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Boost Converter
7
Boost Converter
7.1
Switching Frequency
Switching frequency for boost is defined as 300 kHz to 2.2 MHz. However, frequency from 250 kHz to 2.2
MHz can be set by the external resistor connected to the FSET pin, thus allowing the device to be
synchronized with the lowest external frequency of 300 kHz (see details in
Switching frequency is defined by resistor R3 connected to FSET pin:
f
= 67600/(R
SW
where
•
f
- switching frequency, kHz
SW
•
R
FSET
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
20
40
By default, jumper J4 is installed on the board connecting R3 = 160 kΩ to the FSET pin which sets the
boost at 400 kHz. An external resistor can be connected to change the switching frequency: remove
jumper J4, short pins 7 and 8 on jumper J17, and connect the external resistor to connector J18. Boost
internal parameters will be adjusted automatically according to chosen frequency during start-up.
7.2
Spread Spectrum and Synchronization
The SYNC pin is used to enable boost spread spectrum or, alternatively, to synchronize boost switching
with an external frequency. To disable spread spectrum functionality this pin should be connected to
ground; to enable spread spectrum SYNC should be connected to VDDIO/EN. Selection is done with
jumper J3. When spread spectrum is enabled, boost switching frequency is modulated ±3% from set
frequency with 1.875 kHz modulation frequency.
If an external frequency is detected at SYNC pin, the LP8861-Q1 starts to synchronize boost with this
frequency. Frequency should be higher than the switching frequency set by R
1.2 V to 1.5 V × f
SW SET
10
LP8861-Q1EVM Evaluation Module
+6.4)
FSET
– frequency setting resistor, kΩ
60
80
100
Figure 6. Switching Frequency vs R
.
Copyright © 2015, Texas Instruments Incorporated
120
140
160
R
(k
)
FSET
FSET
www.ti.com
Section
7.2).
180
200
220
C010
resistor and be in range
FSET
SNVU456 – July 2015
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