LTC1538-AUX_15 Datasheet, PDF (15/32 Pages) Linear Technology – Dual High Efficiency, Low Noise, Synchronous Step-Down Switching Regulators

LTC1538-AUX/LTC1539
APPLICATIONS INFORMATION
higher price. Once the ESR requirement for C_OUT has been
met, the RMS current rating generally far exceeds the
I_RIPPLE(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.
INTV_CC/ 5V Standby Regulator
An internal P-channel low dropout regulator produces 5V
at the INTV_CC pin from the V_IN supply pin. INTV_CC powers
the drivers and internal circuitry within the LTC1538-AUX/
LTC1539, as well as any “wake-up” circuitry tied externally
to the INTV_CC pin. The INTV_CC pin regulator can supply
40mA and must be bypassed to ground with a minimum
of 2.2µF 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 10Ω and 10µF 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_CC source. The gate charge is depen-
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:
T_J = 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 V_IN.
EXTV_CC Connection
The LTC1538-AUX/LTC1539 contain an internal P-chan-
nel MOSFET switch connected between the EXTV_CC and
INTV_CC pins. When the voltage applied to EXTV_CC rises
above 4.8V, the internal regulator is turned off and an
internal switch closes, connecting the EXTV_CC pin to the
INTV_CC pin thereby supplying internal power to the IC. The
switch remains closed as long as the voltage applied to
EXTV_CC remains above 4.5V. This allows the MOSFET
driver and control power to be derived from the output
during normal operation (4.8V < V_OUT < 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 EXTV_CC pin and ensure that EXTV_CC ≤ V_IN.
Significant efficiency gains can be realized by powering
INTV_CC from the output, since the V_IN 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 EXTV_CC pin directly to V_OUT.
However, for 3.3V and other lower voltage regulators,
additional circuitry is required to derive INTV_CC power
from the output.
The following list summarizes the four possible connec-
tions for EXTV_CC:
1. EXTV_CC left open (or grounded). This will cause INTV_CC
to be powered from the internal 5V regulator resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTV_CC connected directly to V_OUT. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTV_CC connected to an output-derived boost network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV_CC to an
output-derived voltage which has been boosted to
15