sp7656en2-l-tr Exar Corporation, sp7656en2-l-tr Datasheet - Page 7

• Manufacturer Part Number: sp7656en2-l-tr
• Description: 600khz High Input Voltage, Internally Compensated Sp7656 Non-synchronous Step-down Dc/dc Voltage Regulator
• Manufacturer: Exar Corporation

Inductor Selection
Select the Inductor for inductance L and
saturation current I
inductor with I
overcurrent level.
The inductance can be calculated from:
where:
V
V
f
I
(nominally set to 30% of I
Keep in mind that a higher inductor ripple
current results in a smaller inductance – and
smaller inductor. A smaller inductor has the
advantages of small size, low DC equivalent
resistance DCR, high saturation current and
allows the use of a lower output capacitance
to meet a given step load transient. A higher
inductor
disadvantages. It increases the output voltage
ripple and increases the current at which
converter enters Discontinuous Conduction
Mode. The output current at which converter
enters DCM is ½ of I
current step load that drives the converter into
DCM will result in a large output voltage
transient. Therefore the lowest current for a
step load should be larger than ½ of I
Restriction on high duty cycle operation
The SP7656 is optimized to provide superior
performance for low duty cycle applications.
For applications with output voltages below
9V, the device will operate normally at the
expected 600kHz switching frequency for
conversions with less than 50% duty cycle.
For applications with output voltages below 9V
and greater than 50% duty cycle, the device
will enter into a pulse skipping mode. This is
due to the FB voltage to internal ramp voltage
ratio of the device, and is an intended
behavior from an architecture optimized for
superior performance in low duty cycle
conversion applications and results in a small
increase in output ripple voltage for any given
circuit. For output voltages above 9V, the
device will operate at a constant 600kHz
switching frequency across the specified duty
cycle range up to 100%.
2/19/08 SP7656 PowerBlox
RIP
IN
OUT
is the switching frequency (300kHz)
L
is converter input voltage
is inductor peak-to-peak current ripple

is converter output voltage

Vin
ripple

SAT
Vout
higher than the programmed
SAT
current




RIP
. Make sure to select an
Vout
Vin
. Note that a negative
OUT



level
)

 

1
f

 

also

 

RIP
Irip
1
.

 

has
7
Layout Suggestions
i)
ii)
iii)
iv)
v)
For more detail on the SP7656 layout see the
SP7656EVB
available on our web site. Each layer is shown
in detail as well as a complete bill of materials
and
Cin
Place the input capacitor(s) as close
as possible to the 7656 IC.
Create a pad under the IC that
connects the power pad (pin 9) to the
inductor. Duplicate this pad through
the pcb layers if present, and on the
bottom side of the PCB. Use multiple
vias to connect these layers to aid in
heat dissipation. Do not oversize this
pad - since the LX node is subjected
to very high dv/dt voltages, the stray
capacitance formed between these
islands and the surrounding circuitry
will tend to couple switching noise
Connect the Schottky diode cathode
as close as possible to the LX node
and inductor input side. Connect the
anode to a large diameter trace or a
copper area that connects the input
ground to the output ground.
The output capacitors should be
placed as close to the load as
possible. Use short wide copper
regions to connect output capacitors
to load to avoid inductance and
resistances.
Keep other sensitive circuits and
traces away from the LX node in
particular and away from the power
supply completely if possible.
performance
(Evaluation
7656
Copyright 2007 EXAR Corporation
Lout
Diode
Board)
characterization.
Cout
Manual