Datasheet

LTC3609
17
3609fb
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 LTC3609 circuits:
1.
DC
I
2
R losses. These arise from the resistance of the
internal resistance 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. The DC I
2
R loss for one MOSFET
can simply be determined by [R
DS(ON)
+ R
L
] • I
O
.
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:
T
ransition Loss ≅ (1.7A
–1
) V
IN
2
I
OUT
C
RSS
f
3. INTV
CC
current. This is the sum of the MOSFET driver
and control currents. This loss can be reduced by supply-
ing INTV
CC
current through the EXTV
CC
pin from a high
efficiency source, such as an output derived boost network
or alternate supply if available.
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 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
current 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
a 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. The I
TH
pin external components shown in Figure 6 will provide
adequate compensation for most applications. For a
detailed explanation of switching control loop theory see
Application Note 76.
3.3V OR 5V RUN/SS
V
IN
INTV
CC
RUN/SS
D1
(5a) (5b)
D2*
C
SS
R
SS
*
C
SS
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
R
SS
*
3609 F05
2N7002
Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated