LTC1539 LINER [Linear Technology], LTC1539 Datasheet - Page 21

• Manufacturer Part Number: LTC1539
• Description: Dual High Efficiency, Low Noise, Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
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 LTC1538-AUX/LTC1539 circuits. LTC1538-AUX/
LTC1539 V
topside MOSFET transition losses.
1. The V
2. INTV
3. I
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
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
2
IN
SENSE
R losses are predicted from the DC resistances of the
T
) to only a few percent.
IN
+ Q
CC
IN
current. This reduces the midcurrent loss from
, but is “chopped” between the topside main
B
current is the sum of the MOSFET driver and
current is the DC supply current given in the
IN
), where Q
CC
CC
current, INTV
to ground. The resulting dQ/dt is a current
CC
which is typically much larger than the
IN
U
current results in approximately 3mA
current resulting from the driver and
CC
T
and Q
from an output-derived source,
INFORMATION
U
IN
CC
current typically results in a
B
are the gate charges of the
current, I
W
2
R losses and
DS(ON)
IN
U
GATECHG
.
, then
=
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
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately (25)(C
Thus a 10 F capacitor would require a 250 s rise time,
limiting the charging current to about 200mA.
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.
Transition Loss 2.5(V
OUT
, causing a rapid drop in V
OUT
OUT
LTC1538-AUX/LTC1539
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
)(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
)(f)
21
LOAD
L
2
2
TH
R
R
).
=