Datasheet

LTM8055/LTM8055-1
7
Rev C
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PIN FUNCTIONS
GND (Bank 1, Pin L1): Tie these GND pins to a local
ground plane below the LTM8055/LTM8055-1 and the
circuit components. In most applications, the bulk of the
heat flow out of the LTM8055/LTM8055-1 is through these
pads, so the printed circuit design has a large impact on
the thermal performance of the part. See the PCB Layout
and Thermal Considerations sections for more details.
Return the R
FB1
/R
FB2
feedback divider to this net.
V
OUT
(Bank 2): Power Output Pins. Apply output filter
capacitors between these pins and GND pins.
V
IN
(Bank 3): Input Power. The V
IN
pin supplies current to
the LTM8055/LTM8055-1’s internal power switches and
to one terminal of the optional input current sense resis
-
tor. This pin must be locally bypassed with an external,
low ESR capacitor; see Table 1 for recommended values.
I
OUT
(Pin D1): Output Current Sense. Tie this pin to the
output current sense resistor. The output average current
sense threshold is 58mV, so the LTM8055/LTM8055-1
will regulate the output current to 58mV/R
SENSE
, where
R
SENSE
is the value of the output current sense resistor
in ohms. The load is powered
through the sense resistor
connected at this pin. Tie this pin to V
OUT
if no output
current sense resistor is used. Keep this pin within ±0.5V
of V
OUT
under all conditions.
LL (Pin F1): Light Load Indicator. This open drain pin
indicates that the output current, as sensed through the
resistor connected between V
OUT
and I
OUT
, is approximately
equivalent to 6mV or less. Its state is meaningful only if a
current sense resistor is applied between V
OUT
and I
OUT
.
This is useful to change the switching behavior of the
LTM8055/LTM8055-1 in light load conditions.
SV
IN
(Pins F10, F11): Controller Power Input. Apply a
separate voltage above 5V if the LTM8055/LTM8055-1
is required to operate when the main power input (V
IN
)
is below 5V. Bypass these pins with a high quality, low
ESR capacitor. If a separate supply is not used, connect
these pins to V
IN
.
CLKOUT (Pin G1): Clock Output. Use this pin as a clock
source when synchronizing other devices to the switching
frequency of the LTM8055/LTM8055-1. When this function
is not used, leave this pin open.
MODE (Pin G2): Switching Mode Input. The LTM8055/
LTM8055-1 operates
in forced continuous mode when
MODE is open, and can operate in discontinuous switch-
ing mode when MODE is low. In discontinuous switching
mode, the LTM8055/LTM8055-1 will block reverse induc-
tor current
. This pin is normally left open or tied to LL.
This pin may be tied to GND for the purpose of blocking
reverse current if no output current sense resistor is used.
RT (Pin H1): Timing Resistor. The RT pin is used to program
the switching frequency of the LTM8055/LTM8055-1 by
connecting a resistor from this pin to ground. The range
of oscillation is 100kHz to 800kHz. The Applications
Information section of the data sheet includes a table
to determine the resistance value based on the desired
switching frequency. Minimize capacitance at this pin.
SYNC (Pin H2): External Synchronization Input. The SYNC
pin has an internal pull-down resistor. See the Synchroni-
zation section
in Applications Information for details. Tie
this pin to GND when not used.
FB (Pin J1): Output Voltage Feedback. The LTM8055/
LTM8055-1 regulates the FB pin to 1.2V. Connect the FB
pin to a resistive divider between the output and GND to
set
the output
voltage. See Table 1 for recommended FB
divider resistor values.
COMP (Pin J2): Compensation Pin. The LTM8055 is
equipped with internal compensation that works well with
most applications, so this pin is usually left open. Some
applications, however, may benefit from a compensation
network other than the one integrated into the LTM8055.
In such cases, use the LTM8055-1, which has no internal
compensation network and apply an appropriate external
compensation network for optimal and proper operation.
SS (Pin K1): Soft-Start. Connect a capacitor from this pin
to GND to increase the soft-start time. Soft-start reduces
the input power source’s surge current by gradually in-
creasing the
controller’s current limit. Larger values of the
soft-start capacitor result in longer soft-start times. If no
soft-start is required, leave this pin open.
CTL (Pin K2): Current Sense Adjustment. Apply a voltage
below 1.2V to reduce the current limit threshold of I
OUT
.
Drive CTL to less than 50mV to stop switching. The CTL
pin has an internal pull-up resistor to 2V. If not used,
leave open.
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