LT3434
16
3434fb
Table 4. Catch Diode Selection Criteria
I
Q
at 125°C EFFICIENCY
LEAKAGE V
IN
=12V V
IN
=12V
V
OUT
= 3.3V V
F
AT 1A V
OUT
= 3.3 V
OUT
= 3.3V
DIODE 25°C 125°C25°C 125°CI
L
= 0A I
L
= 1A
IR 10BQ100 0.0µA59µA 0.72V 0.58V 125µA 78.2%
Diodes Inc. 0.1µA 242µA 0.48V 0.41V 215µA 82%
B260SMA
Diodes Inc. 0.2µA 440µA 0.45V 0.36V 270µA 82.8%
B360SMB
IR 1µA 1.81mA 0.42V 0.34V 821µA 82.7%
MBRS360TR
IR 30BQ100 1.7µA 2.64mA 0.40V 0.32V 1088µA 80.3%
APPLICATIO S I FOR ATIO
WUUU
If a no load condition can be anticipated, the supply current
can be further reduced by cycling the SHDN pin at a rate
higher than the natural no load burst frequency. Figure 6
shows Burst Mode operation with the SHDN pin. V
OUT
burst ripple is maintained while the average supply current
drops to 15µA. The PG pin will be active low during the
“on” portion of the SHDN waveform due to the C
T
capaci-
tor discharge when SHDN is taken low. See the Power
Good section for further information.
lack of a significant reverse recovery time. Schottky diodes
are generally available with reverse voltage ratings of 60V
and even 100V and are price competitive with other types.
The effect of reverse leakage and forward drop on effi-
ciency for various Schottky diodes is shown in Table 4. As
can be seen these are conflicting parameters and the user
must weigh the importance of each specification in choos-
ing the best diode for the application.
The use of so-called “ultrafast” recovery diodes is gener-
ally not recommended. When operating in continuous
mode, the reverse recovery time exhibited by “ultrafast”
diodes will result in a slingshot type effect. The power
internal switch will ramp up V
IN
current into the diode in an
attempt to get it to recover. Then, when the diode has
finally turned off, some tens of nanoseconds later, the V
SW
node voltage ramps up at an extremely high dV/dt, per-
haps 5V to even 10V/ns! With real world lead inductances
the V
SW
node can easily overshoot the V
IN
rail. This can
result in poor RFI behavior and, if the overshoot is severe
enough, damage the IC itself.
BOOST PIN
For most applications the boost components are a 0.68µF
capacitor and a MMSD914 diode. The anode is typically
connected to the regulated output voltage to generate a
voltage approximately V
OUT
above V
IN
to drive the output
stage (Figure 7a). However, the output stage discharges
the boost capacitor during the on time of the switch. The
output driver requires at least 2.5V of headroom through-
out this period to keep the switch fully saturated. If the
Figure 6. Burst Mode with Shutdown Pin
V
OUT
50mV/DIV
V
SHDN
2V/DIV
I
SW
500mA/DIV
V
IN
= 12V TIME (50ms/DIV) 3434 G16
V
OUT
= 3.3V
I
Q
= 15µA
CATCH DIODE
The catch diode carries load current during the SW off
time. The average diode current is therefore dependent on
the switch duty cycle. At high input to output voltage ratios
the diode conducts most of the time. As the ratio ap-
proaches unity the diode conducts only a small fraction of
the time. The most stressful condition for the diode is
when the output is short circuited. Under this condition the
diode must safely handle I
PEAK
at maximum duty cycle.
To maximize high and low load current efficiency a fast
switching diode with low forward drop and low reverse
leakage should be used. Low reverse leakage is critical to
maximize low current efficiency since its value over tem-
perature can potentially exceed the magnitude of the
LT3434 supply current. Low forward drop is critical for
high current efficiency since the loss is proportional to
forward drop.
These requirements result in the use of a Schottky type
diode. DC switching losses are minimized due to its low
forward voltage drop and AC behavior is benign due to its