D =

VOUT + VD + VDCR

VIN + VD + VDCR - VSW

D =

VOUT + VD

VIN + VD - VSW

K =

POUT

POUT + PLOSS

K =

POUT

PIN

x R2

R1 =

VREF

VOUT

- 1

LM2830, LM2830-Q1

www.ti.com

SNVS454E – AUGUST 2006 – REVISED DECEMBER 2014

capacitances in the inductor to the output. A ceramic capacitor will bypass this noise while a tantalum will not.

Because the output capacitor is one of the two external components that control the stability of the regulator

control loop, most applications will require a minimum of 22 µF of output capacitance. Capacitance often, but not

always, can be increased significantly with little detriment to the regulator stability. Like the input capacitor,

recommended multilayer ceramic capacitors are X7R or X5R types.

8.2.1.2.4

Catch Diode

The catch diode (D1) conducts during the switch off-time. A Schottky diode is recommended for its fast switching

times and low forward voltage drop. The catch diode should be chosen so that its current rating is greater than:

ID1 = IOUT × (1-D)

(11)

The reverse breakdown rating of the diode must be at least the maximum input voltage plus appropriate margin.

To improve efficiency, choose a Schottky diode with a low forward voltage drop.

8.2.1.2.5

Output Voltage

The output voltage is set using Equation 12, where R2 is connected between the FB pin and GND, and R1 is

connected between VO and the FB pin. A good value for R2 is 10 kΩ. When designing a unity gain converter (Vo

= 0.6 V), R1 should be between 0

Ω and 100 Ω, and R2 should be equal or greater than 10 kΩ.

(12)

VREF = 0.60 V

(13)

8.2.1.2.6

Calculating Efficiency, and Junction Temperature

The complete LM2830 DC-DC converter efficiency can be calculated in the following manner.

(14)

Or

(15)

Calculations for determining the most significant power losses are shown below. Other losses totaling less than

2% are not discussed.

Power loss (PLOSS) is the sum of two basic types of losses in the converter: switching and conduction.

Conduction losses usually dominate at higher output loads, whereas switching losses remain relatively fixed and

dominate at lower output loads. The first step in determining the losses is to calculate the duty cycle (D):

(16)

VSW is the voltage drop across the internal PFET when it is on, and is equal to:

VSW = IOUT × RDSON

(17)

VD is the forward voltage drop across the Schottky catch diode. It can be obtained from the diode manufacturer's

Electrical Characteristics section. If the voltage drop across the inductor (VDCR) is accounted for, the equation

becomes:

(18)

The conduction losses in the free-wheeling Schottky diode are calculated as follows:

PDIODE = VD × IOUT × (1-D)

(19)

Often this is the single most significant power loss in the circuit. Care should be taken to choose a Schottky

diode that has a low forward voltage drop.

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