LM43602-Q1_15 Datasheet, PDF (17/43 Pages) Texas Instruments – LM43602-Q1 SIMPLE SWITCHER® 3.5 V to 36 V 2-A Synchronous Step-Down Voltage Converter

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Feature Description (continued)
250
LM43602-Q1
SNVSA83A – APRIL 2015 – REVISED MAY 2015
200
150
100
50
0
0
500
1000
1500
2000
2500
Switching Frequency (kHz)
C008
Figure 31. RT vs Frequency Curve
Table 1. Typical Frequency Setting R_T Resistance
F_S (kHz)
200
R_T (kΩ)
200
350
115
500
78.7
750
53.6
1000
39.2
1500
26.1
2000
19.6
2200
17.8
The LM43602-Q1 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
R_TERM
Figure 32. 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 LM43602-Q1 will switch at the frequency programmed by the R_T
resistor after a time-out period. It is recommended to connect a resistor R_T to the RT pin such that the internal
oscillator frequency is the same as the target clock frequency when the LM43602-Q1 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 T_ON-MIN or T_OFF-MIN.
Copyright © 2015, Texas Instruments Incorporated
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