LM46002
SNVSA13B –APRIL 2014–REVISED SEPTEMBER 2014
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Feature Description (continued)
The LM46002 switching action can also be synchronized to an external clock from 200 kHz to 2.2 MHz. Connect
an external clock to the SYNC pin, with proper high speed termination, to avoid ringing. The SYNC pin should be
grounded if not used.
SYNC
EXT CLOCK
RTERM
Figure 41. Frequency Synchronization
The recommendations for the external clock include high level no lower than 2 V, low level no higher than 0.4 V,
duty cycle between 10% and 90% and both positive and negative pulse width no shorter than 80 ns. When the
external clock fails at logic high or low, the LM46002 will switch at the frequency programmed by the RT resistor
after a time-out period. It is recommended to connect a resistor RT to the RT pin such that the internal oscillator
frequency is the same as the target clock frequency when the LM46002 is synchronized to an external clock.
This allows the regulator to continue operating at approximately the same switching frequency if the external
clock fails.
The choice of switching frequency is usually a compromise between conversion efficiency and the size of the
circuit. Lower switching frequency implies reduced switching losses (including gate charge losses, switch
transition losses, etc.) and usually results in higher overall efficiency. However, higher switching frequency allows
use of smaller LC output filters and hence a more compact design. Lower inductance also helps transient
response (higher large signal slew rate of inductor current), and reduces the DCR loss. The optimal switching
frequency is usually a trade-off in a given application and thus needs to be determined on a case-by-case basis.
It is related to the input voltage, output voltage, most frequent load current level(s), external component choices,
and circuit size requirement. The choice of switching frequency may also be limited if an operating condition
triggers TON-MIN or TOFF-MIN
.
8.3.8 Minimum ON-Time, Minimum OFF-Time and Frequency Foldback at Drop-Out Conditions
Minimum ON-time, TON-MIN, is the smallest duration of time that the HS switch can be on. TON-MIN is typically 125
ns in the LM46002. Minimum OFF-time, TOFF-MIN, is the smallest duration that the HS switch can be off. TOFF-MIN
is typically 200 ns in the LM46002.
In CCM operation, TON-MIN and TOFF-MIN limits the voltage conversion range given a selected switching frequency.
The minimum duty cycle allowed is
DMIN = TON-MIN × FS
(4)
And the maximum duty cycle allowed is
DMAX = 1 - TOFF-MIN × FS
(5)
Given fixed TON-MIN and TOFF-MIN, the higher the switching frequency the narrower the range of the allowed duty
cycle. In the LM46002, frequency foldback scheme is employed to extend the maximum duty cycle when TOFF-MIN
is reached. The switching frequency will decrease once longer duty cycle is needed under low VIN conditions.
The switching frequency can be decreased to approximately 1/10 of the programmed frequency by RT or the
synchronization clock. Such wide range of frequency foldback allows the LM46002 output voltage to stay in
regulation with a much lower supply voltage VIN. This leads to a lower effective drop-out voltage. Please refer to
Typical Characteristics for more details.
Given an output voltage, the choice of the switching frequency affects the allowed input voltage range, solution
size and efficiency. The maximum operatable supply voltage can be found by
VIN-MAX = VOUT / (FS * TON-MIN
)
(6)
At lower supply voltage, the switching frequency will decrease once TOFF-MIN is tripped. The minimum VIN without
frequency foldback can be approximated by
VIN-MIN = VOUT / (1 - FS * TOFF-MIN
)
(7)
Taking considerations of power losses in the system with heavy load operation, VIN-MIN is higher than the result
calculated in Equation 7 . With frequency foldback, VIN-MIN is lowered by decreased FS. Figure 42 gives an
example of how FS decreases with decreasing supply voltage VIN at drop-out operation.
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