LTC1707 LINER [Linear Technology], LTC1707 Datasheet - Page 10

• Manufacturer Part Number: LTC1707
• Description: High Efficiency Monolithic Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1707
D1 in Figure 5 reduces the start delay but allows C
ramp up slowly providing the soft-start function. This
diode can be deleted if soft-start is not needed.
Efficiency Considerations
The 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. Efficiency can be expressed as:
where L1, L2, etc. are the individual losses as a percentage
of input power.
Although all dissipative elements in the circuit produce
losses, two main sources usually account for most of the
losses in LTC1707 circuits: V
losses. The V
efficiency loss at very low load currents whereas the I
loss dominates the efficiency loss at medium to high load
currents. In a typical efficiency plot, the efficiency curve at
very low load currents can be misleading since the actual
power lost is of no consequence as illustrated in Figure 6.
1. The V
2. I
10
Efficiency = 100% – (L1 + L2 + L3 + ...)
DC bias current as given in the electrical characteristics
and the internal main switch and synchronous switch
gate charge currents. The gate charge current results
from switching the gate capacitance of the internal power
MOSFET switches. Each time the gate is switched from
high to low or from low to high, a packet of charge dQ
moves from V
current out of V
current. In continuous mode, I
Q
bottom switches. Both the DC bias and gate charge losses
are proportional to V
pronounced at higher supply voltages.
internal switches R
continuous mode the average output current flowing
through inductor L is “chopped” between the main
switch and the synchronous switch. Thus, the series
resistance looking into SW pin from L is a function of
2
T
R losses are calculated from the resistances of the
and Q
IN
quiescent current is due to two components: the
B
are the gate charges of the internal top and
IN
IN
quiescent current loss dominates the
IN
U
to ground. The resulting dQ/dt is the
that is typically larger than the DC bias
IN
SW
and thus their effects will be more
U
and external inductor R
IN
quiescent current and I
GATECHG
W
= f(Q
T
+ Q
U
B
) where
SS
L
. In
2
2
to
R
R
Other losses including C
MOSFET switching losses and inductor core and copper
losses generally account for less than 2% total additional
loss.
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
charge C
regulator loop then acts to return V
value. During this recovery time, V
for overshoot or ringing that would indicate a stability
problem. The internal compensation provides adequate
compensation for most applications. But if additional
compensation is required, the I
external compensation as shown in Figure 7 (the 47pF
capacitor, C
both top and bottom MOSFET R
cycle (DC) as follows:
The R
be obtained from the Typical Performance Characteris-
tics curves. Thus, to obtain I
to R
current.
R
L
SW
DS(ON)
and multiply by the square of the average output
OUT
= (R
LOAD
0.001
C2
, which generates a feedback error signal. The
Figure 6. Power Lost vs Load Current
0.01
0.1
, is typically needed for noise decoupling).
1
DS(ON)TOP
for both the top and bottom MOSFETs can
OUT
1
• ESR), where ESR is the effective series
. I
V
V
V
OUT
OUT
OUT
OUT
LOAD
IN
= 1.5V
= 3.3V
= 5V
LOAD CURRENT (mA)
immediately shifts by an amount
)(DC) + (R
and C
10
also begins to charge or dis-
2
OUT
R losses, simply add R
TH
ESR dissipative losses,
OUT
OUT
100
DS(ON)BOT
pin can be used for
DS(ON)
to its steady-state
can be monitored
V
IN
1707 F06
= 6V
and the duty
1000
)(1 – DC)
SW