'VOUT = 3A x

1

8 x 250 kHz x 320

0.4 m:

2 +

2

PF

'VOUT = 4.8 mV

VOUT = IPP x

1

8 x fSW x COUT

ESR

2 +

P

2

10V/V x 10 m:

5 PA/V x 6 PH

CRAMP =

= 300 pF

A x RS

gm x L

CRAMP ,

5V

7A +

0.11V

RS

2 x 6 PH x 250 kHz

5V

7V

1 +

x

0.011:

d

d

VOUT

IO +

VCS(TH)

RS

2 x L x fSW

VOUT

VIN(MIN)

1 +

x

@

ILIM =

VCS(TH)

RS

LM5116-HT

SLVSBS8B – OCTOBER 2013 – REVISED JANUARY 2014

www.ti.com

The nearest standard value of 6 µH will be used. The inductor must be rated for the peak current to prevent

saturation. During normal operation, the peak current occurs at maximum load current plus maximum ripple.

During overload conditions with properly scaled component values, the peak current is limited to VCS(TH) / RS

(See next section). At the maximum input voltage with a shorted output, the valley current must fall below VCS(TH)

/ RS before the high-side MOSFET is allowed to turn on. The peak current in steady state will increase to VIN(MAX)

x tON(min) / L above this level. The chosen inductor must be evaluated for this condition, especially at elevated

temperature where the saturation current rating may drop significantly.

CURRENT SENSE RESISTOR

The current limit is set by the current sense resistor value (RS).

(10)

For a 5-V output, the maximum current sense signal occurs at the minimum input voltage, so RS is calculated

from:

(11)

For this example VCCX = 0 V, so VCS(TH) = 0.11 V. The current sense resistor is calculated as:

(12)

The next lowest standard value of 10 m

Ω was chosen for RS.

RAMP CAPACITOR

With the inductor and sense resistor value selected, the value of the ramp capacitor (CRAMP) necessary for the

emulation ramp circuit is:

(13)

Where L is the value of the output inductor in Henrys, gm is the ramp generator transconductance (5 µA/V), and

A is the current sense amplifier gain (10 V/V). For the 5-V output design example, the ramp capacitor is

calculated as:

(14)

The next lowest standard value of 270 pF was selected for CRAMP. A COG type capacitor with 5% or better

tolerance is recommended.

OUTPUT CAPACITORS

The output capacitors smooth the inductor ripple current and provide a source of charge for transient loading

conditions. For this design example, five 100-µF ceramic capacitors where selected. Ceramic capacitors provide

very low equivalent series resistance (ESR), but can exhibit a significant reduction in capacitance with DC bias.

From the manufacturer’s data, the ESR at 250 kHz is 2 m

Ω / 5 = 0.4 mΩ, with a 36% reduction in capacitance at

5 V. This is verified by measuring the output ripple voltage and frequency response of the circuit. The

fundamental component of the output ripple voltage is calculated as:

(15)

With typical values for the 5-V design example:

(16)

20

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