LT3782A Datasheet, PDF (14/20 Pages) Linear Technology – 2-Phase Step-Up DC/DC Controller

LT3782A
APPLICATIONS INFORMATION
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 sufficient 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 ≤ ^ΔVOUT
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).
Efficiency Considerations
The efficiency of a switching regulator is equal to the out-
put power divided by the input power (Â¥100%). Percent
efficiency can be expressed as:
% Efficiency = 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 efficiency and which change would produce the most
improvement. Although all dissipative elements in the
circuit produce losses, four main sources usually account
for the majority of the losses in LT3782A 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)
2. Power MOSFET switching and conduction losses:
P_FET


=^
I_O(MAX)
2
^{1– DMAX}
²



•
R_DS(ON)
•
D_MAX
•

_T
I_O(MAX)
+k
•
V_O2
•
2
1– D_MAX
• C_RSS
•
f
LT3782A _GBIAS
GBIAS1
+
12V
GBIAS2
3782A F07
2μF
Figure 7
3782af
14