BQ25120A Datasheet, PDF (28/67 Pages) Texas Instruments – Low IQ Highly Integrated Battery Charge Management Solution for Wearables and IoT

bq25120A
SLUSD08 – MAY 2017
www.ti.com
9.3.21 Buck (PWM) Output
The device integrates a low quiscent current switching regulator with DCS control allowing high efficiency down
to 10-µA load currents. DCS control combines the advantages of hysteretic and voltage mode control. The
internally compensated regulation network achieves fast and stable operation with small external components
and low ESR capacitors. During PWM mode, it operates in continuous conduction mode, with a frequency up to
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 + 0.7V.
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 6 shows the
recommended LC output filter combinations.
INDUCTOR VALUE (µH)
2.2
Table 6. Recommended Output Filter
4.7
Possible
OUTPUT CAPACITOR VALUE (µF)
10
Recommended
22
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 7 shows recommended inductor series from different suppliers.
INDUCTANCE (µH)
2.2
2.2
DCR (Ω)
0.300
0.170
Table 7. Inductor Series
DIMENSIONS
(mm³)
1.6 x 0.8 x 0.8
1 .6 x 0.8 x 0.8
INDUCTOR TYPE
MDT1608CH2R2N
GLFR1608T2R2M
SUPPLIER ⁽¹⁾
TOKO
TDK
COMMENT
Smallest size, 75mA max
Smallest size, 150mA max
(1) See Third-party Products Disclaimer
28
Submit Documentation Feedback
Product Folder Links: bq25120A
Copyright © 2017, Texas Instruments Incorporated