NB669, 24V, HIGH CURRENT SYNCHRONOUS BUCK CONVERTER WITH LDO

NB669 Rev. 1.01

www.MonolithicPower.com

15

7/23/2013

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APPLICATION INFORMATION

Input Capacitor

The input current to the step-down converter is

discontinuous and therefore requires a capacitor

to supply the AC current to the step-down

converter while maintaining the DC input voltage.

Ceramic capacitors are recommended for best

performance and should be placed as close to

the VIN pin as possible. Capacitors with X5R and

X7R ceramic dielectrics are recommended

because they are fairly stable with temperature

fluctuations.

The capacitors must also have a ripple current

rating greater than the maximum input ripple

current of the converter. The input ripple current

can be estimated as follows:

OUT

OUT

CIN

OUT

IN

IN

VV

II

(1

)

VV

=×

× −

(7)

where:

OUT

CIN

I

I

2

=

(8)

For simplification, choose the input capacitor with

an RMS current rating greater than half of the

maximum load current.

The input capacitance value determines the input

voltage ripple of the converter. If there is an input

voltage ripple requirement in the system, choose

the input capacitor that meets the specification.

The input voltage ripple can be estimated as

follows::

OUT

OUT

OUT

IN

SW

IN

IN

IN

IV

V

V(1

)

FC

V

V

Δ=

×

× −

×

(9)

OUT

IN

SW

IN

I

1

V

4F

C

Δ= ×

×

(10)

Output Capacitor

The output capacitor is required to maintain the

DC output voltage. Ceramic or POSCAP

capacitors are recommended. The output voltage

ripple can be estimated as:

OUT

OUT

OUT

ESR

SW

IN

SW

OUT

VV

1

V(1

) (R

)

FL

V

8 F

C

Δ=

× −

×

+

××

×

(11)

In the case of ceramic capacitors, the impedance

at the switching frequency is dominated by the

capacitance. The output voltage ripple is mainly

caused by the capacitance. For simplification, the

output voltage ripple can be estimated as:

OUT

OUT

OUT

2

SW

OUT

IN

VV

V(1

)

8F

L C

V

Δ=

× −

×× ×

(12)

In the case of POSCAP capacitors, the ESR

dominates the impedance at the switching

frequency. The ramp voltage generated from the

ESR is high enough to stabilize the system.

Therefore, an external ramp is not needed. A

minimum ESR value around 12mΩ is required to

ensure stable operation of the converter. For

simplification,

the

output

ripple

can

be

approximated as:

OUT

OUT

OUT

ESR

SW

IN

VV

V(1

) R

FL

V

Δ=

× −

×

×

(13)

Maximum output capacitor limitation should be

also considered in design application. NB669 has

an around 1.8ms soft-start time period. If the

output capacitor value is too high, the output

voltage can’t reach the design value during the

soft-start time, and then it will fail to regulate. The

limited approximately by:

O_ MAX

LIM _ AVG

OUT

ss

OUT

C(I

I

) T / V

=

−×

(14)

Inductor

The inductor is necessary to supply constant

current to the output load while being driven by

the switched input voltage. A larger-value

inductor will result in less ripple current that will

result in lower output ripple voltage. However, a

larger-value inductor will have a larger physical

footprint, higher series resistance, and/or lower

saturation current. A good rule for determining

the inductance value is to design the peak-to-

peak ripple current in the inductor to be in the

range of 30% to 40% of the maximum output