LTC1700 Linear Technology, LTC1700 Datasheet - Page 12

• Manufacturer Part Number: LTC1700
• Description: No RSENSE Synchronous Step-Up DC/DC Controller
• Manufacturer: Linear Technology

LTC1700
APPLICATIONS
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power ( 100%).
Percent efficiency can be expressed as:
where L1, L2, etc. are the individual losses as a percentage
of input power. 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 usually account for most of the
losses in LTC1700 circuits:
1. LTC1700 supply current. This DC supply current, given
in the electrical characteristics, excludes MOSFET drivers
and control current. This supply current results in a small
loss which increases with V
2. MOSFETs gate charge current results from switching
the gate capacitance of the power MOSFETs. Each time a
MOSFET gate is switched on and then off, a packet of gate
charge Q
current out from V
control circuit current. In continuous mode, I
f(Q
loss becomes increasing important.
3. DC I
DC I
MOSFETs and inductor. In continuous mode, the average
current flows through the inductor but is “chopped”
between the synchronous P-channel MOSFET and the
main N-channel MOSFET. If the two MOSFETs have ap-
proximately the same R
MOSFET can simply be summed with the resistance of the
inductor to obtain the DC I
R
is 0.2 . This results in losses ranging from 2% to 8% as
the output current increases from 0.5A to 2A for a 5V
output. I
output currents.
12
DS(ON)
% Efficiency = 100%–(L1 + L2 + L3 + ...)
g(TOP)
2
R losses arise only from the resistances of the
2
R Losses. Since there is no sense resistor needed,
= 0.05 and R
2
g
+ Q
R losses cause the efficiency to drop at high
moves from V
g(BOT)
). At high switching frequencies, this
OUT
U
L
is typically much larger than the
DS(ON)
= 0.15 , then the total resistance
INFORMATION
U
OUT
2
OUT
R loss. For example, if each
, then the resistance of one
to ground. The resulting
.
W
GATECHG
U
=
4. Transition losses apply to the main external MOSFET
and increase at higher operating frequencies and output
voltages. Transition losses can be estimated from:
Other losses including C
losses, and inductor core losses, generally account for
less than 2% total loss.
Run/Soft Start Function
The RUN/SS pin is a dual purpose pin that provides the
soft-start function and a means to shut down the LTC1700.
Soft-start reduces input surge current from V
ally increasing the internal current limit. Power supply
sequencing can also be accomplished using this pin.
An internal 3.8 A current source charges up an external
capacitor C
reaches 0.7V, the LTC1700 begins operating. As the
voltage on RUN/SS continues to ramp from 0.7V to 1V, the
internal current limit is also ramped at a proportional linear
rate. The current limit begins near 0A (at V
and ends at 0.078/R
current thus ramps up slowly, reducing the starting surge
current required from the input power supply. If the RUN/
SS has been pulled all the way to ground, there will be a
delay before the current limit starts increasing and is given
by:
For input voltages less than 2.3V during the start-up
duration, the soft-start function has no effect on the
internal 60mA current limit. Therefore to fully take advan-
tage of this feature, the soft-start capacitor has to be sized
accordingly to account for the time it takes V
2.3V. An approximate mathematical representation for the
time it takes V
by:
Transition Loss = 2.5(V
t
t
DELAY
POWER UP
= 1.13C
SS
OUT
. When the voltage on the RUN/SS pin
SS
to reach 2.3V upon powering up is given
C
OUT
/I
2 3
DS(ON)
CHG
260
. –
( . –
2 3
( )
L
V
OUT
IN
IN
(V
and C
RUN/SS
)
V
–
2
IN
I
I
OUT
O(MAX)
–
V
OUT
D
)
C
2.2V). The output
RSS
ESR dissipative
RUN/SS
(f)
OUT
IN
by gradu-
to reach
= 0.7V)