LTC1439 Linear Technology, LTC1439 Datasheet - Page 15

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

APPLICATIONS
The selection of C
series resistance (ESR). Typically, once the ESR require-
ment is satisified the capacitance is adequate for filtering.
The output ripple ( V
where f = operating frequency, C
and I
is highest at maximum input voltage since I
with input voltage. With I
ripple will be less than 100mV at max V
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest (ESR size)
product of any aluminum electrolytic at a somewhat
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 LTC1438/
LTC1439. The INTV
must be bypassed to ground with a minimum of 2.2 F
tantalum or low ESR electrolytic capacitor. Good bypass-
ing is necessary to supply the high transient currents
required by the MOSFET gate drivers.
RIPPLE(P-P)
C
OUT
V
CC
OUT
L
Regulator
= ripple current in the inductor. The output ripple
Required ESR < 2R
CC
requirement.
pin from the V
I ESR
L
OUT
U
CC
OUT
pin regulator can supply 40mA and
is driven by the required effective
INFORMATION
) is approximated by:
4
U
fC
SENSE
IN
L
1
OUT
= 0.4I
supply pin. INTV
OUT
W
= output capacitance
OUT(MAX)
IN
assuming:
OUT
L
the output
CC
U
increases
has been
powers
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC1438/LTC1439
to be exceeded. The IC supply current is dominated by the
gate charge supply current when not using an output
derived EXTV
operating frequency as discussed in the Efficiency Consid-
erations section. The junction temperature can be esti-
mated by using the equations given in Note 1 of the
Electrical Characteristics. For example, the LTC1439 is
limited to less than 21mA from a 30V supply:
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 LTC1438/LTC1439 contain an internal P-channel
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
T
to be powered from the internal 5V regulator resulting
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
Connection
remains above 4.5V. This allows the MOSFET
CC
left open (or grounded). This will cause INTV
pin and ensure that EXTV
CC
CC:
source. The gate charge is dependent on
LTC1438/LTC1439
OUT
CC
IN
pin directly to V
CC
< 9V) and from the
current resulting
V
CC
IN
IN
.
pin to the
CC
.
CC
15
CC
power
rises
OUT
and
CC
.