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LTM8008
APPLICATIONS INFORMATION
In an actual application, most of the INTV CC supply current
is used to drive the gate capacitance of the power MOSFET.
The power dissipation can be a significant concern when
the internal MOSFET is being driven at a high frequency and
the V IN voltage is high. It is important to limit the power
dissipation of the MOSFET and/or adjust the operating
frequency so the LTM8008 does not exceed its maximum
junction temperature rating.
Operating Frequency and Synchronization
The choice of operating frequency may be determined
by on-chip power dissipation, otherwise it is a trade-off
between efficiency and component size. Low frequency
operation improves efficiency by reducing gate drive cur-
rent and MOSFET and diode switching losses. However,
lower frequency operation requires a physically larger
inductor. Switching frequency also has implications for loop
compensation. The LTM8008 uses a constant-frequency
architecture that can be programmed over a 100kHz to
1MHz range with a single external resistor from the RT
pin to GND. The RT pin must have an external resistor
to GND for proper operation of the LTM8008. A table for
selecting the value of R T for a given operating frequency
is shown in Table 1.
Table 1. Timing Resistor (R T ) Value
OSCILLATOR FREQUENCY (kHz) R T (kΩ)
should have a minimum pulse width of 200ns. Tie the
SYNC pin to GND if this feature is not used.
Duty Cycle Considerations
Switching duty cycle is a key variable defining converter
operation. As such, its limits must be considered. Minimum
on-time is the smallest time duration that the LTM8008
is capable of turning on the power MOSFET. This time is
generally about 220ns (typical) (see Minimum On-Time
in the Electrical Characteristics table). In each switching
cycle, the LTM8008 keeps the power switch off for at least
220ns (typical) (see Minimum Off-Time in the Electrical
Characteristics table).
Soft-Start
The LTM8008 contains several features to limit peak switch
currents and output voltage (V OUT ) overshoot during
start-up or recovery from a fault condition. The primary
purpose of these features is to prevent damage to external
components or the load. High peak switch currents during
start-up may occur in switching regulators. Since V OUT
is far from its final value, the feedback loop is saturated
and the regulator tries to charge the output capacitor
as quickly as possible, resulting in large peak currents.
A large surge current may cause inductor saturation or
power switch failure.
100
200
300
400
500
600
700
800
900
1000
140
63.4
41.2
30.9
24.3
19.6
16.5
14
12.1
10.5
The LTM8008 addresses this mechanism with the SS pin.
The SS circuit reduces the power MOSFET current by pull-
ing down the V C pin. In this way the SS allows the output
capacitor to charge gradually toward its final value while
limiting the start-up peak currents. The inductor current
slew rate is limited by the soft-start function.
Besides start-up, soft-start can also be invoked by the
following faults:
1. INTV CC overvoltage
The operating frequency of the LTM8008 can be synchro-
nized to an external clock source. By providing a digital
clock signal into the SYNC pin, the LTM8008 will operate
at the SYNC clock frequency. If this feature is used, an R T
resistor should be chosen to program a switching frequency
20% slower than SYNC pulse frequency. The SYNC pulse
2. INTV CC undervoltage
3. Thermal lockout
Any of these three faults will cause the LTM8008 to stop
switching immediately and discharge the SS pin. When
all faults are cleared and the SS pin has been discharged
below 0.2V, a 10μA current source will start charging the
8008fa
9
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