LTM8028
13
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applicaTions inForMaTion
Reverse Voltage
The LTM8028 incorporates a circuit that detects if BKV
decreases below V
OUT
. If this voltage condition is detected,
internal circuitry turns off the drive to the internal linear
regulator’s pass transistor, thereby turning off the output.
This circuit’s intent is to limit and prevent back-feed current
from V
OUT
to V
IN
if the input voltage collapses due to a
fault or overload condition. Do not apply a voltage to BKV.
Programming Switching Frequency
The LTM8028 has an operational switching frequency range
between 200kHz and 1MHz. This frequency is programmed
with an external resistor from the RT pin to ground. Do
not leave this pin open under any condition. The RT pin
is also current limited to 60μA. See Table 3 for resistor
values and the corresponding switching frequencies.
Table 3. R
T
Resistor Values and Their Resultant Switching
Frequencies
SWITCHING FREQUENCY (MHz) R
T
(kΩ)
1 40.2
0.750 53.6
0.5 82.5
0.3 143
0.2 200
Switching Frequency Trade-Offs
It is recommended that the user apply the optimal R
T
value
given in Table 1 for the input and output operating condi-
tion. System level or other considerations, however, may
n
ec
essitate another operating frequency. A higher switching
frequency, for example, will yield a smaller output ripple,
while a lower frequency will reduce power loss. Switch
-
ing too fast, however, can generate excessive heat and
even possibly damage the LTM8028 in fault conditions.
Switching too slow can result in a final design that has too
muc
h output capacitance or sub-harmonic oscillations that
cause excessive ripple. In all cases, stay below the stated
maximum frequency (f
MAX
) given in Table 1.
Switching Frequency Synchronization
The nominal switching frequency of the LTM8028 is
determined by the resistor from the RT pin to GND and
may be set from 200kHz to 1MHz. The internal oscillator
may also be synchronized to an external clock through
the SYNC pin. The external clock applied to the SYNC pin
must have a logic low below 0.25V and a logic high greater
than 1.25V. The input frequency must be 20% higher than
the frequency determined by the resistor at the RT pin.
The duty cycle of the input signal needs to be greater than
10% and less than 90%. Input signals outside of these
specified parameters will cause erratic switching behavior
and subharmonic oscillations. When synchronizing to an
external clock, please be aware that there will be a fixed
delay from the input clock edge to the edge of switch. The
SYNC pin must be tied to GND if the synchronization to an
external clock is not required. When SYNC is grounded,
the switching frequency is determined by the resistor at
the RT pin.
Soft-Start
The soft-start function controls the slew rate of the power
supply output voltage during start-up. A controlled output
voltage ramp minimizes output voltage overshoot, reduces
inrush current from the V
IN
supply, and facilitates supply
sequencing. A capacitor connected from the SS pin to
GND programs the slew rate. The capacitor is charged
from an internal 11μA current source to produce a ramped
output voltage.
Maximum Output Current Adjust
To adjust the regulated load current, an analog voltage
is applied to the I
MAX
pin. Varying the voltage between
0V and 1.5V adjusts the maximum current between the
minimum and the maximum current, 5.6A typical. Above
1.5V, the control voltage has little effect on the regulated
inductor current. A graph of the output current versus I
MAX
voltage is given in the Typical Performance Characteristics
