LTC1435A Linear Technology, LTC1435A Datasheet - Page 14

• Manufacturer Part Number: LTC1435A
• Description: High Efficiency Low Noise Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

LTC1435A
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 sev-
eral cycles to respond to a step in DC (resistive) load cur-
rent. When a load step occurs, V
an amount equal to ( I
fective series resistance of C
charge or discharge C
signal. The regulator loop then acts to return V
steady-state value. During this recovery time V
monitored for overshoot or ringing, which would indicate
a stability problem. The I
14
APPLICATIONS
By powering EXTV
additional V
control currents will be scaled by a factor of
Duty Cycle/Efficiency. For example, in a 20V to 5V ap-
plication, 10mA of INTV
mately 3mA of V
loss from 10% or more (if the driver was powered di-
rectly from V
MOSFET, inductor and current shunt. 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
R
tance is 0.25 . This results in losses ranging from 3%
to 10% as the output current increases from 0.5A to
2A. I
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
L
R losses are predicted from the DC resistances of the
Transition Loss = 2.5 (V
= 0.15 , and R
2
R losses cause the efficiency to drop at high output
, but is “chopped” between the topside main
IN
IN
current resulting from the driver and
) to only a few percent.
IN
U
CC
current. This reduces the midcurrent
OUT
SENSE
from an output-derived source, the
LOAD
TH
which generates a feedback error
INFORMATION
U
external components shown in
CC
= 0.05 , then the total resis-
)(ESR), where ESR is the ef-
IN
OUT
IN
current results in approxi-
and C
)
OUT
1.85
. I
immediately shifts by
W
(I
SENSE
OUT
LOAD
MAX
DS(ON)
ESR dissipative
)(C
also begins to
to obtain I
DS(ON)
RSS
OUT
= 0.05 ,
U
OUT
)(f)
can be
, then
to its
2
R
the Figure 1 circuit will provide adequate 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.
Automotive Considerations:
Plugging into the Cigarette Lighter
As battery-powered devices go mobile, there is a natural
interest in plugging into the cigarette lighter in order to
conserve or even recharge battery packs during operation.
But before you connect, be advised: you are plugging into
the supply from hell. The main battery line in an automo-
bile is the source of a number of nasty potential transients,
including load dump, reverse battery and double battery.
Load dump is the result of a loose battery cable. When the
cable breaks connection, the field collapse in the alternator
can cause a positive spike as high as 60V which takes several
hundred milliseconds to decay. Reverse battery is just what
it says, while double battery is a consequence of tow truck
operators finding that a 24V jump start cranks cold engines
faster than 12V.
The network shown in Figure 9 is the most straightforward
approach to protect a DC/DC converter from the ravages
of an automotive battery line. The series diode prevents
current from flowing during reverse battery, while the
transient suppressor clamps the input voltage during load
dump. Note that the transient suppressor should not
OUT
Figure 9. Automotive Application Protection
, causing a rapid drop in V
12V
GENERAL INSTRUMENT
TRANSIENT VOLTAGE
50A I PK RATING
SUPPRESSOR
1.5KA24A
V
OUT
IN
LTC1435A
. No regulator can
1435A F09
LOAD
).