Datasheet

LT3434
17
3434fb
APPLICATIO S I FOR ATIO
WUUU
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
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 7c). Tying BOOST to V
IN
or an
independent supply may reduce efficiency but it will re-
duce 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
LT3434 sees can be reduced by placing a Zener diode in
series with the BOOST diode (Figure 7a option).
A 0.68µF 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, 65mA boost current and 0.7V discharge ripple. The
boost capacitor value could be reduced under less de-
manding conditions but this will not improve circuit opera-
tion 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 LT3434 controls the operation of the
IC. When the voltage on the SHDN pin is below the 1.2V
shutdown threshold the LT3434 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 of 3µA.
In addition to the shutdown feature, the LT3434 has an
undervoltage lockout function. When the input voltage is
below 2.4V, switching will be disabled. The undervoltage
lockout threshold doesn’t have any hysteresis and is
mainly used to insure that all internal voltages are at the
correct level before switching is enabled. If an undervolt-
age lockout function with hysteresis is needed to limit
input current at low V
IN
to V
OUT
ratios refer to Figure 8 and
the following:
VR
V
R
V
R
IV
V
VR
R
UVLO
SHDN SHDN
SHDN SHDN
HYST
OUT
=++
⎛
⎝
⎜
⎞
⎠
⎟
+
=
()
1
32
1
3
R1 should be chosen to minimize quiescent current during
normal operation by the following equation:
R
VV
I
IN
SHDN MAX
1
2
15
=
()
()
–
.
()
BOOST
LT3434
V
BOOST
– V
SW
= V
OUT
V
BOOST(MAX)
= V
IN
+ V
OUT
V
IN
V
OUT
OPTIONAL
(7a)
V
IN
SWGND
BOOST
LT3434
V
BOOST
– V
SW
= V
DC
V
BOOST(MAX)
= V
DC
+ V
IN
V
IN
V
DC
D
SS
3434 F07
V
OUT
(7c)
V
IN
SWGND
BOOST
LT3434
V
BOOST
– V
SW
= V
IN
V
BOOST(MAX)
= 2V
IN
V
IN
V
OUT
(7b)
V
IN
SWGND
Figure 7. BOOST Pin Configurations