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LTC1553LCSW#TRPBF

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13
LTC1553L
APPLICATIONS INFORMATION
WUU
U
R
P
DC Q I
VP
VI
R
P
DC Q I
VP
VV I
DS ON Q
MAX Q
MAX
IN MAX Q
OUT MAX
DS ON Q
MAX Q
MAX
IN MAX Q
IN OUT MAX
()
()
()
()
()
()
=
()
[]
()
=
()
()()
=
()
[]
()
=
()
−
()()
1
1
2
1
2
2
2
2
2
2
1
2
P
MAX
should be calculated based primarily on required
efficiency or allowable thermal dissipation. A typical high
efficiency circuit designed for Pentium II with a 5V input
and a 2.8V, 11.2A output might allow no more than 4%
efficiency loss at full load for each MOSFET. Assuming
roughly 90% efficiency at this current level, this gives a
P
MAX
value of:
[(2.8)(11.2A/0.9)(0.04)] = 1.39W per FET
and a required R
DS(ON)
of:
R
VW
VA
R
VW
VV A
DS ON Q
DS ON Q
()
()
=
()( )
()( )
=
=
()( )
−
()()
=
1
2
2
2
5139
2 8 11 2
0 019
5139
528112
0 025
.
..
.
.
..
.
Ω
Ω
Note also that while the required R
DS(ON)
values suggest
large MOSFETs, the dissipation numbers are only 1.39W
per device or less––large TO-220 packages and heat sinks
are not necessarily required in high efficiency applica-
tions. Siliconix Si4410DY or International Rectifier IRF7413
(both in SO-8) or Siliconix SUD50N03 or Motorola
MTD20N03HDL (both in D PAK) are small footprint sur-
face mount devices with R
DS(ON)
values below 0.03Ω at 5V
of gate drive that work well in LTC1553L circuits. With
higher output voltages, the R
DS(ON)
of Q1 may need to be
significantly lower than that for Q2. These conditions can
often be met by paralleling two MOSFETs for Q1 and using
a single device for Q2. Note that using a higher P
MAX
value
in the R
DS(ON)
calculations will generally decrease MOSFET
cost and circuit efficiency while increasing MOSFET heat
sink requirements.
Q1
G1
Q2
0.1µF
L
O
V
OUT
1553L F07
C
OUT
C
IN
V
IN
PV
CC
1N5243B
13V
1N5817
OPTIONAL FOR V
IN
> 5V
G2
LTC1553L
20
1
2
+
+
Figure 7. Doubling Charge Pump
If the OUTEN pin is low, G1 and G2 are both held low to
prevent output voltage undershoot. As V
CC
and PV
CC
power up from a 0V condition, an internal undervoltage
lockup circuit prevents G1 and G2 from going high until
V
CC
reaches about 3.5V. If V
CC
powers up while PV
CC
is at
ground potential, the SS is forced to ground potential
internally. SS clamps the COMP pin low and prevents the
drivers from turning on. On power-up or recovery from
thermal shutdown, the drivers are designed such that G2
is held low until G1 first goes high.
Power MOSFETs
Two N-channel power MOSFETs are required for most
LTC1553L circuits. Logic level MOSFETs should be used
and they should be selected based on on-resistance con-
siderations. R
DS(ON)
should be chosen based on input and
output voltage, allowable power dissipation and maxi-
mum required output current. In a typical LTC1553L buck
converter circuit the average inductor current is equal to
the output load current. This current is always flowing
through either Q1 or Q2 with the power dissipation split up
according to the duty cycle:
DC Q
V
V
DC Q
V
V
VV
V
OUT
IN
OUT
IN
IN OUT
IN
1
21
()
=
()
=− =
−
()
The R
DS(ON)
required for a given conduction loss can now
be calculated by rearranging the relation P = I
2
R.
14
LTC1553L
APPLICATIONS INFORMATION
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Note: Please refer to the manufacturer’s data sheet for testing conditions
and detail information.
Inductor Selection
The inductor is often the largest component in the LTC1553L
design and should be chosen carefully. Inductor value and
type should be chosen based on output slew rate require-
ments, output ripple requirements and expected peak
current. Inductor value is primarily controlled by the
required current slew rate. The maximum rate of rise of
current in the inductor is set by its value, the input-to-
output voltage differential and the maximum duty cycle of
the LTC1553L. In a typical 5V input, 2.8V output applica-
tion, the maximum current slew rate will be:
DC
VV
LL
A
s
MAX
IN OUT
−
()
=
183.
µ
where L is the inductor value in µH. With proper frequency
compensation, the combination of the inductor and output
capacitor will determine the transient recovery time. In
general, a smaller value inductor will improve transient
response at the expense of increased output ripple voltage
and inductor core saturation rating. A 2µH inductor would
have a 0.9A/µs rise time in this application, resulting in a
5.5µs delay in responding to a 5A load current step. During
this 5.5µs, the difference between the inductor current and
the output current must be made up by the output capaci-
tor, causing a temporary voltage droop at the output. To
minimize this effect, the inductor value should usually be
in the 1µH to 5µH range for most typical 5V input LTC1553L
circuits. To optimize performance, different combinations
of input and output voltages and expected loads may
require different inductor values.
Once the required value is known, the inductor core type
can be chosen based on peak current and efficiency
requirements. Peak current in the inductor will be equal to
the maximum output load current plus half of the peak-to-
peak inductor ripple current. Ripple current is set by the
inductor value, the input and output voltage and the
operating frequency. The ripple current is approximately
equal to:
I
VV V
fLV
RIPPLE
IN OUT OUT
OSC O IN
=
−
()()
()()()
f
OSC
= LTC1553L oscillator frequency = 300kHz
L
O
= Inductor value
Table 5. Recommended MOSFETs for LTC1553L Applications
TYPICAL INPUT
R
DS(ON)
CAPACITANCE
PARTS AT 25°C (mΩ) RATED CURRENT (A) C
ISS
(pF) θ
JC
(°C/W) T
JMAX
(°C)
Siliconix SUD50N03-10 19 15 at 25°C 3200 1.8 175
TO-252 10 at 100°C
Siliconix Si4410DY 20 10 at 25°C 2700 — 150
SO-8 8 at 75°C
Motorola MTD20N03HDL 35 20 at 25°C 880 1.67 150
D PAK 16 at 100°C
SGS-Thomson STD20N03L 23 20 at 25°C 2300 2.5 175
D PAK 14 at 100°C
Motorola MTB75N03HDL 7.5 75 at 25°C 4025 1.0 150
DD PAK 59 at 100°C
IRF IRL3103S 14 56 at 25°C 1600 1.8 175
DD PAK 40 at 100°C
IRF IRLZ44 28 50 at 25°C 3300 1.0 175
TO-220 36 at 100°C
Fuji 2SK1388 37 35 at 25°C 1750 2.08 150
TO-220
15
LTC1553L
APPLICATIONS INFORMATION
WUU
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The output capacitor in a buck converter sees much less
ripple current under steady-state conditions than the input
capacitor. Peak-to-peak current is equal to that in the
inductor, usually 10% to 40% of the total load current.
Output capacitor duty places a premium not on power
dissipation but on ESR. During an output load transient,
the output capacitor must supply all of the additional load
current demanded by the load until the LTC1553L can
adjust the inductor current to the new value. Output
capacitor ESR results in a step in the output voltage equal
to the ESR value multiplied by the change in load current.
An 11A load step with a 0.05Ω ESR output capacitor will
result in a 550mV output voltage shift; this is 19.6% of the
output voltage for a 2.8V supply! Because of the strong
relationship between output capacitor ESR and output
load transient response, the output capacitor is usually
chosen for ESR, not for capacitance value; a capacitor with
suitable ESR will usually have a larger capacitance value
than is needed for energy storage.
Electrolytic capacitors rated for use in switching power
supplies with specified ripple current ratings and ESR can
be used effectively in LTC1553L applications. OS-CON
electrolytic capacitors from SANYO and other manufac-
turers give excellent performance and have a very high
performance/size ratio for electrolytic capacitors. Surface
mount applications can use either electrolytic or dry
tantalum capacitors. Tantalum capacitors must be surge
tested and specified for use in switching power supplies.
Low cost, generic tantalums are known to have very short
lives followed by explosive deaths in switching power
supply applications. AVX TPS series surface mount
devices are popular surge tested tantalum capacitors that
work well in LTC1553L applications.
A common way to lower ESR and raise ripple current
capability is to parallel several capacitors. A typical
LTC1553L application might exhibit 5A input ripple cur-
rent. SANYO OS-CON part number 10SA220M (220µF/
10V) capacitors feature 2.3A allowable ripple current at
85°C; three in parallel at the input (to withstand the input
ripple current) will meet the above requirements. Simi-
larly, AVX TPSE337M006R0100 (330µF/6V) have a rated
maximum ESR of 0.1Ω; seven in parallel will lower the net
output capacitor ESR to 0.014Ω. For low cost application,
SANYO MV-GX series of capacitors can be used with
acceptable performance.
Solving this equation with our typical 5V to 2.8V applica-
tion with a 2µH inductor, we get:
22 056
300 2
2
..
()( )
()()
=
kHz H
A
µ
P-P
Peak inductor current at 11.2A load:
11 2
2
2
12 2..A
A
A+=
The ripple current should generally be between 10% and
40% of the output current. The inductor must be able to
withstand this peak current without saturating, and the
copper resistance in the winding should be kept as low as
possible to minimize resistive power loss. Note that in
circuits not employing the current limit function, the
current in the inductor may rise above this maximum
under short circuit or fault conditions; the inductor should
be sized accordingly to withstand this additional current.
Inductors with gradual saturation characteristics are often
the best choice.
Input and Output Capacitors
A typical LTC1553L design puts significant demands on
both the input and the output capacitors. During constant
load operation, a buck converter like the LTC1553L draws
square waves of current from the input supply at the
switching frequency. The peak current value is equal to the
output load current plus 1/2 peak-to-peak ripple current,
and the minimum value is zero. Most of this current is
supplied by the input bypass capacitor. The resulting RMS
current flow in the input capacitor will heat it up, causing
premature capacitor failure in extreme cases. Maximum
RMS current occurs with 50% PWM duty cycle, giving an
RMS current value equal to I
OUT
/2. A low ESR input
capacitor with an adequate ripple current rating must be
used to ensure reliable operation.
Note that capacitor manufacturers’ ripple current ratings
are often based on only 2000 hours (three months)
lifetime at rated temperature. Further derating of the input
capacitor ripple current beyond the manufacturer’s speci-
fication is recommended to extend the useful life of the
circuit. Lower operating temperature will have the largest
effect on capacitor longevity.
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LTC1553LCSW#TRPBF

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Description:
Switching Voltage Regulators 5-B Progmable Sync Sw Reg Cntr for Penti
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