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LT1956IFE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P28
13
LT1956/LT1956-5
1956f
APPLICATIO S I FOR ATIO
WUUU
where:
f = switching frequency
t
ON
= switch minimum on time
V
F
= diode forward voltage
V
IN
= input voltage
I • R = inductor I • R voltage drop
If this condition is not observed, the current will not be
limited at I
PK
, but will cycle-by-cycle ratchet up to some
higher value. Using the nominal LT1956 clock frequency
of 500KHz, a V
IN
of 12V and a (V
F
+ I • R) of say 0.7V, the
maximum t
ON
to maintain control would be approximately
116ns, an unacceptably short time.
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
decreases roughly linearly down to a limit of about 100kHz.
This lower oscillator frequency during short-circuit condi-
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 V
IN
= 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 V
F
(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:
V
IN
= (0.63V + 0.128V)/(100kHz • 300ns)
V
IN(MAX)
= 25V
Increasing the DCR of the inductor will increase the maxi-
mum V
IN
allowed during an output short to ground but will
also drop overall efficiency during normal operation.
Every time the converter wakes up from shutdown or
undervoltage lockout to begin switching, the output
capacitor may potentially be starting from 0V. This re-
quires that the part obey the overall duty cycle demanded
by the loop, related to V
IN
and V
OUT
, as the output voltage
rises to its target value. It is recommended that for [V
IN
/
(V
OUT
+ V
F
)] ratios > 4, 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 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
capacitor is 0.05Ω to 0.2Ω. A typical output capacitor is an
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.
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E CASE SIZE ESR (MAX,
Ω
) RIPPLE CURRENT (A)
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1
D CASE SIZE
AVX TPS, Sprague 593D 0.1 to 0.3 0.7 to 1.1
C CASE SIZE
AVX TPS 0.2 (typ) 0.5 (typ)
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 125mA
RMS
. The formula
to calculate this is:
14
LT1956/LT1956-5
1956f
Output capacitor ripple current (RMS):
I
VVV
LfV
RIPPLE RMS
OUT IN OUT
IN
()
.–
=
()( )
()()( )
029
Ceramic Capacitors
Ceramic capacitors 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
C
can be placed
in series with the V
C
compensation capacitor C
C
. Care
must be taken however, since this resistor sets the high
frequency gain of the error amplifier, including the gain
at the switching frequency. If the gain of the error
amplifier is high enough at the switching frequency,
output ripple voltage (although smaller for a ceramic
output capacitor) may still affect the proper operation of
the regulator. A filter capacitor C
F
in parallel with the
R
C
/C
C
network is suggested to control possible ripple at
the V
C
pin. The LT1956 can be stabilized for V
OUT
= 5V at
1A using a 22µF ceramic output capacitor and V
C
com-
ponent values of C
C
= 4700pF, R
C
␣ =␣ 4.7k and C
F
= 220pF.
INPUT CAPACITOR
Step-down regulators draw current from the input supply
in pulses. The rise and fall times of these pulses are very
fast. The input capacitor is required to reduce the voltage
ripple this causes at the input of LT1956 and force the
switching current into a tight local loop, thereby minimiz-
ing EMI. The RMS ripple current can be calculated from:
ICI
VVV
V
RIPPLE RMS IN OUT
OUT IN OUT
IN
()
–
=
()
2
Ceramic capacitors are ideal for input bypassing. At 500kHz
switching frequency, the energy storage requirement of
the input capacitor suggests that values in the range of
2.2µF to 10µF are suitable for most applications. If opera-
tion is required close to the minimum input required by the
output of the LT1956, a larger value may be required. This
is to prevent excessive ripple causing dips below the mini-
mum operating voltage resulting in erratic operation.
Depending on how the LT1956 circuit is powered up you
may need to check for input voltage transients.
The input voltage transients may be caused by input
voltage steps or by connecting the LT1956 converter to an
already powered up source such as a wall adapter. The
sudden application of input voltage will cause a large
surge of current in the input leads that will store energy in
the parasitic inductance of the leads. This energy will
cause the input voltage to swing above the DC level of input
power source and it may exceed the maximum voltage
rating of input capacitor and LT1956.
The easiest way to suppress input voltage transients is to
add a small aluminum electrolytic capacitor in parallel with
the low ESR input capacitor. The selected capacitor needs
to have the right amount of ESR in order to critically
dampen the resonant circuit formed by the input lead
inductance and the input capacitor. The typical values of
ESR will fall in the range of 0.5Ω to 2Ω and capacitance will
fall in the range of 5µF to 50µF.
If tantalum capacitors are used, values in the 22µF to
470µF range are generally needed to minimize ESR and
meet ripple current and surge ratings. Care should be
taken to ensure the ripple and surge ratings are not
exceeded. The AVX TPS and Kemet T495 series are surge
rated. AVX recommends derating capacitor operating
voltage by 2 for high surge applications.
CATCH DIODE
Highest efficiency operation requires the use of a Schottky
type diode. DC switching losses are minimized due to its
low forward voltage drop, and AC behavior is benign due
to its lack of a significant reverse recovery time. Schottky
diodes are generally available with reverse voltage ratings
of up to 60V and even 100V, and are price competitive with
other types.
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
APPLICATIO S I FOR ATIO
WUUU
15
LT1956/LT1956-5
1956f
internal switch will ramp up V
IN
current into the diode in an
attempt to get it to recover. Then, when the diode has
finally turned off, some tens of nanoseconds later, the V
SW
node voltage ramps up at an extremely high dV/dt, per-
haps 5 to even 10V/ns! With real world lead inductances,
the V
SW
node can easily overshoot the V
IN
rail. This can
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 10MQ060N Schottky. It is rated at 1.5A average
forward current and 60V reverse voltage. Typical forward
voltage is 0.63V at 1A. The diode conducts current only
during switch off time. Peak reverse voltage is equal to
regulator input voltage. Average forward current in normal
operation can be calculated from:
I
D(AVG)
= I
OUT
(1 – DC)
This formula will not yield values higher than 1.5A with
maximum load current of 1.5A. The only reason to
consider a larger diode is the worst-case condition of a
high input voltage and shorted output. With a shorted
condition, diode current will increase to a typical value of
2A, determined by peak switch current limit. This is safe
for short periods of time, but it would be prudent to check
with the diode manufacturer if continuous operation
under these conditions must be tolerated.
BOOST␣ PIN␣
For most applications, the boost components are a 0.1µF
capacitor and an MMSD914TI diode. The anode is typi-
cally connected to the regulated output voltage to generate
a voltage approximately V
OUT
above V
IN
to drive the output
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 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
IN
boost voltage from exceeding the BOOST pin absolute
maximum rating. The additional voltage across the switch
driver also increases power loss, reducing efficiency. If
available, an independent supply can be used with a local
bypass capacitor.
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
1800ns on time, 42mA 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 LT1956. 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 5.5µA 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
LO
10k or less. If shutdown
current is an issue, R
LO
can be raised to 100k, but the error
due to initial bias current and changes with temperature
should be considered.
Rk
R
RV V
VR A
LO
HI
LO IN
LO
=
()
=
−
()
−
()
10
238
238 55
to 100k 25k suggested
.
..µ
V
IN
= minimum input voltage
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
resistor values are used, the shutdown pin should be
APPLICATIO S I FOR ATIO
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LT1956IFE#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Hi V, 1.5A, 500kHz Buck Sw Regs
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