Datasheet

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I
SKIP
+ 66 mA )
V
IN
160 W
(5)
I
PEAK
+ 66 mA )
V
IN
80 W
(6)
PFM Mode at Light Load
Comparator High
Comparator Low
Comparator Low 2
PWM Mode at Medium to Full Load
1.6%
0.8%
V
OUT
Dynamic Voltage Positioning
Soft-Start
bq25010
bq25011
bq25012
SLUS615B – DECEMBER 2004 – REVISED JULY 2007
During the power save mode, the output voltage is monitored with the comparator by the thresholds comp low
and comp high. As the output voltage falls below the comp low threshold (set to typically 0.8% above VOUT
nominal), the P-channel switch turns on. The P-channel switch is turned off as the peak switch current is
reached. The typical peak switch current can be calculated as:
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 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 μ A 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 μ F
and still have a low absolute voltage drop during heavy load transient changes. Refer to Figure 7 as well for
detailed operation of the power save mode.
Figure 7. 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.
As described in the power save mode operation section and as detailed in Figure 7 , 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.
The bq2501x 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 30 mA, 60 mA, 120 mA and
then the typical switch current limit of 230 mA. Therefore the startup time depends mainly on the output capacitor
and load current. Typical startup time with a 10- μ F output capacitor and a 100-mA load current is 1.6 ms.
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