LT3430 Linear Technology, LT3430 Datasheet - Page 13

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

APPLICATIO S I FOR ATIO
If this condition is not observed, the current will not be
limited at I
higher value. Using the nominal LT3430 clock frequency
of 200KHz, a V
maximum t
90ns, an unacceptably short time.
The solution to this dilemma is to slow down the oscilla-
tor when the FB pin voltage is abnormally low thereby
indicating some sort of short-circuit condition. Oscillator
frequency is unaffected until FB voltage drops to about 2/3
of its normal value. Below this point the oscillator fre-
quency decreases roughly linearly down to a limit of about
40kHz. This lower oscillator frequency during short-cir-
cuit conditions can then maintain control with the effective
minimum on time.
It is recommended that for [V
a soft-start circuit should be used to control the output
capacitor charge rate during start-up or during recovery
from an output short circuit, thereby adding additional
control over peak inductor current. See Buck Converter
with Adjustable Soft-Start later in this data sheet.
OUTPUT CAPACITOR
The output capacitor is normally chosen by its effective
series resistance (ESR), because this is what determines
output ripple voltage. To get low ESR takes volume , so
physically smaller capacitors have high ESR. The ESR
range for typical LT3430 applications is 0.05 to 0.2 . A
typical output capacitor is an AVX type TPS, 100 F at 10V,
with a guaranteed ESR less than 0.1 . This is a “D” size
surface mount solid tantalum capacitor. TPS capacitors
are specially constructed and tested for low ESR, so they
give the lowest ESR for a given volume. The value in
microfarads is not particularly critical, and values from
22 F to greater than 500 F work well, but you cannot
cheat mother nature on ESR. If you find a tiny 22 F solid
tantalum capacitor, it will have high ESR, and output ripple
voltage will be terrible. Table 3 shows some typical solid
tantalum surface mount capacitors.
PK
ON
, but will cycle-by-cycle ratchet up to some
to maintain control would be approximately
IN
of 40V and a (V
U
U
IN
/(V
F
+ I • R) of say 0.7V, the
OUT
W
+ V
F
)] ratios > 10,
U
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E Case Size
AVX TPS, Sprague 593D
D Case Size
AVX TPS, Sprague 593D
C Case Size
AVX TPS
Many engineers have heard that solid tantalum capacitors
are prone to failure if they undergo high surge currents.
This is historically true, and type TPS capacitors are
specially tested for surge capability, but surge ruggedness
is not a critical issue with the output capacitor. Solid
tantalum capacitors fail during very high turn-on surges,
which do not occur at the output of regulators. High
discharge surges, such as when the regulator output is
dead shorted, do not harm the capacitors.
Unlike the input capacitor, RMS ripple current in the
output capacitor is normally low enough that ripple cur-
rent rating is not an issue. The current waveform is
triangular with a typical value of 250mA
to calculate this is:
Output capacitor ripple current (RMS):
Ceramic Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available. They are generally chosen for their
good high frequency operation, small size and very low
ESR (effective series resistance). Their low ESR reduces
output ripple voltage but also removes a useful zero in the
loop frequency response, common to tantalum capaci-
tors. To compensate for this, a resistor R
in series with the V
must be taken however, since this resistor sets the high
frequency gain of the error amplifier, including the gain at
I
RIPPLE RMS
0 29
.
C
compensation capacitor C
ESR (Max.,
V
OUT
0.1 to 0.3
0.1 to 0.3
0.2 (typ)
L f V
V
IN
IN
)
V
OUT
RMS
Ripple Current (A)
C
can be placed
. The formula
0.7 to 1.1
0.7 to 1.1
0.5 (typ)
LT3430
sn3430 3430is
13
C
. Care