Datasheet

D =
V
OUT
+ V
D
+ V
DCR
V
IN
+ V
D
- V
SW
D =
V
OUT
+ V
D
V
IN
+ V
D
- V
SW
K =
P
OUT
P
OUT
+ P
LOSS
K =
P
OUT
P
IN
x R2
R1 =
V
REF
V
OUT
- 1
LMR10530
SNVS814A –JUNE 2012–REVISED APRIL 2013 www.ti.com
larger output capacitance is required to ensure sufficient phase margin. Capacitance can often, but not always,
be increased significantly with little detriment to the regulator stability. Like the input capacitor, recommended
multilayer ceramic capacitors are X7R or X5R types. Again, verify actual capacitance at the desired operating
voltage and temperature. Check the RMS current rating of the capacitor. The maximum RMS current rating of the
capacitor is:
(13)
One may select a 1206 size MLCC for output capacitor, since its current rating is typically above 1A, more than
enough for the requirement.
CATCH DIODE
The catch diode conducts during the switch off-time. A Schottky diode is recommended for its fast switching time
and low forward voltage drop. The catch diode should be chosen such that its current rating is greater than:
I
D
= I
OUT
x (1-D) (14)
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.
OUTPUT VOLTAGE
The output voltage is set using the following equation where R2 is connected between the FB pin and GND, and
R1 is connected between V
OUT
and the FB pin. A good value for R2 is 2kΩ.
(15)
V
REF
= 0.60V (16)
EFFICIENCY ESTIMATION
The complete LMR10530 DC/DC converter efficiency can be calculated in the following manner:
(17)
Or
(18)
Calculations for determining the most significant power losses are shown below. Other losses totaling less than
2% are not discussed.
The main power loss (P
LOSS
) in the converter includes two basic types of losses: switching loss and conduction
loss. In addition, there is loss associated with the power required for the internal circuitry of IC. Conduction
losses usually dominate at higher output loads, whereas switching losses dominate at lower output loads. The
first step in determining the losses is to calculate the duty cycle (D):
(19)
V
SW
is the voltage drop across the internal power switch when it is on, and is equal to:
V
SW
= I
OUT
x R
DS(ON)
(20)
V
D
is the forward voltage drop across the catch diode. It can be obtained from the diode manufactures Electrical
Characteristics section. If the DC voltage drop across the inductor (V
DCR
) is accounted for, the equation
becomes:
(21)
The conduction losses in the catch diode are calculated as follows:
P
DIODE
= V
D
x I
OUT
x (1-D) (22)
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