200
400
600
800
1000
1200
1400
1600
1800
2000
2200
20
40
60
80
100
120
140
160
180
200
220
f
S
W
(
k
Hz
)
R
FSET
(k
)
C010
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
SW
= 67600/(R
FSET
+6.4)
where
•
f
SW
- switching frequency, kHz
•
R
FSET
– frequency setting resistor, k
Ω
(1)
Figure 6. Switching Frequency vs R
FSET
By default, jumper J4 is installed on the board connecting R3 = 160 k
Ω
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
FSET
resistor and be in range
1.2 V to 1.5 V × f
SW SET
.
10
LP8861-Q1EVM Evaluation Module
SNVU456 – July 2015
Copyright © 2015, Texas Instruments Incorporated