LTC1538CG-AUX Linear Technology, LTC1538CG-AUX Datasheet - Page 11

• Manufacturer Part Number: LTC1538CG-AUX
• Description: IC SW REG STP-DN SYNC DUAL28SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1538CG-AUX

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 1.19 ~ 9 V
• Current - Output: 50mA
• Frequency - Switching: 125kHz ~ 240kHz
• Voltage - Input: 3.5 ~ 36 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS
OPERATION
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
R
power MOSFETs and D1 are selected. Finally, C
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).
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 2
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.
SENSE
is known, C
U
U
OSC
(Refer to Functional Diagram)
and L can be chosen. Next, the
INFORMATION
U
16
(65536) oscillator cycles.
W
SENSE
IN
U
and C
. Once
OUT
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
Power for the top and bottom MOSFET drivers and most
of the other LTC1538-AUX/LTC1539 circuitry is derived
from the INTV
also connected to INTV
open, an internal 5V low dropout regulator supplies INTV
power. If EXTV
turned off and an internal switch is turned on to connect
EXTV
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
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
voltage applied to the EXTV
to the EXTV
applied voltage is required to be less than the voltage
applied to the V
lers are shut down. This prevents a voltage backfeed
situation from the source applied to the EXTV
V
output, the nominal INTV
guaranteed range of 4.7V to 5.2V.
R
R
The LTC1538-AUX/LTC1539 current comparator has a
maximum threshold of 150mV/R
mon mode range of SGND to INTV
parator threshold sets the peak of the inductor current,
yielding a maximum average output current I
the peak value less half the peak-to-peak ripple current, I
IN
SENSE
SENSE
pin. If the EXTV
CC
CC
/ EXTV
Selection for Output Current
is chosen based on the required output current.
to INTV
CC
CC
CC
LTC1538-AUX/LTC1539
CC
IN
pin is taken above 4.8V. The externally
CC
pin. The bottom MOSFET driver supply is
Power
pin at all times, even when both control-
is taken above 4.8V, the 5V regulator is
. This allows the INTV
CC
CC
regulator can be used as a standby
pin is tied to the first controller’s 5V
CC
CC
. When the EXTV
CC
pin when the voltage applied
pin voltage will stay in the
SENSE
CC
CC
. The current com-
and an input com-
pin will follow the
CC
CC
power to be
MAX
CC
pin is left
pin to the
equal to
11
CC
L
.