LT1956/LT1956-5
W U U
APPLICATIO S I FOR ATIO
U
capacitor may potentially be starting from 0V. This re-
quires that the part obey the overall duty cycle demanded
by the loop, related to VIN and VOUT, as the output voltage
rises to its target value. It is recommended that for [VIN/
(VOUT + VF)] ratios > 4, a soft-start circuit should be used
tocontroltheoutputcapacitorchargerateduringstart-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.
where:
f = switching frequency
tON = switch minimum on time
VF = diode forward voltage
VIN = input voltage
I • R = inductor I • R voltage drop
If this condition is not observed, the current will not be
limited at IPK, but will cycle-by-cycle ratchet up to some
higher value. Using the nominal LT1956 clock frequency
of 500KHz, a VIN of 12V and a (VF + I • R) of say 0.7V, the
maximum tON to maintain control would be approximately
116ns, an unacceptably short time.
OUTPUT CAPACITOR
The LT1956 will operate with either ceramic or tantalum
output capacitors. The output capacitor is normally cho-
sen by its effective series resistance (ESR), because this
is what determines output ripple voltage. The ESR range
for typical LT1956 applications using a tantalum output
capacitoris0.05Ωto0.2Ω. Atypicaloutputcapacitorisan
AVX type TPS, 100µF at 10V, with a guaranteed ESR less
than 0.1Ω. 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 criti-
cal, and values from 22µF to greater than 500µF work well,
but you cannot cheat mother nature on ESR. If you find a
tiny 22µF 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.
The solution to this dilemma is to slow down the oscillator
when the FB pin voltage is abnormally low thereby indicat-
ing some sort of short-circuit condition. Oscillator fre-
quency is unaffected until FB voltage drops to about 2/3 of
its normal value. Below this point the oscillator frequency
decreasesroughlylinearlydowntoalimitofabout100kHz.
Thisloweroscillatorfrequencyduringshort-circuitcondi-
tions can then maintain control with the effective mini-
mum on time. Even with frequency foldback, however, the
LT1956 will not survive a permanent output short at the
absolute maximum voltage rating of VIN = 60V; this is
defined solely by internal semiconductor junction break-
down effects.
For the maximum input voltage allowed during an output
short to ground, the previous equation defining minimum
on-time can be used. Assuming VF (D1 catch diode) =
0.63V at 1A (short-circuit current is folded back to typical
switch current limit • 0.5), I (inductor) • DCR = 1A • 0.128
= 0.128V (L = CDRH6D28-22), typical f = 100kHz (folded
back) and typical minimum on-time = 300ns, the maxi-
mum allowable input voltage during an output short to
ground is typically:
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E CASE SIZE
ESR (MAX,
Ω
)
RIPPLE CURRENT (A)
AVX TPS, Sprague 593D
D CASE SIZE
0.1 to 0.3
0.1 to 0.3
0.2 (typ)
0.7 to 1.1
AVX TPS, Sprague 593D
C CASE SIZE
0.7 to 1.1
0.5 (typ)
AVX TPS
VIN = (0.63V + 0.128V)/(100kHz • 300ns)
VIN(MAX) = 25V
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 125mARMS. The formula
to calculate this is:
Increasing the DCR of the inductor will increase the maxi-
mumVIN allowedduringanoutputshorttogroundbutwill
also drop overall efficiency during normal operation.
Every time the converter wakes up from shutdown or
undervoltage lockout to begin switching, the output
1956f
13
Linear [ Linear ]
