LTC3828_15 Datasheet, PDF (18/32 Pages) Linear Technology – Dual, 2-Phase Step-Down Controller with Tracking

LTC3828
APPLICATIONS INFORMATION
an ideal candidate for highest efficiency battery operated
systems. Also consider parallel ceramic and high quality
electrolytic capacitors as an effective means of achieving
ESR and bulk capacitance goals.
In continuous mode, the source current of the top N-chan-
nel MOSFET is a square wave of duty cycle V_OUT/V_IN. To
prevent large voltage transients, a low ESR input capaci-
tor sized for the maximum RMS current of one channel
must be used. The maximum RMS capacitor current is
given by:
[ ^{( )}]^1/2
V_OUT V_IN − V_OUT
C_INRequiredI_RMS ≈ I_MAX
V_IN
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS = I_OUT/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on only 2000 hours
of life. This makes it advisable to further derate the capaci-
tor, or to choose a capacitor rated at a higher temperature
than required. Several capacitors may also be paralleled
to meet size or height requirements in the design. Always
consult the manufacturer if there is any question.
The benefit of the LTC3828 multiphase clocking can be
calculated by using the equation above for the higher
power controller and then calculating the loss that would
have resulted if both controller channels switched on at
the same time. The total RMS power lost is lower when
both controllers are operating due to the interleaving of
current pulses through the input capacitor’s ESR. This is
why the input capacitor’s requirement calculated above for
the worst-case controller is adequate for the dual controller
design. Remember that input protection fuse resistance,
battery resistance and PC board trace resistance losses are
also reduced due to the reduced peak currents in a multi-
phase system. The overall benefit of a multiphase design
will only be fully realized when the source impedance of
the power supply/battery is included in the efficiency test-
ing. The drains of the two top MOSFETS should be placed
within 1cm of each other and share a common C_IN(s).
Separating the drains and C_IN may produce undesirable
voltage and current resonances at V_IN.
18
The selection of C_OUT is driven by the required output
voltage ripple and load transient response. Both the ca-
pacitor effective series resistance (ESR) and capacitance
determine the output ripple:
ΔV_OUT
≈
ΔI_L
•
⎛
⎝⎜
ESR
+
1⎞
8fC_OUT ⎠⎟
where f = operating frequency, C_OUT = output capacitance
and ΔI_L = ripple current in the inductor. The output ripple
is highest at maximum input voltage since ΔI_L increases
with input voltage.
Usually, ceramic capacitors are used to minimize the output
voltage ripple because of their ultralow ESR. Currently,
multilayer ceramic capacitors have capacitor values up to
hundreds of μF. However, the capacitance of the ceramic
capacitors usually decreases with increased DC bias volt-
age and ambient temperature. In general, X5R or X7R type
capacitors are recommended for high performance solu-
tions. The OPTI-LOOP current mode control of LTC3828
provides stable, high performance transient response
even with all ceramic output capacitors. Manufactures
such as TDK, Taiyo Yuden, Murata and AVX provide high
performance ceramic capacitors.
When high capacitance is needed, especially for load
transient requirement, low ESR polymerized electrolytic
capacitors such as Sanyo POSCAP or Panasonic SP capaci-
tor can be used in parallel with ceramic capacitors. Other
high performance electolytic capacitor manufacturers
include AVX, KEMET and NEC. With LTC3828, a com-
bination of ceramic and low ESR electrolytic capacitors
can provide a low ripple, fast transient, high density and
cost-effective solution. Consult manufacturers for specific
recommendations.
INTV_CC Regulator
An internal P-channel low dropout regulator produces 5V
at the INTV_CC pin from the V_IN supply pin. INTV_CC powers
the drivers and internal circuitry within the IC. The INTV_CC
pin regulator can supply a peak current of 50mA and must
be bypassed to ground with a minimum of 4.7μF tantalum,
10μF special polymer, or low ESR type electrolytic capaci-
tor. A 1μF ceramic capacitor placed directly adjacent to the
INTV_CC and PGND IC pins is highly recommended. Good
3828fc