LT3437 LINER [Linear Technology], LT3437 Datasheet - Page 16

• Manufacturer Part Number: LT3437
• Description: High Voltage 500mA, 200kHz Step-Down Switching Regulator with 100uA Quiescent Current
• Manufacturer: LINER [Linear Technology]

LT3437
APPLICATIO S I FOR ATIO
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. When the diode has finally
turned off, some tens of nanoseconds later, the V
voltage ramps up at an extremely high dV/dt, perhaps 5V
to even 10V/ns! With real world lead inductances, the V
node can easily overshoot the V
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
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
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
independent supply may reduce efficiency, but it will
reduce the minimum V
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
16
U
IN
OUT
U
required to start-up with light
above V
IN
current into the diode in an
IN
BOOST
W
rail. This can result in
IN
to drive the output
= V
IN
U
• 2) from
SW
IN
or an
node
SW
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.
SHUTDOWN FUNCTION AND UNDERVOLTAGE
LOCKOUT
The SHDN pin on the LT3437 controls the operation of the
IC. When the voltage on the SHDN pin is below the 1.2V
shutdown threshold, the LT3437 is placed in a “zero”
supply current state. Driving the SHDN pin above the
shutdown threshold enables normal operation. The SHDN
pin has an internal sink current with a typical value of 5µA.
In addition to the shutdown feature, the LT3437 has an
undervoltage lockout function. When the input voltage is
V
V
V
IN
IN
IN
Figure 6. BOOST Pin Configurations
V
V
BOOST(MAX)
BOOST(MAX)
V
V
V
V
BOOST
BOOST
BOOST
BOOST(MAX)
V
GND
V
GND
V
GND
IN
IN
IN
LT3437
LT3437
LT3437
– V
– V
– V
BOOST
BOOST
BOOST
SW
= V
SW
= V
SW
SW
SW
SW
IN
= 2V
= V
DC
= V
= V
(6a)
(6b)
(6c)
+ V
OUT
+ V
DC
IN
IN
OUT
IN
OPTIONAL
D
SS
3437 F06
V
V
V
V
OUT
OUT
DC
OUT
3437f