Datasheet

r =
'i
L
l
OUT
D =
V
OUT
+ V
D
V
IN
+ V
D
- V
SW
D =
V
OUT
V
IN
LMR10530
SNVS814A –JUNE 2012–REVISED APRIL 2013 www.ti.com
CURRENT LIMIT
The LMR10530 uses cycle-by-cycle current limiting to protect the internal power switch. During each switching
cycle, a current limit comparator detects if the power switch current exceeds 4.4A (typical), and turns off the
switch until the next switching cycle begins.
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off the internal power switch when the IC junction
temperature typically exceeds 165°C. After thermal shutdown occurs, the power switch does not turn on again
until the junction temperature drops below approximately 150°C.
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the ratio of output voltage (V
OUT
) to input voltage (V
IN
):
(3)
The catch diode (D1) forward voltage drop and the voltage drop across the internal PMOS must be included to
calculate a more accurate duty cycle. Calculate D by using the following formula:
(4)
V
SW
can be approximated by:
V
SW
= I
OUT
x R
DS(ON)
where
• I
OUT
is output load current. (5)
The diode forward drop (V
D
) can range from 0.3V to 0.7V depending on the quality of the diode. The lower the
V
D
, the higher the operating efficiency of the converter.
The inductor value determines the output ripple current (Δi
L
, as defined in Figure 23). Lower inductor values
decrease the size of the inductor, but increase the output ripple current. An increase in the inductor value will
decrease the output ripple current. In general, the ratio of ripple current to the output current is optimized when it
is set between 0.2 and 0.4 for output currents above 2A. This ratio r is defined as:
(6)
One must ensure that the minimum current limit (3.4A) is not exceeded, so the peak current in the inductor must
be calculated. The peak current (I
LPK
) in the inductor is calculated by:
I
LPK
= I
OUT
+ Δi
L
/2 (7)
When the designed maximum output current is reduced, the ratio r can be increased. At a current of 0.1A, r can
be made as high as 0.9. The ripple ratio can be increased at lighter loads because the net ripple is actually quite
low, and if r remains constant the inductor value can be made quite large. An equation empirically developed for
the maximum ripple ratio at any current below 2A is:
r = 0.387 x I
OUT
-0.3667
(8)
Note that this is just a guideline, and it needs to be combined with two important factors for proper selection of
inductance values at any operating condition. The first consideration is at output voltage above 2.5V, one needs
to ensure that the inductance given by the above guideline should not be less than 1µH for the LMR10530X or
0.5µH for the LMR10530Y. Since the LMR10530 has a fixed internal corrective ramp signal, a very low
inductance value at high output voltage will generate a very steep down slope of inductor current, which will
result in an insufficient slope compensation, and cause instability known as sub-harmonic oscillation. Another
consideration is at low load current, one needs to ensure that the inductance value given by the guideline should
not exceed 10µH for the LMR10530X and 4.7µH for the LMR10530Y, since too much inductance effectively
flattens the down slope of the inductor current, and may significantly limit the system bandwidth and phase
margin resulting in instability.
16 Submit Documentation Feedback Copyright © 2012–2013, Texas Instruments Incorporated
Product Folder Links: LMR10530