FTD1601

2021. 03. 15

7/9

Revision No : 0

Function Description

The FTD1601 regulates input voltages from 9V to 60V down

to an output voltage as low as 1.23V, and supplies up to 3A

of load current.

The FTD1601 uses current-mode control to regulate the

output voltage. The output voltage is measured at FB

through a resis�ve voltage divider and amplified through the

internal trans-conductance error amplifier. The output

voltage of the error amplifier is compared to the switch

current (measured internally) to control the output voltage.

Setting the Output Voltage

The output voltage is set using a resis�ve voltage divider

connected from the output voltage to FB. The voltage

divider divides the output voltage down to the feedback

voltage by the ra�o:

V =V ×( RR+R)

Thus the output voltage is:

V =1.23×(R +RR)

Inductor

The inductor is required to supply constant current to the

load while being driven by the switched input voltage. A

larger value inductor will result in less ripple current that will

in turn result in lower output ripple voltage. However, the

larger value inductor will have a larger physical size, higher

series resistance, and/or lower satura�on current. A good

rule for determining inductance is to allow the peak-to-peak

ripple current to be approximately 30% or the maximum

switch current limit. Also, make sure that the peak inductor

current is below the maximum switch current limit.

The inductance value can be calculated by:

L= Vf×∆I ×(1−VV )

is the switching frequency, and ∆IL is the peak-to-peak

inductor ripple current.

Choose an inductor that will not saturate under the

maximum inductor peak current, calculated by:

L =I + V2×f ×L×(1−VV )

The choice of which style inductor to use mainly depends on

the price vs. size requirements and any EMI constraints.

Input Capacitor

The input current to the step-down converter is

discon�nuous, therefore a capacitor is required to supply

the AC current while maintaining the DC input voltage. Use

low ESR capacitors for the best performance. Ceramic

capacitors are preferred, but tantalum or low-ESR

electroly�c capacitors will also suffice. Choose X5R or X7R

dielectrics when using ceramic capacitors. Since the input

capacitor (C1) absorbs the input switching current, it

requires and adequate ripple current ra�ng. The RMS

current in the input capacitor can be es�mated by:

I =I × VV ×(1−VV )

current ra�ng greater than half of the maximum load

current. The input capacitor can be electroly�c, tantalum or

ceramic. When using electroly�c or tantalum capacitor, be

placed as close to the IC as possible. When using ceramic

capacitors, make sure that they have enough capacitance to

provide sufficient charge to prevent excessive voltage ripple

at input. The input voltage ripple for low ESR capacitors can

be es�mated by:

∆V =IC ×f×VV (1−VV )

where C1 is the input capacitance Value.

Output Capacitor

The output capacitor (C2) is required to maintain the DC