LT3782
14
3782fg
For a given V
IN
and V
OUT
, we can calculate the duty cycle D
and then derive the output RMS ripple current from Figure
6. After choosing output capacitors with suffi cient RMS
ripple current rating, we also need to consider the ESR
requirement if electrolytic caps, tantulum caps, POSCAPs
or SP CAPs are selected. Given the required output ripple
voltage spec ΔV
OUT
(in RMS value) and the calculated RMS
ripple current ΔI
OUT
, one can estimate the ESR value of
the output capacitor to be
ESR≤
ΔV
OUT
ΔI
OUT
External Regulator to Bias Gate Drivers
For applications with V
IN
higher than 24V, the IC temperature
may get too high. To reduce heat, an external regulator
between 12V to 14V should be used to override the internal
V
GBIAS
regulator to supply the current needed for BGATE1
and BGATE2 (see Figure 7).
Effi ciency Considerations
The effi ciency of a switching regulator is equal to the out-
put power divided by the input power (¥100%). Percent
effi ciency can be expressed as:
% Effi ciency = 100% – (L1 + L2 + L3 + …),
where L1, L2, etc. are the individual loss components
as a percentage of the input power. It is often useful to
analyze individual losses to determine what is limiting
the effi ciency and which change would produce the most
improvement. Although all dissipative elements in the
circuit produce losses, four main sources usually ac-
count for the majority of the losses in LT3782 application
circuits:
1. The supply current into V
IN
. The V
IN
current is the sum
of the DC supply current I
Q
(given in the Electrical Char-
acteristics) and the MOSFET driver and control currents.
The DC supply current into the V
IN
pin is typically about
7mA and represents a small power loss (much less
than 1%) that increases with V
IN
. The driver current
results from switching the gate capacitance of the power
MOSFET; this current is typically much larger than the
DC current. Each time the MOSFET is switched on and
then off, a packet of gate charge Q
G
is transferred from
GBIAS to ground. The resulting dQ/dt is a current that
must be supplied to the GBIAS capacitor through the
V
IN
pin by an external supply. In normal operation:
I
Q(TOT)
≈ I
Q
= f • Q
G
P
IC
= V
IN
• (I
Q
+ f • Q
G
)
Figure 7
3782 F07
2μF
12V
+
GBIAS
GBIAS1
GBIAS2
LT3782
APPLICATIONS INFORMATION