Datasheet

r =
'i
L
l
OUT
D =
V
OUT
+ V
D
V
IN
+ V
D
- V
SW
D =
V
OUT
V
IN
LM2833
SNVS505E –MAY 2008–REVISED APRIL 2013 www.ti.com
OUTPUT OVER-VOLTAGE PROTECTION
The LM2833 has a built in output over-voltage comparator that compares the FB pin voltage to a threshold
voltage that is 15% higher than the internal reference V
REF
. Once the FB pin voltage exceeds this threshold level
(typically 0.69V), the internal PMOS power switch is turned off, which allows the output voltage to decrease
towards regulation.
UNDER-VOLTAGE LOCKOUT
Under-voltage lockout (UVLO) prevents the LM2833 from operating until the input voltage exceeds 2.70V
(typical). The UVLO threshold has approximately 350mV of hysteresis, so the part will operate until V
IN
drops
below 2.35V (typical). Hysteresis prevents the part from turning off during power up if V
IN
is non-monotonic.
CURRENT LIMIT
The LM2833 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 26). 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)
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