LTC3812-5_15 Datasheet, PDF (27/34 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3812-5
APPLICATIONS INFORMATION
EFFICIENCY CONSIDERATIONS
The percent efficiency of a switching regulator is equal to
the output power divided by the input power times 100%.
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
account for most of the losses in LTC3812-5 circuits:
1. DC I²R losses. These arise from the resistances of the
MOSFETs, inductor and PC board traces and cause the
efficiency to drop at high output currents. In continuous
mode the average output current flows through L, but is
chopped between the top and bottom MOSFETs. If the two
MOSFETs have approximately the same R_DS(ON), then
the resistance of one MOSFET can simply be summed
with the resistances of L and the board traces to obtain
the DC I²R loss. For example, if R_DS(ON) = 0.01Ω and
R_L = 0.005Ω, the loss will range from 15mW to 1.5W
as the output current varies from 1A to 10A.
2. Transition loss. This loss arises from the brief amount
of time the top MOSFET spends in the saturated region
during switch node transitions. It depends upon the
input voltage, load current, driver strength and MOSFET
capacitance, among other factors. The loss is significant
at input voltages above 20V and can be estimated from
the second term of the P_MAIN equation found in the Power
MOSFET Selection section. When transition losses are
significant, efficiency can be improved by lowering the
frequency and/or using a top MOSFET(s) with lower
C_RSS at the expense of higher R_DS(ON).
3. INTV_CC current. This is the sum of the MOSFET
driver and control currents. Control current is typically
about 3mA and driver current can be calculated by:
I_GATE = f(Q_G(TOP) + Q_G(BOT)), where Q_G(TOP) and Q_G(BOT)
are the gate charges of the top and bottom MOSFETs.
This loss is proportional to the supply voltage that
INTV_CC is derived from, i.e., V_IN for the external NMOS
linear regulator, V_OUT for the internal EXTV_CC regula-
tor, or V_EXT when an external supply is connected to
INTV_CC.
4. C_IN loss. The input capacitor has the difficult job of fil-
tering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I²R loss
and sufficient capacitance to prevent the RMS current
from causing additional upstream losses in fuses or
batteries.
Other losses, including C_OUT ESR loss, Schottky diode D1
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss. When
making adjustments to improve efficiency, the input cur-
rent is the best indicator of changes in efficiency. If you
make a change and the input current decreases, then the
efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
CHECKING TRANSIENT RESPONSE
The regulator loop response can be checked by looking
at the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
load step occurs, V_OUT immediately shifts by an amount
equal to ∆I_LOAD (ESR), where ESR is the effective series
resistance of C_OUT. ∆I_LOAD also begins to charge or dis-
charge C_OUT generating a feedback error signal used by the
regulator to return V_OUT to its steady-state value. During
this recovery time, V_OUT can be monitored for overshoot
or ringing that would indicate a stability problem.
DESIGN EXAMPLE
As a design example, take a supply with the following
specifications: V_IN = 12V to 60V, V_OUT = 5V ±5%, I_OUT(MAX)
= 6A, f = 250kHz. First, calculate the timing resistor:
R_ON
=
2.4V
•
5V
250kHz
•
76pF
=
110k
and choose the inductor for about 40% ripple current at
the maximum V_IN:
L
=
5V
250kHz • 0.4
•
6A


1

5V
60V


=
7.6μH
With a 7.7μH inductor, ripple current will vary from 1.5A
to 2.4A (25% to 40%) over the input supply range.
Next, choose the bottom MOSFET switch. Since the
drain of the MOSFET will see the full supply voltage 60V
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