ltc3711 Linear Technology Corporation, ltc3711 Datasheet - Page 17

• Manufacturer Part Number: ltc3711
• Description: 5-bit Adjustable, Wide Operating Range, No Rsense? Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
current, but requires a diode to isolate C
network must be able to pull RUN/SS above the 4.2V
maximum threshold of the latchoff circuit and overcome
the 4 A maximum discharge current.
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 LTC3711 circuits:
1. DC I
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
resistance of one MOSFET can simply be summed with the
resistances of L and the board traces to obtain the DC I
loss. For example, if R
loss will range from 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:
TG
Transition Loss (1.7A
2
10
R losses. These arise from the resistances of the
BOOST
SW
Figure 7. Optional External Gate Driver
Q1
FMMT619
Q2
FMMT720
U
DS(ON)
GATE
OF M1
U
–1
= 0.01 and R
) V
BG
IN
2
I
W
OUT
10
INTV
PGND
C
SS
DS(ON)
RSS
Q3
FMMT619
Q4
FMMT720
L
CC
= 0.005 , the
. Any pull-up
f
U
, then the
GATE
OF M2
3711 F07
2
R
3. INTV
and control currents. This loss can be reduced by supply-
ing INTV
efficiency source, such as an output derived boost net-
work or alternate supply if available.
4. C
filtering the large RMS input current to the regulator. It
must have a very low ESR to minimize the AC I
sufficient capacitance to prevent the RMS current from
causing additional upstream losses in fuses or batteries.
Other losses, including C
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
equal to I
resistance of C
discharge C
by the regulator to return V
During this recovery time, V
overshoot or ringing that would indicate a stability
problem. The I
3.3V OR 5V
Figure 8. RUN/SS Pin Interfacing with Latchoff Defeated
IN
loss. The input capacitor has the difficult job of
CC
CC
current. This is the sum of the MOSFET driver
LOAD
current through the EXTV
D1
OUT
(8a)
V
TH
(ESR), where ESR is the effective series
OUT
generating a feedback error signal used
IN
R
SS
pin external components shown in
. I
OUT
*
RUN/SS
LOAD
immediately shifts by an amount
OUT
C
SS
OUT
ESR loss, Schottky diode D1
OUT
also begins to charge or
to its steady-state value.
can be monitored for
*OPTIONAL TO OVERRIDE
OVERCURRENT LATCHOFF
INTV
CC
CC
R
pin from a high
D2*
LTC3711
SS
(8b)
*
RUN/SS
2
R loss and
3711 F08
17
C
SS
3711f