LTC3731HG Linear Technology, LTC3731HG Datasheet - Page 16

• Manufacturer Part Number: LTC3731HG
• Description: IC REG SW SYNC 3PH STPDWN 36SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3731HG

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 6 V
• Frequency - Switching: 225kHz ~ 680kHz
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: -40°C ~ 140°C
• Mounting Type: Surface Mount
• Package / Case: 36-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output: -
• Power - Output: -
• Lead Free Status / Rohs Status: Not Compliant

LTC3731H
applicaTions inForMaTion
reduced by the factor, N, due to the effective increase in
the frequency of the current pulses.
Ceramic capacitors are becoming very popular for small
designs but several cautions should be observed. “X7R”,
“X5R” and “Y5V” are examples of a few of the ceramic
materials used as the dielectric layer, and these different
dielectrics have very different effect on the capacitance
value due to the voltage and temperature conditions ap-
plied. Physically, if the capacitance value changes due to
applied voltage change, there is a concommitant piezo
effect which results in radiating sound! A load that draws
varying current at an audible rate may cause an attendant
varying input voltage on a ceramic capacitor, resulting in
an audible signal. A secondary issue relates to the energy
flowing back into a ceramic capacitor whose capacitance
value is being reduced by the increasing charge. The volt-
age can increase at a considerably higher rate than the
constant current being supplied because the capacitance
value is decreasing as the voltage is increasing! Never-
theless, ceramic capacitors, when properly selected and
used, can provide the lowest overall loss due to their
extremely low ESR.
The selection of C
series resistance (ESR). Typically once the ESR requirement
is satisfied the capacitance is adequate for filtering. The
steady-state output ripple (∆V
where f = operating frequency of each stage, N is the
number of output stages, C
∆I
highest at maximum input voltage since ∆I

L
∆
= ripple current in each inductor. The output ripple is
V
OUT
≈
∆
I
RIPPLE
OUT



is driven by the required effective
ESR
OUT
+
OUT
8
NfC
= output capacitance and
) is determined by:
1
OUT



L
increases with
input voltage. The output ripple will be less than 50mV at
max V
and
The emergence of very low ESR capacitors in small, surface
mount packages makes very small physical implementa-
tions possible. The ability to externally compensate the
switching regulator loop using the I
wider selection of output capacitor types. The impedance
characteristics of each capacitor type is significantly differ-
ent than an ideal capacitor and therefore requires accurate
modeling or bench evaluation during design.
Manufacturers such as Nichicon, Nippon Chemi-Con and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo and the Panasonic SP
surface mount types have a good (ESR)(size) product.
Once the ESR requirement for C
RMS current rating generally far exceeds the I
requirement. Ceramic capacitors from AVX, Taiyo Yuden,
Murata and Tokin offer high capacitance value and very
low ESR, especially applicable for low output voltage
applications.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum
electrolytic and dry tantalum capacitors are both available
in surface mount configurations. New special polymer
surface mount capacitors offer very low ESR also but
have much lower capacitive density per unit volume. In
the case of tantalum, it is critical that the capacitors are
surge tested for use in switching power supplies. Several
excellent choices are the AVX TPS, AVX TPSV, the KEMET
T510 series of surface-mount tantalums or the Panasonic
SP series of surface mount special polymer capacitors
C
C
OUT
OUT
IN
required ESR < N • R
> 1/(8Nf)(R
with ∆I
L
= 0.4I
SENSE
OUT(MAX)
)
SENSE
assuming:
OUT
TH
has been met, the
pin allows a much
RIPPLE(P-P)
3731Hfb