LTC1778EGN Linear Technology, LTC1778EGN Datasheet - Page 16

• Manufacturer Part Number: LTC1778EGN
• Description: IC CONTROLLR STP-DWN SYNC 16SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1778EGN

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Current - Output: 2A
• Voltage - Input: 4 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -
• Power - Output: -
• Frequency - Switching: -

APPLICATIO S I FOR ATIO
LTC1778/LTC1778-1
reduces the start delay while allowing C
slowly for the soft-start function.
After the controller has been started and given adequate
time to charge up the output capacitor, C
short-circuit timer. After the RUN/SS pin charges above
4V, if the output voltage falls below 75% of its regulated
value, then a short-circuit fault is assumed. A 1.8µA cur-
rent then begins discharging C
persists until the RUN/SS pin drops to 3.5V, then the con-
troller turns off both power MOSFETs, shutting down the
converter permanently. The RUN/SS pin must be actively
pulled down to ground in order to restart operation.
The overcurrent protection timer requires that the soft-start
timing capacitor C
that the output is in regulation by the time C
the 4V threshold. In general, this will depend upon the size
of the output capacitance, output voltage and load current
characteristic. A minimum soft-start capacitor can be
estimated from:
Generally 0.1µF is more than sufficient.
Overcurrent latchoff operation is not always needed or
desired. Load current is already limited during a short-
circuit by the current foldback circuitry and latchoff
operation can prove annoying during troubleshooting.
The feature can be overridden by adding a pull-up current
greater than 5µA to the RUN/SS pin. The additional
current prevents the discharge of C
also shortens the soft-start period. Using a resistor to V
as shown in Figure 8a is simple, but slightly increases
shutdown current. Connecting a resistor to INTV
16
C
3.3V OR 5V
Figure 8. RUN/SS Pin Interfacing with Latchoff Defeated
SS
> C
OUT
D1
(8a)
V
V
OUT
IN
R
SS
SS
U
R
*
RUN/SS
be made large enough to guarantee
SENSE
U
C
SS
(10
– 4
SS
. If the fault condition
[F/V s])
W
*OPTIONAL TO OVERRIDE
SS
OVERCURRENT LATCHOFF
INTV
during a fault and
2N7002
CC
SS
R
D2*
SS
SS
(8b)
SS
*
to charge up
is used as a
has reached
RUN/SS
U
1778 F08
C
SS
CC
as
IN
shown in Figure 8b eliminates the additional shutdown
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 4µA 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 LTC1778 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 15mW to 1.5W as the output current
varies from 1A to 10A.
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:
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.
Transition Loss ≅ (1.7A
IN
loss. The input capacitor has the difficult job of
2
CC
R losses. These arise from the resistances of the
CC
current. This is the sum of the MOSFET driver
current through the EXTV
DS(ON)
–1
= 0.01Ω and R
) V
IN
2
I
OUT
CC
C
SS
pin from a high
RSS
DS(ON)
L
= 0.005Ω, the
. Any pull-up
f
2
R loss and
, then the
1778fb
2
R