ltc3612 Linear Technology Corporation, ltc3612 Datasheet - Page 16

• Manufacturer Part Number: ltc3612
• Description: 3a, 4mhz Monolithic Synchronous Step-down Dc/dc Converter
• Manufacturer: Linear Technology Corporation

LTC3612
APPLICATIONS INFORMATION
where f
tance and ΔI
ripple is highest at maximum input voltage since ΔI
increases with input voltage.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or RMS
current handling requirement of the application. Aluminum
electrolytic, special polymer, ceramic and dry tantalum
capacitors are all available in surface mount packages.
Tantalum capacitors have the highest capacitance density,
but can have higher ESR and must be surge tested for
use in switching power supplies. Aluminum electrolytic
capacitors have signifi cantly higher ESR, but can often
be used in extremely cost-sensitive applications provided
that consideration is given to ripple current ratings and
long-term reliability.
Ceramic Input and Output Capacitors
Ceramic capacitors have the lowest ESR and can be cost
effective, but also have the lowest capacitance density,
high voltage and temperature coeffi cients, and exhibit
audible piezoelectric effects. In addition, the high Q of
ceramic capacitors along with trace inductance can lead
to signifi cant ringing.
They are attractive for switching regulator use because
of their very low ESR, but great care must be taken when
using only ceramic input and output capacitors.
Ceramic capacitors are prone to temperature effects
which require the designer to check loop stability over
the operating temperature range. To minimize their large
temperature and voltage coeffi cients, only X5R or X7R
ceramic capacitors should be used.
When a ceramic capacitor is used at the input and the power
is being supplied through long wires, such as from a wall
adapter, a load step at the output can induce ringing at the
V
be mistaken as loop instability. At worst, the ringing at the
input can be large enough to damage the part.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfi ll a charge storage
requirement. During a load step, the output capacitor
16
IN
pin. At best, this ringing can couple to the output and
OSC
= operating frequency, C
L
= ripple current in the inductor. The output
OUT
= output capaci-
L
must instantaneously supply the current to support the
load until the feedback loop raises the switch current
enough to support the load. The time required for the
feedback loop to respond is dependent on the compensa-
tion components and the output capacitor size. Typically,
3 to 4 cycles are required to respond to a load step, but
only in the fi rst cycle does the output drop linearly. The
output droop, V
linear drop of the fi rst cycle; however, this behavior can
vary depending on the compensation component values.
Thus, a good place to start is with the output capacitor
size of approximately:
This is only an approximation; more capacitance may
be needed depending on the duty cycle and load step
requirements.
In most applications, the input capacitor is merely required
to supply high frequency bypassing, since the impedance
to the supply is very low.
Output Voltage Programming
The output voltage is set by an external resistive divider
according to the following equation:
The resistive divider allows pin V
the output voltage as shown in Figure 1.
Burst Clamp Programming
If the voltage on the MODE pin is less than 0.8V, Burst
Mode operation is enabled.
If the voltage on the MODE pin is less than 0.3V, the internal
default burst clamp level is selected. The minimum voltage
on the ITH pin is typically 525mV (internal clamp).
If the voltage is between 0.45V and 0.8V, the voltage on
the MODE pin (V
on the ITH pin (external clamp) and determines the burst
clamp level I
C
V
OUT
OUT
≈
= 0.6 • 1+
f
SW
3.5 • ΔI
BURST
• V
⎛
⎝ ⎜
DROOP
BURST
DROOP
(typically from 0A to 3.5A).
OUT
R2
R1
, is usually about 2 to 4 times the
) is equal to the minimum voltage
⎞
⎠ ⎟
V
FB
to sense a fraction of
3612f