Datasheet

LT3957A
20
3957afa
APPLICATIONS INFORMATION
The ESR can be minimized by using high quality X5R or
X7R dielectric ceramic capacitors. In many applications,
ceramic capacitors are sufficient to limit the output volt-
age ripple.
The RMS ripple current rating of the output capacitor
needs to be greater than:
I
RMS(COUT)
> 0.3 • ΔI
L2
Inverting Converter: Selecting the DC Coupling Capacitor
The DC voltage rating of the DC coupling capacitor (C
DC
,
as shown in Figure 10) should be larger than the maximum
input voltage minus the output voltage (negative voltage):
V
CDC
> V
IN(MAX)
– V
OUT
C
DC
has nearly a rectangular current waveform. During
the switch off-time, the current through C
DC
is I
IN
, while
approximately –I
O
flows during the on-time. The RMS
rating of the coupling capacitor is determined by the fol-
lowing equation:
I
RMS(CDC)
>I
O(MAX)
D
MAX
1D
MAX
A low ESR and ESL, X5R or X7R ceramic capacitor works
well for C
DC
.
Board Layout
The high power and high speed operation of the LT3957A
demands careful attention to board layout and component
placement. Careful attention must be paid to the internal
power dissipation of the LT3957A at high input voltages,
high switching frequencies, and high internal power switch
currents to ensure that a junction temperature of 125°C is
not exceeded. This is especially important when operat-
ing at high ambient temperatures. Exposed pads on the
bottom of the package are SGND and SW terminals of the
IC, and must be soldered to a SGND ground plane and a
SW plane respectively. It is recommended that multiple
vias in the printed circuit board be used to conduct heat
away from the IC and into the copper planes with as much
area as possible.
To prevent radiation and high frequency resonance prob-
lems, proper layout of the components connected to the
Figure 8. A Simplified Inverting Converter
C
DC
V
IN
C
IN
L1
D1
C
OUT
V
OUT
3757A F08
+
GND
LT3957A
SW
L2
+
+
+
Inverting Converter: Switch Duty Cycle and Frequency
For an inverting converter operating in CCM, the duty
cycle of the main switch can be calculated based on the
negative output voltage (V
OUT
) and the input voltage (V
IN
).
The maximum duty cycle (D
MAX
) occurs when the converter
has the minimum input voltage:
D
MAX
=
V
OUT
V
D
V
OUT
V
D
V
IN(MIN)
Inverting Converter: Output Diode and Input Capacitor
Selections
The selections of the inductor, output diode and input
capacitor of an inverting converter are similar to those of
the SEPIC converter. Please refer to the corresponding
SEPIC converter sections.
Inverting Converter: Output Capacitor Selection
The inverting converter requires much smaller output
capacitors than those of the boost, flyback and SEPIC
converters for similar output ripples. This is due to the fact
that, in the inverting converter, the inductor L2 is in series
with the output, and the ripple current flowing through the
output capacitors are continuous. The output ripple voltage
is produced by the ripple current of L2 flowing through
the ESR and bulk capacitance of the output capacitor:
ΔV
OUT(PP)
I
L2
•ESR
COUT
+
1
8•ƒ•C
OUT
After specifying the maximum output ripple, the user can
select the output capacitors according to the preceding
equation.