LT3430 Linear Technology, LT3430 Datasheet - Page 15

• Manufacturer Part Number: LT3430
• Description: High Voltage/ 3A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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 5 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.
The suggested catch diode (D1) is an International Recti-
fier 30BQ060 Schottky. It is rated at 3A average forward
current and 60V reverse voltage. Typical forward voltage
is 0.52V at 3A. The diode conducts current only during
switch off time. Peak reverse voltage is equal to regulator
input voltage. Average forward current in normal opera-
tion can be calculated from:
This formula will not yield values higher than 3A with
maximum load current of 3A.
BOOST PIN
For most applications, the boost components are a 0.68 F
capacitor and a MMSD914TI diode. The anode is typically
connected to the regulated output voltage to generate a
voltage approximately V
stage. However, the output stage discharges the boost
capacitor during the on time of the switch. The output
driver requires at least 3V of headroom throughout 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, although, care must be taken to
prevent the 2 V
BOOST pin absolute maximum rating. The additional
voltage across the switch driver also increases power
loss, reducing efficiency. If available, and independent
supply can be used with a local bypass capacitor.
A 0.68 F boost capacitor is recommended for most appli-
cations. Almost any type of film or ceramic capacitor is
I
D AVG
(
SW
node can easily overshoot the V
)
I
OUT
IN
V
U
IN
V
IN
boost voltage from exceeding the
–
OUT
V
OUT
U
above V
W
IN
to drive the output
IN
rail. This can
U
SW
suitable, but the ESR should be <1 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, 80mA 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.
SHUTDOWN FUNCTION AND UNDERVOLTAGE
LOCKOUT
Figure 4 shows how to add undervoltage lockout (UVLO)
to the LT3430. Typically, UVLO is used in situations where
the input supply is current limited , or has a relatively high
source resistance. A switching regulator draws constant
power from the source, so source current increases as
source voltage drops. This looks like a negative resistance
load to the source and can cause the source to current limit
or latch low under low source voltage conditions. UVLO
prevents the regulator from operating at source voltages
where these problems might occur.
Threshold voltage for lockout is about 2.38V. A 5.5 A bias
current flows out of the pin at this threshold. The internally
generated current is used to force a default high state on
the shutdown pin if the pin is left open. When low shut-
down current is not an issue, the error due to this current
can be minimized by making R
current is an issue, R
due to initial bias current and changes with temperature
should be considered.
Keep the connections from the resistors to the shutdown
pin short and make sure that interplane or surface capaci-
tance to the switching nodes are minimized. If high
V
R
R
IN
LO
HI
= Minimum input voltage
2 38
10
.
R
k
LO
to 100k 25k suggested
V R
V
IN
LO
LO
2 38
can be raised to 100k, but the error
.
5 5
.
V
A
LO
10k or less. If shutdown
LT3430
sn3430 3430is
15