Texas Instruments BQ25123, Manual

Page: 26 / 72

2 MHz. If the load current decreases, the converter enters a power save mode to maintain high efficiency down
to light loads. In this mode, the device generates a single switching pulse to ramp up the inductor current and
recharge the output capacitor, followed by a sleep period where most of the internal circuits are shut down to
achieve a low quiescent current. The duration of the sleep period depends on the load current and the inductor
peak current. For optimal operation and maximum power delivery allow V
PMID
> V
SYS
+ 0.7 V.
The output voltage is programmable using the SYS_SEL and SYS_VOUT bits in the SYS VOUT control register.
The SW output is enabled using the EN_SYS_OUT bit in the register. This bit is for testing and debug only and
not intended to be used in the final system. When the device is enabled, the internal reference is powered up
and the device enters softstart, starts switching, and ramps up the output voltage. When SW is disabled, the
output is in shutdown mode in a low quiescent state. The device provides automatic output voltage discharge so
the output voltage will ramp up from zero once the device in enabled again. Once SYS has been disabled, either
V
IN
needs to be connected or the MR button must be held low for the t
RESET
duration to re-enable SYS.
The output is optimized for operation with a 2.2-µH inductor and 10-µF output capacitor. Table 9-6 shows the
recommended LC output filter combinations.
Table 9-6. Recommended Output Filter
INDUCTOR VALUE (µH) OUTPUT CAPACITOR VALUE (µF)
4.7 10 22
2.2 Possible Recommended Possible
The inductor value affects the peak-to-peak ripple current, the PWM-to-PFM transition point where the part
enters and exits Pulse Frequency Modulation to lower the power consumed at low loads, the output voltage
ripple and the efficiency. The selected inductor must be selected for its DC resistance and saturation current.
The inductor ripple current (ΔI
L
) can be estimated according to Equation 7 .
ΔI
L
= V
SYS
x (1-(V
SYS
/V
PMID
))/(L x f)
(7)
Use Equation 8 to calculate the maximum inductor current under static load conditions. The saturation current of
the inductor should be rated higher than the maximum inductor current. As the size of the inductor decreases,
the saturation “knee” must be carefully considered to ensure that the inductance does not decrease during
higher load condition or transient. This is recommended because during a heavy load transient the inductor
current rises above the calculated value. A more conservative way is to select the inductor saturation current
above the high-side MOSFET switch current.
I
L
(max) = I
SYS
(max) + ΔI
L
/ 2
(8)
Where
• F = Switching Frequency
• L = Inductor Value
• ΔI
L
= Peak-to-Peak Inductor Ripple Current
• I
L
(max) = Maximum Inductor Current
In DC/DC converter applications, the efficiency is affected by the inductor AC resistance and by the inductor
DCR value.
Table 9-7 shows recommended inductor series from different suppliers.
Table 9-7. Inductor Series
INDUCTANCE (µH) DCR (Ω)
DIMENSIONS
(mm
3
)
INDUCTOR TYPE SUPPLIER
(1)
COMMENT
2.2 0.300 1.6 x 0.8 x 0.8 MDT1608CH2R2N TOKO Smallest size, 75mA max
2.2 0.170 1 .6 x 0.8 x 0.8 GLFR1608T2R2M TDK Smallest size, 150mA max
2.2 0.245 2.0 x 1.2 x 1.0 MDT2012CH2R2N TOKO Small size, high efficiency
2.2 0.23 2.0 x 1.2 x 1.0 MIPSZ2012 2R2 TDK
2.2 0.225 2.0 x 1.6 x 1.0 74438343022 Wurth
BQ25123
SLUSCZ6A – JANUARY 2018 – REVISED MAY 2021
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