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7
Boost Converter
7.1
Switching Frequency
Switching frequency for boost is defined as 300 kHz to 2.2 MHz. However, frequency can be set from 250
kHz to 2.2 MHz with the external resistor connected to FSET pin, thus allowing the LP8862-Q to be
synchronized with an external frequency of 300 kHz (see details in
Switching frequency is defined by resistor R3 connected to FSET pin:
ƒ
= 67600/(R
SW
where
•
ƒ
- 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 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.
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 and 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 device starts to synchronize boost with this
frequency. Frequency should be higher than the switching frequency set by R
1.2...1.5 × ƒ
.
SW SET
SNVU478A – July 2015 – Revised October 2015
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+ 6.4)
FSET
– frequency setting resistor, kΩ
60
80
100
Figure 6. Switching Frequency vs R
Copyright © 2015, Texas Instruments Incorporated
Section
120
140
160
R
(k
)
FSET
FSET
LP8862-Q1EVM Evaluation Module
Boost Converter
7.2).
180
200
220
C010
resistor and be in range
FSET
(1)
9
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