18
LTC3709
3709fb
to the output of supply 1. This resistor divider is selected
to be the same as the divider across supply 2’s output. The
TRACK pin of supply 2 is connected to this extra resistor
divider. For the ratiometric tracking, simply connect the
TRACK pin of supply 2 to the V
FB
pin of supply 1. Figure
7 shows this implementation. Note that in the coincident
tracking, output voltage of supply 1 has to be set higher
than output voltage of supply 2.
Note that since the shutdown trip point varies from part to
part, the “slave” part’s RUN/SS pin will need to be con-
nected to V
CC
. This eliminates the possibility that different
LTC3709s may shut down at different times.
If output sequencing is not needed, connect the TRACK
pins to V
CC
.
Do Not Float these pins
.
APPLICATIO S I FOR ATIO
WUUU
Figure 7. Setup for Coincident and Ratiometric Tracking
R2
R1
SUPPLY 1
R6
3709 F07
R5
R4
R3
V
OUT1
V
OUT2
V
FB
SUPPLY 2
TRACK
LTC3709
V
FB
R3
R4
=
R5
R6
V
OUT2
COINCIDENTLY TRACKS V
OUT1
R3
R4
=
R1
R2
RATIOMETRIC POWER UP
BETWEEN V
OUT1
AND V
OUT2
resistance of one MOSFET can simply be summed with the
resistances of L and the board traces to obtain the DC I
2
R
loss. For example, if R
DS(ON)
= 0.01Ω and R
L
= 0.005Ω, the
loss will range from 0.1% up to 10% as the output current
varies from 1A to 10A for a 1.5V output.
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 capaci-
tance, among other factors. The loss is significant at input
voltages above 20V and can be estimated from:
TransitionLoss V I C f
R
DRV V V
IN OUT RSS
DS ON DRV
CC GS TH GS TH
≈
−
+
⎛
⎝
⎜
⎞
⎠
⎟
(.)• • • • •
()_
() ()
05
11
2
3. Gate driver supply current. The driver current supplies
the gate charge Q
G
required to switch the power MOSFETs.
This current is typically much larger than the control
circuit current. In continuous mode operation:
I
GATECHG
= f (Q
g(TOP)
+ Q
g(BOT)
)
4. C
IN
loss. The input capacitor has the difficult job of
filtering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I
2
R 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 conduc-
tion loss during dead time and inductor core loss generally
account for less than 2% additional loss.
When making any adjustments to improve efficiency, the
final arbiter is the total input current for the regulator at
your operating point. If you make a change and the input
current decreases, then you improved the efficiency. 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
a load step occurs, V
OUT
immediately shifts by an amount
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 LTC3709 circuits:
1. DC I
2
R 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
