LTC1649CS#PBF Linear Technology, LTC1649CS#PBF Datasheet - Page 9

• Manufacturer Part Number: LTC1649CS#PBF
• Description: IC BUCK/SW CAP SYNC ADJ 16SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Switched Capacitor (Charge Pump)r

Specifications of LTC1649CS#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.26 ~ 2.5 V
• Current - Output: 20A
• Frequency - Switching: 200kHz
• Voltage - Input: 2.7 ~ 5 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS
ages and expected loads may require different 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 added to half the peak-
to- peak inductor ripple current. Ripple current is set by the
inductor value, the input and output voltage and the
operating frequency. If the efficiency is high and can be
considered approximately equal to 1, the ripple current is
approximately equal to:
Solving this equation with our typical 3.3V to 2.5V appli-
cation, we get:
Peak inductor current at 10A load:
The inductor core must be adequate 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 the current may rise above
this maximum level in circuits under current limit or under
fault conditions in unlimited circuits; the inductor should
be sized to withstand this additional current.
Input and Output Capacitors
A typical LTC1649 design puts significant demands on
both the input and output capacitors. Under normal steady
load operation, a buck converter like the LTC1649 draws
square waves of current from the input supply at the
switching frequency, with the peak value equal to the
output current and the minimum value near zero. Most of
this current must come from the input bypass capacitor,
since few raw supplies can provide the current slew rate to
DC =
f
L = inductor value
10A +
(200kHz)(2 H)
OSC
I =
(0.8)(0.76)
= LTC1649 oscillator frequency = 200kHz
(V
V
1.5A
(f
V
IN
OUT
2
OSC
IN
– V
= 10.8A
)(L)
OUT
= 1.5A
U
)
DC
INFORMATION
U
P–P
W
U
feed such a load directly. 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
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
(3 months) lifetime; further derating of the input capacitor
ripple current beyond the manufacturer’s specification is
recommended to extend the useful life of the circuit.
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 a fraction of the total load current. Output
capacitor duty places a premium not on power dissipation
but on low ESR. During an output load transient, the
output capacitor must supply all of the additional load
current demanded by the load until the LTC1649 can
adjust the inductor current to the new value. ESR in the
output capacitor results in a step in the output voltage
equal to the ESR value multiplied by the change in load
current. A 5A load step with a 0.05 ESR output capacitor
will result in a 250mV output voltage shift; this is a 10%
output voltage shift for a 2.5V 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 to control steady-state
output ripple.
Electrolytic capacitors rated for use in switching power
supplies with specified ripple current ratings and ESR can
be used effectively in LTC1649 applications. OS-CON
electrolytic capacitors from Sanyo give excellent perfor-
mance and have a very high performance/size ratio for an
electrolytic capacitor. 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 tantalum capaci-
OUT
/2. A low ESR input capacitor with an
LTC1649
9