LTC1439 Linear Technology, LTC1439 Datasheet - Page 21

• Manufacturer Part Number: LTC1439
• Description: Dual High Efficiency/ Low Noise/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIONS
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, four main sources usually account for most of the
losses in LTC1438/LTC1439 circuits. LTC1438/LTC1439
V
transition losses.
1. The V
2. INTV
IN
Efficiency = 100% – (L1 + L2 + L3 + ...)
Electrical Characteristics which excludes MOSFET driver
and control currents. V
small (<< 1%) loss which increases with V
control currents. The MOSFET driver current results
from switching the gate capacitance of the power
MOSFETs. Each time a MOSFET gate is switched from
low to high to low again, a packet of charge dQ moves
from INTV
out of INTV
control circuit current. In continuous mode, I
f(Q
topside and bottom side MOSFETs. It is for this reason
that the large topside and synchronous MOSFETs are
turned off during low current operation in favor of the
small topside MOSFET and external Schottky diode,
allowing efficient, constant-frequency operation at low
output currents.
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,
10mA of INTV
of V
10% or more (if the driver was powered directly from
V
current, INTV
IN
T
) to only a few percent.
IN
+ Q
CC
IN
current. This reduces the midcurrent loss from
B
current is the sum of the MOSFET driver and
current is the DC supply current given in the
), where Q
CC
CC
to ground. The resulting dQ/dt is a current
CC
CC
which is typically much larger than the
IN
current, I
U
current results in approximately 3mA
current resulting from the driver and
CC
T
and Q
from an output-derived source,
INFORMATION
U
IN
2
R losses and topside MOSFET
current typically results in a
B
are the gate charges of the
W
IN
U
GATECHG
.
=
3. I
4. Transition losses apply only to the topside MOSFET(s)
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 DC (resistive) load
current. When a load step occurs, V
amount equal to ( I
series resistance of C
discharge C
forces the regulator loop to adapt to the current change
and return V
recovery time V
ringing which would indicate a stability problem. The I
external components shown in Figure 1 will prove ad-
equate compensation for most applications.
A second, more severe transient is caused by switching in
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
MOSFET, inductor and current sense R. In continuous
mode the average output current flows through L and
R
MOSFET and the synchronous MOSFET. If the two
MOSFETs have approximately the same R
the resistance of one MOSFET can simply be summed
with the resistances of L and R
losses. For example, if each R
0.15 and R
0.25 . This results in losses ranging from 3% to 10%
as the output current increases from 0.5A to 2A. I
losses cause the efficiency to roll off at high output
currents.
and only when operating at high input voltages (typically
20V or greater). Transition losses can be estimated from:
Other losses including C
losses, Schottky conduction losses during dead-time,
and inductor core losses, generally account for less
than 2% total additional loss.
2
SENSE
R losses are predicted from the DC resistances of the
Transition Loss 2.5(V
OUT
, causing a rapid drop in V
, but is “chopped” between the topside main
OUT
OUT
SENSE
generating the feedback error signal which
OUT
to its steady-state value. During this
LOAD
can be monitored for overshoot or
OUT
= 0.05 , then the total resistance is
LTC1438/LTC1439
)(ESR) where ESR is the effective
. I
IN
LOAD
IN
and C
)
1.85
also begins to charge or
DS(ON)
(I
OUT
SENSE
OUT
MAX
. No regulator can
OUT
)(C
ESR dissipative
= 0.05 , R
to obtain I
RSS
shifts by an
DS(ON)
)(f)
21
, then
L
2
2
TH
R
R
=