R34DS0034EU0101 Rev.1.01

Page 23

Mar 13, 2024

ISL73006SLH Datasheet

The temperature coefficient of the ESR can be significant and cause difficulty with this. You need careful

evaluation for wide temperature range operations. Consider a combination of Tantalum and MLCC capacitors to

achieve high total capacitance with lower ESR.

5.14

Input Capacitor Selection

Use a mix of input bypass capacitors to control the voltage overshoot and undershoot across the internal

MOSFETs of the synchronous buck regulator. Use small low ESR ceramic capacitors for high-frequency

decoupling and bulk capacitors to supply the current needed each time the upper MOSFET turns on. Place the

small ceramic capacitors physically close to the IC between the PVIN and PGND pins.

The critical parameters for the bulk input capacitance are the voltage and RMS current ratings. For reliable

operation, select bulk capacitors with voltage and current ratings above the maximum input voltage and largest

RMS current required by the circuit. Their voltage rating should be at least 1.5 times greater than the maximum

input voltage, while a voltage rating of 2.5 times is a conservative guideline when considering voltage derating

performance to 125°C. Consult the capacitor datasheets for temperature derating tables. For most cases, the

RMS current rating requirement for the input capacitor of a buck regulator is approximately 1/2 the DC load

current.

Use Equation 11 to closely approximate the maximum RMS current through the input capacitors.

The minimum recommended input capacitance for the ISL73006SLH is 22µF. Place these high-frequency,

low-ESR capacitors close to the VIN and PGND pins. These capacitors provide the instantaneous current into the

buck regulator during the high-frequency switching transitions.

5.15

Output Capacitor Selection

An output capacitor is required to filter the inductor ripple current and supply the load transient current. The

filtering requirements are a function of the switching frequency and the ripple current. The load transient

requirements are a function of the slew rate (di/dt) and the magnitude of the transient load current. These

requirements are generally achieved with a combination of bulk and decoupling capacitors with a careful layout.

High-frequency, low ESR ceramic capacitors initially supply the transient load current and reduce the current load

slew rate seen by the bulk capacitors. The Effective Series Resistance (ESR) and voltage rating requirements

generally determine the bulk filter capacitor values rather than actual capacitance requirements. Place

high-frequency decoupling capacitors as close to the power pins of the load as physically possible. Be careful not

to add inductance in the circuit board wiring that could cancel the usefulness of these low inductance components.

The shape of the output voltage waveform during a load transient that represents the worst-case loading

conditions ultimately determines the number of output capacitors and their type. When this load transient is

applied to the regulator, most of the current required by the load is initially contributed by the output capacitors.

This is due to the finite amount of time required for the inductor current to slew up or down to the level of the

output current required by the load. This results in a momentary undershoot or overshoot in the output voltage. At

the initial edge of the transient undershoot or overshoot, the Equivalent Series Inductance (ESL) of each capacitor

induces a spike that adds on top of the voltage drop due to the ESR. After the initial spike, the output voltage dips

down (load step on) or peaks up (load step off) as the output capacitor sources or sinks the transient load current

until the output inductor current reaches the load current. Figure 41 shows a typical response of the output voltage

to a transient load current.

(EQ. 11)

ICINrms

VOUT

VIN

----------------xIOUT

MAX

2

x1

VOUT

VIN

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

–







1

12

------x

VIN VOUT

–

LxfOSC

--------------------------------x

VOUT

VIN

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





 2

+







=