BQ25120_15 Datasheet, PDF (26/68 Pages) Texas Instruments – bq25120 700-nA Low IQ Highly Integrated Battery Charge Management Solution for Wearables and IoT

BQ25120
SLUSBZ9A – AUGUST 2015 – REVISED AUGUST 2015
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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
3 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.
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. 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. The recommended LC
output filter combinations are:
INDUCTOR VALUE (µH)
2.2
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 has to be rated for its DC resistance and saturation current. The
inductor ripple current (ΔI_L) decreases with higher inductance and increases with higher V_IN or V_OUT and can be
estimated according to Equation 7.
ΔI_L = V_OUT x (1-(V_OUT/V_IN))/(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 the select the inductor saturation current above the high-
side MOSFET switch current.
I_L(max) = I_OUT(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. Increasing the inductor fault produces lower RMS currents, but degrades transient response.
The following inductor series from different suppliers are recommended:
INDUCTANCE (µH)
2.2
2.2
2.2
2.2
DCR (Ω)
0.300
0.170
0.245
0.23
26
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Table 6. Inductor Series
DIMENSIONS
(mm³)
1.6 x 0.8 x 0.8
1 .6 x 0.8 x 0.8
2.0 x 1.2 x 1.0
2.0 x 1.2 x 1.0
INDUCTOR TYPE
MDT1608CH2R2N
GLFR1608T2R2M
MDT2012CH2R2N
MIPSZ2012 2R2
SUPPLIER
TOKO
TDK
TOKO
TDK
COMMENT
Smallest size, 75mA max
Smallest size, 150mA max
Small size, high efficiency
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