LTC1775 LINER [Linear Technology], LTC1775 Datasheet - Page 17

• Manufacturer Part Number: LTC1775
• Description: High Power No RSENSE TM Current Mode Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
cycle skipping can occur with correspondingly larger
current and voltage ripple.
Efficiency Considerations
The efficiency of a switching regulator is equal to the
output power divided by the input power ( 100%). Per-
cent 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 LTC1775 circuits:
1. INTV
2. DC I
%Efficiency = 100% – (L1 + L2 + L3 + ...)
and control currents. The driver 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
ground. The resulting current out of INTV
much larger than the control circuit current. In continu-
ous mode, I
By powering EXTV
the additional V
control currents will be scaled by a factor of Duty Cycle/
Efficiency. For example, in a 20V to 5V application at
400mA load, 10mA of INTV
proximately 3mA of V
from 10% (if the driver was powered directly from V
to about 3%.
tor, DC I
MOSFETs and inductor. In continuous mode the aver-
age output current flows through L, but is “chopped”
between the top MOSFET and the bottom MOSFET. If
the two MOSFETs have approximately the same R
then the resistance of one MOSFET can simply be
summed with the resistance of L to obtain the DC I
loss. For example, if each R
0.15 , then the total resistance is 0.2 . This results in
2
CC
R Losses. Since there is no separate sense resis-
current. This is the sum of the MOSFET driver
2
R losses arise only from the resistances of the
GATECHG
IN
U
current resulting from the driver and
CC
= f(Q
IN
from an output-derived source,
U
current. This reduces the loss
g(TOP)
g
DS(ON)
CC
moves from INTV
W
current results in ap-
+ Q
= 0.05 and R
g(BOT)
CC
).
U
is typically
DS(ON)
CC
L
IN
2
to
R
=
)
,
3. Transition losses apply only to the topside MOSFET,
4. LTC1775 V
Other losses including C
losses, Schottky conduction losses during dead time and
inductor core 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. The regulator loop acts on the resulting feedback
error signal to return V
this recovery time V
ringing which would indicate a stability problem. The I
pin external components shown in Figure 1 will provide
adequate compensation for most applications.
A second, more severe transient is caused by connecting
loads with large (> 1 F) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive in order
to limit the inrush current to the load.
losses ranging from 2% to 8% as the output current
increases from 0.5A to 2A for a 5V output. I
cause the efficiency to drop at high output currents.
and only when operating at high input voltages (typi-
cally 20V or greater). Transition losses can be esti-
mated from:
supply current to the controller excluding MOSFET gate
drive current. Total supply current is typically about
850 A. If EXTV
draw only 330 A from V
come from EXTV
(< 1%) loss which increases with V
Transition Loss = (1.7)(V
OUT
, causing a rapid drop in V
LOAD
IN
OUT
supply current. The V
)(ESR), where ESR is the effective series
CC
, and C
OUT
is connected to 5V, the LTC1775 will
OUT
CC
OUT
. V
can be monitored for overshoot or
immediately shifts by an amount
to its steady-state value. During
IN
IN
IN
OUT
and the remaining 520 A will
and C
current results in a small
IN
begins to charge or dis-
2
)(I
OUT
OUT
O(MAX)
IN
IN
. No regulator can
.
current is the DC
ESR dissipative
LTC1775
)(C
RSS
2
R losses
)(f)
17
TH