LTC3828_15 Datasheet, PDF (22/32 Pages) Linear Technology – Dual, 2-Phase Step-Down Controller with Tracking

LTC3828
APPLICATIONS INFORMATION
The resistive load reduces the DC loop gain while main-
taining the linear control range of the error amplifier.
The maximum output voltage deviation can theoretically
be reduced to half or alternatively the amount of output
capacitance can be reduced for a particular application. A
complete explanation is included in Design Solutions 10.
(See www.linear.com)
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. Percent efficiency can
be expressed as:
%Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, four main sources usually account for most of the
losses in LTC3828 circuits: 1) IC V_IN current, 2) INTV_CC
regulator current, 3) I²R losses, 4) Topside MOSFET
transition losses.
1. The V_IN current has two components: the first is the DC
supply current given in the Electrical Characteristics table,
which excludes MOSFET driver and control currents; V_IN
current typically results in a small (<0.1%) loss.
2. INTV_CC current is the sum of the MOSFET driver and
control currents. The MOSFET driver current results from
switching the gate capacitance of the power MOSFETs.
Each time a MOSFET gate is switched from low to high
to low again, a packet of charge dQ moves from INTV_CC
to ground. The resulting dQ/dt is a current out of INTV_CC
that is typically much larger than the control circuit current.
In continuous mode, I_GATECHG =f(Q_T + Q_B), where Q_T and
Q_B are the gate charges of the topside and bottom side
MOSFETs.
3. I²R losses are predicted from the DC resistances of
the fuse (if used), MOSFET, inductor, current sense resis-
tor, and input and output capacitor ESR. In continuous
mode the average output current flows through L and
R_SENSE, but is “chopped” between the topside MOSFET
and the synchronous MOSFET. 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, R_SENSE and ESR to obtain I²R losses. For example, if
each R_DS(ON) = 30mΩ, R_L = 50mΩ, R_SENSE = 10mΩ and
R_ESR = 40mΩ (sum of both input and output capacitance
losses), then the total resistance is 130mΩ. This results
in losses ranging from 3% to 13% as the output current
increases from 1A to 5A for a 5V output, or a 4% to 20%
loss for a 3.3V output. Efficiency varies as the inverse
square of V_OUT for the same external components and
output power level. The combined effects of increasingly
lower output voltages and higher currents required by
high performance digital systems is not doubling but
quadrupling the importance of loss terms in the switching
regulator system!
4. Transition losses apply only to the topside MOSFET(s),
and become significant only when operating at high input
voltages (typically 15V or greater). Transition losses can
be estimated from:
( ) ( ) ^{Transition Loss =}
V_IN
2
•
⎛
⎝⎜
I_MAX
2
⎞
⎠⎟
R_DR
•
( )( ) ^CMILLER
f
⎛
⎝⎜
5V
1
– V_TH
+
1^⎞
V_TH ⎠⎟
Other “hidden” losses such as copper trace and internal
battery resistances can account for an additional 5% to
10% efficiency degradation in portable systems. It is very
important to include these “system” level losses during
the design phase. The internal battery and fuse resistance
losses can be minimized by making sure that C_IN has ad-
equate charge storage and very low ESR at the switching
frequency. A 25W supply will typically require a minimum of
20μF to 40μF of capacitance having a maximum of 20mΩ to
50mΩ of ESR. The LTC3828 2-phase architecture typically
halves this input capacitance requirement over competing
solutions. Other losses including Schottky conduction
losses during dead-time and inductor core losses generally
account for less than 2% total additional loss.
3828fc
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