lt1976 Linear Technology Corporation, lt1976 Datasheet - Page 19

• Manufacturer Part Number: lt1976
• Description: High Voltage 1.5a, 200khz Step-down Switching Regulator With 100?a Quiescent Current
• Manufacturer: Linear Technology Corporation

Table 5. Catch Diode Selection Criteria
DIODE
IR 10BQ100 0.0μA
Diodes Inc.
B260SMA
Diodes Inc.
B360SMB
IR
MBRS360TR
IR 30BQ100 1.7μA 2.64mA 0.40V
APPLICATIO S I FOR ATIO
drops to 15μA. The PG pin will be active low during the
“on” portion of the SHDN waveform due to the C
tor discharge when SHDN is taken low. See the Power
Good section for further information.
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
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
LT1976 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
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.
0.1μA
0.2μA
25°C
1μA
V
LEAKAGE
OUT
= 3.3V
1.81mA 0.42V
242μA 0.48V
440μA 0.45V
125°C
59μA
U
0.72V
25°C
U
PEAK
V
F
AT 1A
125°C
0.58V
0.41V
0.36V
0.34V
0.32V
at maximum duty cycle.
W
I
V
Q
V
OUT
1088μA
at 125°C EFFICIENCY
IN
I
125μA
215μA
270μA
821μA
L
= 0A
=12V
= 3.3 V
U
T
OUT
V
I
74.1%
82.8%
83.6%
83.7%
84.5%
L
IN
capaci-
= 1A
=12V
= 3.3V
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
attempt to get it to recover. Then, when the diode has
finally turned off, some tens of nanoseconds later, the V
node voltage ramps up at an extremely high dV/dt, per-
haps 5V to even 10V/ns! With real world lead inductances
the V
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 0.33μF
capacitor and a MMSD914 diode. The anode is typically
connected to the regulated output voltage to generate a
voltage approximately V
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
output voltage is less than 3.3V it is recommended that an
alternate boost supply is used. The boost diode can be
connected to the input (Figure 7b) but care must be taken
to prevent the boost voltage (V
exceeding the BOOST pin absolute maximum rating. The
additional voltage across the switch driver also increases
power loss and reduces efficiency. If available, an inde-
pendent supply can be used to generate the required
BOOST voltage (Figure 7c). Tying BOOST to V
independent supply may reduce efficiency but it will re-
duce the minimum V
loads. If the generated BOOST voltage dissipates too
much power at maximum load, the BOOST voltage the
LT1976 sees can be reduced by placing a Zener diode in
series with the BOOST diode (Figure 7a option).
SW
node can easily overshoot the V
IN
OUT
LT1976/LT1976B
required to start-up with light
above V
IN
current into the diode in an
BOOST
IN
to drive the output
= V
IN
rail. This can
IN
• 2) from
IN
19
or an
1976bfg
SW