel with RIMON to filter out the ripple such

that VIMON represents the total average input current of the

2-phase boost. VIMON can be calculated using Equation 14.

As shown in Figure 3 on page 7, VIMON is sent to inputs of Gm2

and comparators of CMP_PD and CMP_OCAVG for the following

functions:

1. VIMON is compared with 1.6V (VREF_CC) at error amplifier

Gm2 inputs to achieve constant current control function. The

CC control threshold for the boost input current is typically set

in a way that the per-phase average inductor current (when CC

control) is lower than the per-phase cycle-by-cycle peak

current limiting (OC1) threshold. Refer to “Constant Current

Control (CC)” on page 35 for detailed descriptions.

2. VIMON is compared with phase dropping thresholds (1.1V

falling to drop Phase 2, 1.15V rising to add Phase 2). Refer to

“Automatic Phase Dropping/Adding” on page 33 for detailed

descriptions.

3. VIMON is compared with 2V for OC_AVG fault protections.

Refer to “Average Overcurrent Fault (OC_AVG) Protection” on

page 36 for detailed descriptions.

FIGURE 60. CURRENT-SENSING BLOCK DIAGRAM

VIN

VOUT

ISENxP

ISENx

ISENxN

RSENx

RBIASxA

RBIASxB

RSETxB

ISENx+112µA

RSETxA

CSA

112µA

IBIAS

ISENx

IC INTERNAL CIRCUITS

CISENx

112µA

RSETx RSETxA RSETxB

(EQ. 8)

RBIASx RBIASxA RBIASxB

(EQ. 9)

RBIASx RSETx

(EQ. 10)

ISENx ILx

RSENx

RSETx

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



(EQ. 11)

IMON

IL1 R



SEN1

RSET1

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

IL2 R



SEN2

RSET2

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













0.125



17 10



–

(EQ. 12)

IMON

IIN

RSEN

RSET

---------------- 0.125



17 10



–

(EQ. 13)

VIMON IMON RIMON



(EQ. 14)

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