LTC1538-AUX Linear Technology, LTC1538-AUX Datasheet - Page 15

• Manufacturer Part Number: LTC1538-AUX
• Description: Dual High Efficiency/ Low Noise/ Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIONS
higher price. Once the ESR requirement for C
met, the RMS current rating generally far exceeds the
I
In surface mount applications multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. In the case of tantalum, it is
critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS series of surface mount tantalums, available in case
heights ranging from 2mm to 4mm. Other capacitor types
include Sanyo OS-CON, Nichicon PL series and Sprague
593D and 595D series. Consult the manufacturer for other
specific recommendations.
INTV
An internal P-channel low dropout regulator produces 5V
at the INTV
the drivers and internal circuitry within the LTC1538-AUX/
LTC1539, as well as any “wake-up” circuitry tied externally
to the INTV
40mA and must be bypassed to ground with a minimum
of 2.2 F tantalum or low ESR electrolytic capacitor. Good
bypassing is necessary to supply the high transient cur-
rents required by the MOSFET gate drivers.
To prevent any interaction due to the high transient gate
currents being drawn from the external capacitor an
additional series filter of 10 and 10 F to SGND can be
added.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC1538-AUX/
LTC1539 to be exceeded. The IC supply current is domi-
nated by the gate charge supply current when not using an
output derived EXTV
dent on operating frequency as discussed in the Efficiency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 1 of the
Electrical Characteristics. For example, the LTC1539 is
limited to less than 21mA from a 30V supply:
RIPPLE(P-P)
CC
/ 5V Standby Regulator
CC
CC
requirement.
pin from the V
pin. The INTV
U
CC
source. The gate charge is depen-
INFORMATION
U
IN
CC
supply pin. INTV
pin regulator can supply
W
OUT
CC
U
has been
powers
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous mode at maximum V
EXTV
The LTC1538-AUX/LTC1539 contain an internal P-chan-
nel MOSFET switch connected between the EXTV
INTV
above 4.8V, the internal regulator is turned off and an
internal switch closes, connecting the EXTV
INTV
switch remains closed as long as the voltage applied to
EXTV
driver and control power to be derived from the output
during normal operation (4.8V < V
internal regulator when the output is out of regulation
(start-up, short circuit). Do not apply greater than 10V to
the EXTV
Significant efficiency gains can be realized by powering
INTV
from the driver and control currents will be scaled by a
factor of Duty Cycle/Efficiency. For 5V regulators this
supply means connecting the EXTV
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTV
from the output.
The following list summarizes the four possible connec-
tions for EXTV
1. EXTV
2. EXTV
3. EXTV
T
to be powered from the internal 5V regulator resulting
in an efficiency penalty of up to 10% at high input
voltages.
connection for a 5V regulator and provides the highest
efficiency.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV
output-derived voltage which has been boosted to
J
CC
CC
CC
= 70 C + (21mA)(30V)(85 C/W) = 124 C
CC
CC
pin thereby supplying internal power to the IC. The
pins. When the voltage applied to EXTV
from the output, since the V
CC
CC
CC
Connection
remains above 4.5V. This allows the MOSFET
CC
left open (or grounded). This will cause INTV
connected to an output-derived boost network.
connected directly to V
pin and ensure that EXTV
LTC1538-AUX/LTC1539
CC:
OUT
OUT
CC
IN
pin directly to V
. This is the normal
CC
< 9V) and from the
current resulting
V
IN
CC
IN
.
pin to the
CC
.
CC
CC
15
CC
power
to an
rises
OUT
and
CC
.