LTC1439 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1439
Linear Technology
'LTC1439' PDF : 32 Pages View PDF
LTC1438/LTC1439
APPLICATIONS INFORMATION
The selection of COUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisified the capacitance is adequate for filtering.
The output ripple (∆VOUT) is approximated by:
∆VOUT
≈
∆IL
⎛
⎝⎜
ESR
+
1
4 fC OUT
⎞
⎠⎟
where f = operating frequency, COUT = output capacitance
and ∆IL = ripple current in the inductor. The output ripple
is highest at maximum input voltage since ∆IL increases
with input voltage. With ∆IL = 0.4IOUT(MAX) the output
ripple will be less than 100mV at max VIN assuming:
COUT Required ESR < 2RSENSE
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 COUT has been
met, the RMS current rating generally far exceeds the
IRIPPLE(P-P) requirement.
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.
INTVCC Regulator
An internal P-channel low dropout regulator produces 5V
at the INTVCC pin from the VIN supply pin. INTVCC powers
the drivers and internal circuitry within the LTC1438/
LTC1439. The INTVCC pin regulator can supply 40mA and
must be bypassed to ground with a minimum of 2.2µF
tantalum or low ESR electrolytic capacitor. Good bypass-
ing is necessary to supply the high transient currents
required by the MOSFET gate drivers.
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 EXTVCC source. The gate charge is dependent on
operating frequency as discussed in the Efficiency Consid-
erations section. The junction temperature can be esti-
mated by using the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC1439 is
limited to less than 21mA from a 30V supply:
TJ = 70°C + (21mA)(30V)(85°C/W) = 124°C
To prevent maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous mode at maximum VIN.
EXTVCC Connection
The LTC1438/LTC1439 contain an internal P-channel
MOSFET switch connected between the EXTVCC and
INTVCC pins. When the voltage applied to EXTVCC rises
above 4.8V, the internal regulator is turned off and an
internal switch closes, connecting the EXTVCC pin to the
INTVCC pin thereby supplying internal power to the IC. The
switch remains closed as long as the voltage applied to
EXTVCC remains above 4.5V. This allows the MOSFET
driver and control power to be derived from the output
during normal operation (4.8V < VOUT < 9V) and from the
internal regulator when the output is out of regulation
(start-up, short circuit). Do not apply greater than 10V to
the EXTVCC pin and ensure that EXTVCC ≤ VIN.
Significant efficiency gains can be realized by powering
INTVCC from the output, since the VIN current resulting
from the driver and control currents will be scaled by a
factor of Duty Cycle/Efficiency. For 5V regulators this
supply means connecting the EXTVCC pin directly to VOUT.
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTVCC power
from the output.
The following list summarizes the four possible connec-
tions for EXTVCC:
1. EXTVCC left open (or grounded). This will cause INTVCC
to be powered from the internal 5V regulator resulting
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