LT3710
U
W U U
APPLICATIO S I FOR ATIO
Output Inductor Selection
The RDS(ON) of the MOSFETs should be selected to deliver
the required current at the desired efficiency as well as to
meet the thermal requirement of the MOSFET package.
The conduction power losses of the MOSFETs are:
The key parameters for choosing the inductor include
inductance, RMS and saturation current ratings and DCR.
The inductance must be selected to achieve a reasonable
value of ripple current, which is determined by:
PM1 IO2 • RDS(ON)M1 • D2
PM2 IO2 • RDS(ON)M2 • (1 – D2)
VOUT2 • 1− D2
(
)
∆IL =
whereIO isthemaximumoutputcurrentofLT3710circuit,
RDS(ON)M1 andRDS(ON)M2 aretheon-resistanceforthetop
and bottom MOSFETs, respectively. The RDS(ON) must be
determined with 6.5V gate drive and the expected operat-
ing temperature.
f •L
Typically, the inductor ripple current is designed to be
20% to 40% of the maximum output current.
TheRMScurrentratingmustbehighenoughtodeliverthe
maximum output current. A sufficient saturation current
rating should prevent the inductor core from saturating.
These two current ratings can be determined by:
A good number of high performance power MOSFET
selections are available from Siliconix, International Rec-
tifier and Fairchild. If the VDSS and RDS(ON) ratings are the
same,theMOSFETswiththelowestgatechargeQG should
be chosen to minimize the power loss associated with the
MOSFET gate drives, the switching transitions and the
controller bias supply.
2
∆ILMAX
2
IRMS ≥ IO
+
12
∆ILMAX
ISAT ≥ IO +
2
Output Capacitor Selection
The selection of the output capacitor is determined by the
output ripple and load transient requirements. In low
output voltage applications, always choose capacitors
with low ESR. The output ripple voltage is approximated
by:
where IO is the maximum output current and ∆ILMAX is the
maximum peak-to-peak inductor ripple current.
To optimize the efficiency, we usually choose the inductor
with the minimum DCR if the inductance and current
ratings are the same.
1
∆VOUT ≈ ∆IL ESR +
8fCOUT
Power MOSFET Selection
The LT3710 drives two external N-channel MOSFETs to
deliver high currents at high efficiency. The gate drive
voltage is typically 6.5V. The key parameters for choos-
ing MOSFETs include drain to source voltage rating VDSS
and RDS(ON) at 6.5V gate drive. Note that the transformer
secondary voltage waveform will overshoot at its rising
edge due to the ringing between transformer leakage
inductance and parasitic capacitance. The VDSS of both
top and bottom MOSFETs must be sufficiently higher
than the maximum overshoot. It is recommended that an
RC snubber or a voltage clamping circuitry be placed
across the transformer secondary winding to limit the VS
overshoot.
where ∆IL is the inductor peak-to-peak ripple current.
A partial list of low ESR high performance capacitor types
includesSPcapacitorsfromPanasonicandCornellDubilier,
POSCAPs and OS-CON capacitors from Sanyo, T510 and
T520 surface mount capacitors from Kemet.
Design Example
Figure 3 shows an application example for the LT3710. It
is a dual output, high efficiency, isolated DC/DC power
supply with 36V to 72V input, 3.3V/10A and 1.8V/10A
outputs. The basic power stage topology is a 2-transistor
3710f
9
Linear [ Linear ]
