BQ25015 Datasheet, PDF (15/28 Pages) Texas Instruments – SINGLE-CHIP CHARGER AND DC/DC CONVERTER IC FOR PORTABLE APPLICATIONS

bq25015
bq25017
www.ti.com
SLUS721A – DECEMBER 2006 – REVISED MARCH 2007
The N-channel rectifier is turned on and the inductor current ramps down. As the inductor current approaches
zero the N-channel rectifier is turned off and the P-channel switch is turned on again starting the next pulse. The
converter continues these pulses until the comp high threshold (set to typically 1.6% above VOUT nominal) is
reached. The converter enters a sleep mode, reducing the quiescent current to a minimum. The converter
wakes up again as the output voltage falls below the comp low threshold again. This control method reduces the
quiescent current to typically to 15 µA and the switching frequency to a minimum, thereby achieving high
converter efficiency. Setting the skip current thresholds to typically 0.8% and 1.6% above the nominal output
voltage at light load current results in a dynamic output voltage achieving lower absolute voltage drops during
heavy load transient changes. This allows the converter to operate with a small output capacitor of only 10 µF
and still have a low absolute voltage drop during heavy load transient changes. Refer to Figure 16 as well for
detailed operation of the power save mode.
1.6%
0.8%
V_OUT
PFM Mode at Light Load
Comparator High
Comparator Low
Comparator Low 2
PWM Mode at Medium to Full Load
Figure 16. Power Save Mode Thresholds and Dynamic Voltage Positioning
The converter enters the fixed-frequency PWM mode again as soon as the output voltage drops below the comp
low 2 threshold.
Dynamic Voltage Positioning
As described in the power save mode operation section and as detailed in Figure 16, the output voltage is
typically 0.8% above the nominal output voltage at light load currents as the device is in power save mode. This
gives additional headroom for the voltage drop during a load transient from light load to full load. During a load
transient from full load to light load the voltage overshoot is also minimized due to active regulation turning on
the N-Channel rectifier switch.
Soft-Start
The bq25015/7 has an internal soft-start circuit that limits the inrush current during startup. This soft-start is
implemented as a digital circuit increasing the switch current in steps of typically 60 mA, 120 mA, 240 mA and
then the typical switch current limit of 480 mA. Therefore the starup time depends mainly on the output capacitor
and load current. Typical startup time with a 10-µF output capacitor and a 100-mA load current is 1.6 ms.
100% Duty Cycle Low Dropout Operation
The bq2501 offers a low input-to-output voltage difference while still maintaining operation with the use of the
100% duty cycle mode. In this mode the P-channel switch is constantly turned on. This is particularly useful in
battery-powered applications to achieve longest operation time by taking full advantage of the whole battery
voltage range. The minimum input voltage to maintain regulation depends on the load current and output voltage
and can be calculated as:
ǒ Ǔ _{VIN(min) + VOUT(max) ) IOUT(max) RDS(on)MAX ) RL}
(7)
where
I_OUT(max) = maximum output current plus indicator ripple current
R_DS(on)MAX = maximum P-channel switch R_DS(on)
R_L = DC resistance of the inductor
V_OUT(max) = nominal output voltage plus maximum output voltage tolerance
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