LTC1709-8_1 Datasheet, PDF (21/28 Pages) Linear Technology – 2-Phase, 5-Bit VID,Current Mode, High Efficiency,Synchronous Step-Down Switching Regulators

LTC1709-8/LTC1709-9
APPLICATIO S I FOR ATIO
1) I²R losses are predicted from the DC resistances of the
fuse (if used), MOSFET, inductor, current sense resistor,
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 approxi-
mately 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) = 10mΩ, R_L = 10mΩ, and R_SENSE = 5mΩ, then the
total resistance is 25mΩ. This results in losses ranging
from 2% to 8% as the output current increases from 3A to
15A per output stage for a 5V output, or a 3% to 12% loss
per output stage 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 increas-
ingly 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!
2) Transition losses apply only to the topside MOSFET(s),
and are significant only when operating at high input
voltages (typically 12V or greater). Transition losses can
be estimated from:
Transition Loss = (1.7) V_IN2 I_O(MAX) C_RSS f
3) 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 = (Q_T + Q_B), where Q_T and Q_B
are the gate charges of the topside and bottom side
MOSFETs.
Supplying INTV_CC power through the EXTV_CC switch input
from an output-derived source will scale the V_IN current
required for the driver and control circuits by the ratio
(Duty Factor)/(Efficiency). For example, in a 20V to 5V
application, 10mA of INTV_CC current results in approxi-
mately 3mA of V_IN current. This reduces the mid-current
loss from 10% or more (if the driver was powered directly
from V_IN) to only a few percent.
4) 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 cur-
rents; the second is the current drawn from the differential
amplifier output. V_IN current typically results in a small
(<0.1%) loss.
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 in the
design of a system. The internal battery and input fuse
resistance losses can be minimized by making sure that
C_IN has adequate charge storage and a very low ESR at
the switching frequency. A 50W supply will typically
require a minimum of 200µF to 300µF of capacitance
having a maximum of 10mΩ to 20mΩ of ESR. The
LTC1709 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.
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 DC (resistive) load
current. When a load step occurs, V_OUT 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
discharge C_OUT generating the feedback error signal that
forces the regulator to adapt to the current change and
return V_OUT to its steady-state value. During this recovery
time V_OUT can be monitored for excessive overshoot or
ringing, which would indicate a stability problem. The
availability of the I_TH pin not only allows optimization of
control loop behavior but also provides a DC coupled and
AC filtered closed loop response test point. The DC step,
rise time, and settling at this test point truly reflects the
closed loop response. Assuming a predominantly second
order system, phase margin and/or damping factor can be
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