Datasheet

18
LT3437
3437fc
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.
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. 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, perhaps 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 0.1µF
capacitor and a BAS21 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 6a). 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 6b), but care must be taken
to prevent the boost voltage (V
BOOST
= V
IN
• 2) from
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 6c). Tying BOOST to V
IN
or an
independent supply may reduce efficiency, but it will
reduce the minimum V
IN
required to start-up with light
loads. If the generated BOOST voltage dissipates too
much power at maximum load, the BOOST voltage the
LT3437 sees can be reduced by placing a Zener diode in
series with the BOOST diode (Figure 6a option).
A 0.1µF boost capacitor is recommended for most appli-
cations. Almost any type of film or ceramic capacitor is
suitable, but the ESR should be <1Ω to ensure it can be
fully recharged during the off time of the switch. The
capacitor value is derived from worst-case conditions of
4700ns on time, 11mA boost current and 0.7V discharge
ripple. The boost capacitor value could be reduced under
less demanding conditions, but this will not improve
circuit operation or efficiency. Under low input voltage and
low load conditions, a higher value capacitor will reduce
discharge ripple and improve start-up operation.
BOOST
LT3437
V
BOOST
– V
SW
= V
OUT
V
BOOST(MAX)
= V
IN
+ V
OUT
V
IN
V
OUT
OPTIONAL
(6a)
V
IN
SWGND
BOOST
LT3437
V
BOOST
– V
SW
= V
DC
V
BOOST(MAX)
= V
DC
+ V
IN
V
IN
V
DC
D
SS
3437 F06
V
OUT
(6c)
V
IN
SWGND
BOOST
LT3437
V
BOOST
– V
SW
= V
IN
V
BOOST(MAX)
= 2V
IN
V
IN
V
OUT
(6b)
V
IN
SWGND
Figure 6. BOOST Pin Configurations
APPLICATIO S I FOR ATIO
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