Datasheet

LTC3735
16
3735fa
APPLICATIONS INFORMATION
Linear Technology Application Note 77. Assuming induc-
tors are selected to have same ripple percentage for both
1-phase and 2-phase configurations, Figure 5 shows the
reduction of output ripple current by 2-phase operation.
Not only the ripple amplitude is more than halved, but
the ripple frequency is also doubled. Compared with the
output voltage ripple for 1-phase:
∆V
OUT
≈ ∆I
RIPPLE
ESR+
1
8 • f •C
OUT






∆V
OUT
of 2-phase is less than 50% of that of 1-phase, given
the same output capacitor ESRs. Or, to have same ∆V
OUT
2-phase only need half the number of output capacitors
that are needed in 1-phase.
The output ripple varies with input voltage since ∆I
L
is a
function of input voltage. The output ripple will be less than
±25mV at max V
IN
with ∆I
L
= 0.4I
OUT(MAX)
/2 assuming:
C
OUT
required ESR < 4(R
SENSE
) and
C
OUT
> 1/(16f)(R
SENSE
)
The LTC3735 employs OPTI-LOOP technique to compen-
sate the switching regulator loop with external components
(through I
TH
pin). OPTI-LOOP compensation speeds
up regulator’s transient response, minimizes output
capacitance and effectively removes constraints on output
capacitor ESR. It opens a much wider selection of output
capacitor types and a variety of capacitor manufactures are
available for high current, low voltage switching regulators.
Manufacturers such as Nichicon, United Chemicon
and Sanyo should be considered for high performance
through-hole capacitors. The OS-CON semiconductor
dielectric capacitor available from Sanyo has the lowest
(ESR)(size) product of any aluminum electrolytic at a
somewhat higher price. An additional ceramic capacitor
in parallel with OS-CON type capacitors is recommended
to reduce the inductance effects.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. New special polymer (SP)
surface mount capacitors from Panasonic offer very low
ESR also but have much lower capacitive density per unit
volume. In the case of tantalum, it is critical that the capaci-
tors are surge tested for use in switching power supplies.
Several excellent choices are the AVX TPS, AVX TPSV or the
KEMET T510 series of surface mount tantalums, available
in case heights ranging from 2mm to 4mm. Other capacitor
types include Sanyo OS-CON, POSCAPs, Kemet AO-CAPs,
Nichicon PL series and Sprague 595D series. Consult
the manufacturer for other specific recommendations. A
combination of capacitors will often result in maximizing
performance and minimizing overall cost and size.
PV
CC
Decoupling
The PV
CC
pin supplies power to the top and bottom gate
drivers and therefore must be bypassed to power ground
with a minimum of 4.7µF ceramic or tantalum capacitor.
Since the gate driving currents are of high amplitude and
high slew rate, this bypassing capacitor should be placed
very close to the PV
CC
and PGND pins to minimize the
parasitic inductance. Do NOT apply greater than 7V to
the PV
CC
pin.
The PV
CC
pin also supplies current to the internal control
circuitry of the LTC3735. This supply current is much
lower
than that of the current for the external MOSFET
gate drive. Ceramic capacitors are very good for high
frequency filtering and a 0.1µF ~ 1µF ceramic capacitor
should be placed adjacent to the PV
CC
and SGND pins.
DUTY FACTOR (V
OUT
/V
IN
)
0.1
30
40
50
0.4 0.6 0.9
3735 F05
20
10
0
0.2 0.3
0.5
0.7 0.8
∆I
RIPPLE
OF 2-PHASE
∆I
RIPPLE
OF 1-PHASE
• 100%
Figure 5. Output Ripple Current Reduction
of 2-Phase Over Single Phase