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LTC3126

3126f

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of the feedforward capacitor in parallel with the upper

resistor divider resistor to restore the full bandwidth of

the feedback loop.

Feedforward Resistor

In applications where there is a long connection between

the feedback resistor divider and the point at which the

output voltage is sensed, it is recommended that a 10k

feedforward resistor (RFF) be added in series with the

feedforward capacitor as shown in Figure 7 below. The

feedforward resistor prevents high frequency noise on

the VOUT trace from coupling into the sensitive FB node.

The addition of a 10k feedforward resistor will have little

impact on the frequency response of the control loop

since the divider pole location is dominated by the values

of resistors RTOP and RBOT.

applicaTions inForMaTion

A reasonable choice for ripple current is ∆IL = 1A which

represents 33% of the minimum current limit. The DC cur-

rent rating of the inductor should be at least equal to the

maximum load current plus half the ripple current in order

to prevent core saturation and loss of efficiency during

operation. To optimize efficiency the inductor should have

a low DC resistance. As a general guideline, the inductor

resistance (ESR) should be approximately equal to the

low side switch resistance (70mΩ) or less.

High values of inductor ripple current will reduce the out-

put current capability of the converter since higher ripple

increases the peak switch current and will therefore trip

the current limit at a lighter load current. The maximum

output current is equal to the current limit minus half the

peak-to-peak ripple current as shown in the following

equation where ILIMIT is the threshold of the high side

switch current limit:

IOUT(MAX) =ILIMIT –

∆IL

In addition, there is a minimum inductor value required

to maintain stability of the current loop as determined by

the fixed internal slope compensation. Specifically, if the

buck converter is going to be utilized at duty cycles over

50%, the inductance value must be at least equal to LMIN

as given by the following equation:

LMIN =

MHz

fSW

•

VOUT

µH

To ensure sufficient slope compensation if the external

synchronization feature is being used, the inductor must

be sized for the lowest possible switching frequency the

part will experience which is determined by the internal

oscillator frequency.

PGOOD Output

The open-drain PGOOD output is driven low whenever FB

is more than +9.8%/–8.7% (typical) from the FB reference

voltage. The PGOOD output is also driven low whenever

the buck converter is disabled. The maximum voltage that

can be applied to the PGOOD output is 5.5V. The PGOOD

comparatorhasadeglitchingdelayofapproximately200µs.

Figure 7. Feedforward Resistor (RFF) for Improved

Noise Robustness

LTC3126

RBOT

3126 F07

RTOP

RFF

10k

CFF

VOUT

GND

Inductor Selection

The choice of inductor value influences both the efficiency

and the magnitude of the output voltage ripple. Larger

inductance values will reduce inductor current ripple and

will therefore lead to lower output voltage ripple. For a

fixedDCresistance,alargervalueinductorwillyieldhigher

efficiency via reduced RMS and core losses. However, a

larger inductor within a given inductor family will generally

have a greater series resistance, thereby counteracting

this efficiency advantage. The peak-to-peak current ripple,

∆IL, given by the following equation will be largest at the

highest input voltage experienced in the application.

∆IL =

VOUT

fSW •L

1–

VOUT

VIN









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