LTC1538-AUX_15 Datasheet, PDF (11/32 Pages) Linear Technology – Dual High Efficiency, Low Noise, Synchronous Step-Down Switching Regulators

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OPERATION ^{(Refer to Functional Diagram)}
Frequency Synchronization
A Phase-Locked Loop (PLL) is available on the LTC1539
to allow the oscillator to be synchronized to an external
source connected to the PLLIN pin. The output of the
phase detector at the PLL LPF pin is also the control input
of the oscillator, which operates over a 0V to 2.4V range
corresponding to – 30% to 30% in frequency. When
locked, the PLL aligns the turn-on of the top MOSFET to
the rising edge of the synchronizing signal. When PLLIN
is left open, PLL LPF goes low, forcing the oscillator to
minimum frequency.
Power-On Reset
The POR1 pin is an open drain output which pulls low
when the main regulator output voltage of the LTC1539
first controller is out of regulation. When the output
voltage rises to within 5% of regulation, a timer is started
which releases POR1 after 2¹⁶ (65536) oscillator cycles.
Auxiliary Linear Regulator
The auxiliary linear regulator in the LTC1538-AUX and
LTC1539 controls an external PNP transistor for operation
up to 500mA. A precise internal AUXFB resistive divider is
invoked when the AUXDR pin is above 9.5V to allow
regulated 12V VPP supplies to be easily implemented.
When AUXDR is below 8.5V an external feedback divider
may be used to set other output voltages. Taking the
AUXON pin low shuts down the auxiliary regulator provid-
ing a convenient logic-controlled power supply.
LTC1538-AUX/LTC1539
The AUX block can be used as a comparator having its
inverting input tied to the internal 1.19V reference. The
AUXDR pin is used as the output and requires an external
pull-up to a supply of less than 8.5V in order to inhibit the
invoking of the internal resistive divider.
INTV_CC / EXTV_CC Power
Power for the top and bottom MOSFET drivers and most
of the other LTC1538-AUX/LTC1539 circuitry is derived
from the INTV_CC pin. The bottom MOSFET driver supply is
also connected to INTV_CC. When the EXTV_CC pin is left
open, an internal 5V low dropout regulator supplies INTV_CC
power. If EXTV_CC is taken above 4.8V, the 5V regulator is
turned off and an internal switch is turned on to connect
EXTV_CC to INTV_CC. This allows the INTV_CC power to be
derived from a high efficiency external source such as the
output of the regulator itself or a secondary winding, as
described in the Applications Information section.
The 5V/20mA INTV_CC regulator can be used as a standby
regulator when the two controllers are in shutdown or
when either or both controllers are on. Irrespective of the
signals on the RUN/SS pins, the INTV_CC pin will follow the
voltage applied to the EXTV_CC pin when the voltage applied
to the EXTV_CC pin is taken above 4.8V. The externally
applied voltage is required to be less than the voltage
applied to the V_IN pin at all times, even when both control-
lers are shut down. This prevents a voltage backfeed
situation from the source applied to the EXTV_CC pin to the
V_IN pin. If the EXTV_CC pin is tied to the first controller’s 5V
output, the nominal INTV_CC pin voltage will stay in the
guaranteed range of 4.7V to 5.2V.
APPLICATIONS INFORMATION
The basic LTC1539 application circuit is shown in Fig-
ure 1. External component selection is driven by the load
requirement and begins with the selection of R_SENSE. Once
R_SENSE is known, C_OSC and L can be chosen. Next, the
power MOSFETs and D1 are selected. Finally, C_IN and C_OUT
are selected. The circuit shown in Figure 1 can be config-
ured for operation up to an input voltage of 28V (limited by
the external MOSFETs).
R_SENSE Selection for Output Current
R_SENSE is chosen based on the required output current.
The LTC1538-AUX/LTC1539 current comparator has a
maximum threshold of 150mV/R_SENSE and an input com-
mon mode range of SGND to INTV_CC. The current com-
parator threshold sets the peak of the inductor current,
yielding a maximum average output current I_MAX equal to
the peak value less half the peak-to-peak ripple current, ∆I_L.
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