DC1307A Datasheet, PDF (11/22 Pages) Linear Technology – Complete Switch Mode Power Supply

LTM8027
Applications Information
For most applications, the design process is straight
forward, summarized as follows:
1. Look at Table 2 and find the row that has the desired
input range and output voltage.
2. Apply the recommended C_IN, C_OUT, R_ADJ and R_T values.
3. Connect the BIAS pins as indicated.
While these component and connection combinations have
been tested for proper operation, it is incumbent upon the
user to verify proper operation over the intended system’s
line, load and environmental conditions.
Capacitor Selection Considerations
The C_IN and C_OUT capacitor values in Table 2 are the
minimum recommended values for the associated oper-
ating conditions. Applying capacitor values below those
indicated in Table 2 is not recommended, and may result
in undesirable operation. Using larger values is generally
acceptable, and can yield improved dynamic response, if
it is necessary. Again, it is incumbent upon the user to
verify proper operation over the intended system’s line,
load and environmental conditions.
Ceramic capacitors are small, robust and have very low
ESR. However, not all ceramic capacitors are suitable.
X5R and X7R types are stable over temperature and ap-
plied voltage and give dependable service. Other types,
including Y5V and Z5U have very large temperature and
voltage coefficients of capacitance. In an application cir-
cuit they may have only a small fraction of their nominal
capacitance resulting in much higher output voltage ripple
than expected.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8027. A
ceramic input capacitor combined with trace or cable
inductance forms a high Q (under damped) tank circuit.
If the LTM8027 circuit is plugged into a live supply, the
input voltage can ring to twice its nominal value, possi-
bly exceeding the device’s rating. This situation is easily
avoided; see the Hot-Plugging Safely section.
Input Power Requirements
The LTM8027 is biased using an internal linear regulator
to generate operational voltages from the V_IN pin. Virtually
all of the circuitry in the LTM8027 is biased via this internal
linear regulator output (INTV_CC on the Block Diagram).
This pin is internally decoupled with a low ESR capacitor
to GND. The INTV_CC regulator generates an 8V output
provided there is ample voltage on the V_IN pin. The INTV_CC
regulator has approximately 1V of dropout, and will follow
the V_IN pin with voltages below the dropout threshold.
The LTM8027 has a typical start-up requirement of V_IN >
7.5V. This assures that the onboard regulator has ample
headroom to bring the internal regulator (INTV_CC) above
its UVLO threshold. The INTV_CC regulator can only source
current, so forcing the BIAS1 pin above 8.5V allows use
of externally derived power for the IC. This effectively
shuts down the internal linear regulator and reduces
power dissipation within the LTM8027. Using the onboard
regulator for start-up, then applying power to BIAS1 from
the converter output or external supply maximizes con-
version efficiencies and is a common practice. If IBIAS1
is maintained above 6.5V using an external source, the
LTM8027 can continue to operate with V_IN as low as 4.5V.
BIAS Power
The internal circuitry of the LTM8027 is powered by the
INTV_CC bus, which is derived either from the afore men-
tioned internal linear regulator or the BIAS1 pin, if it is
greater than 8.5V. Since the internal linear regulator is by
nature dissipative, deriving INTV_CC from an external source
through the BIAS pins reduces the power lost within the
LTM8027 and can increase overall system efficiency.
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