Datasheet

LT3988
14
3988f
applicaTions inForMaTion
Input Capacitor Selection
Bypass the input of the LT3988 circuit with a 4.7μF or higher
ceramic capacitor of X7R or X5R type. A lower value or
a less expensive Y5V type will work if there is additional
bypassing provided by bulk electrolytic capacitors, or if the
input source impedance is low. The following paragraphs
describe the input capacitor considerations in more detail.
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input ca-
pacitor is required to reduce the resulting voltage ripple at
the LT3988 input and to force this switching current into a
tight local loop, minimizing EMI. The input capacitor must
have low impedance at the switching frequency to do this
effectively and it must have an adequate ripple current rat-
ing. With two switchers operating at the same frequency
but with different phases and duty cycles, calculating the
input capacitor RMS current is not simple; however, a
conservative value is the RMS input current for the phase
delivering the most power (V
OUT
• I
OUT
):
I
IN(RMS)
= I
OUT
•
V
OUT
V
IN
– V
OUT
( )
V
IN
<
I
OUT
2
and is largest when V
IN
= 2V
OUT
(50% duty cycle). As
the second, lower power channel draws input current,
the input capacitor’s RMS current actually decreases as
the out-of-phase current cancels the current drawn by the
higher power channel. Considering that the maximum load
current from a single phase (if SW1 and SW2 are both at
maximum current) is ~1A, RMS ripple current will always
be less than 0.5A.
The high frequency of the LT3988 reduces the energy
storage requirements of the input capacitor, so that the
capacitance required is often less than 10μF. The combi-
nation of small size and low impedance (low equivalent
series resistance or ESR) of ceramic capacitors makes
them the preferred choice. The low ESR results in very
low voltage ripple. Ceramic capacitors can handle larger
magnitudes of ripple current than other capacitor types
of the same value.
An alternative to a high value ceramic capacitor is a lower
value along with a larger electrolytic capacitor, for example
a 1μF ceramic capacitor in parallel with a low ESR tantalum
capacitor. For the electrolytic capacitor, a value larger than
10μF will be required to meet the ESR and ripple current
requirements. Because the input capacitor is likely to see
high surge currents when the input source is applied, tan-
talum capacitors should be surge rated. The manufacturer
may also recommend operation below the rated voltage
of the capacitor. Be sure to place the 1μF ceramic as close
as possible to the V
IN
and GND pins on the IC for optimal
noise immunity.
A final caution is in order regarding the use of ceramic
capacitors at the input. A ceramic input capacitor can
combine with stray inductance to form a resonant tank
circuit. If power is applied quickly (for example by plugging
the circuit into a live power source), this tank can ring,
doubling the input voltage and damaging the LT3988. The
solution is to either clamp the input voltage or dampen the
tank circuit by adding a lossy capacitor in parallel with the
ceramic capacitor. For details, see Application Note 88.
Frequency Compensation
The LT3988 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT3988 does not depend on the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. The LT3988
is internally compensated with the RC network tied to the
VC node. The internal compensation network is optimized
to provide stability over the full frequency range. Figure 5
shows an equivalent circuit for the LT3988 control loop.
The error amplifier is a transconductance amplifier with
0.75V
LT3988
3988 F05
R1
OUT
R
ESR
C
C
40pF
R
C
300k
V
C
7M
ERROR
AMPLIFIER
FB
R2
C
OUT
CURRENT MODE
POWER STAGE
C
PL
g
m
= 2A/V
g
m
= 40µA/V
Figure 5. Model For Loop Response