BQ25570_14 Datasheet, PDF (22/41 Pages) Texas Instruments – Ultra Low Power Harvester Power Management IC with Boost Charger, and Nano-Powered Buck Converter

bq25570
SLUSBH2C – MARCH 2013 – REVISED JANUARY 2014
DETAILED DESCRIPTION
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Boost Charger Overview
The bq25570 includes both an ultra low quiescent current, efficient synchronous boost charger and buck
converter. The boost charger is intended to be powered from a high impedance DC source, such as a solar
panel, TEG or piezoelectric module; therefore, it regulates its input voltage (VIN_DC) in order to prevent the input
source from collapsing. The boost charger monitors its output voltage (VSTOR) and stops switching when
VSTOR reaches a resistor programmable threshold level. The buck converter is powered from VSTOR. Both
converters are based on a switching regulator architecture which maximizes efficiency while minimizing start-up
and operation power. Both use pulse frequency modulation (PFM) to maintain efficiency, even under light load
conditions. In addition, the boost charger implements battery protection features so that either rechargeable
batteries or capacitors can be used as energy storage elements at the storage element output (VBAT). Figure 6
is a high-level functional block diagram which highlights most of the major functional blocks inside the bq25570.
Enable Controls
There are two enable pins for the bq25570 in order to maximize the flexibility of control for the system. EN high
voltage is relative to VBAT. VOUT_EN high voltage is relative to VSTOR. When taken high (relative to VBAT),
the EN pin shuts down the IC completely including the boost charger, battery management circuitry and buck
converter. It also turns off the PFET that connects VBAT to VSTOR. This mode can be described as ship mode,
because it will put the IC in the lowest leakage state and provide a long storage period without discharging the
battery attached to VBAT. If it is not desired to control EN, it is recommended that this pin be tied to VSS, or
system ground. When EN is low, VOUT_EN is used to enable and disable the buck converter. The table below
summarizes the functionailty.
EN PIN
LOGIC
LEVEL
0
0
1
Table 1. Enable Functionality Table
VOUT_EN
PIN LOGIC
LEVEL
FUNCITONAL MODE
0
Buck standby mode. Boost charger and VBAT_OK are enabled. Buck converter is disabled.
1
Boost charger, buck converter and VBAT_OK enabled.
x
Ship mode. Boost charger, buck converter and VBAT_OK indication are disabled. (ship mode)
Startup Operation
The bq25570 has two circuits for boosting the input voltage, a low-power cold-start circuit, drawing power
excluxively from VIN_DC when ≥ VIN(CS), and the high efficiency main boost charger, with the bias rails drawing
power from VSTOR when ≥ VSTOR_CHGEN and the power stage drawing power from VIN_DC when ≥ VIN(DC)
minimum. When EN = 0 and VSTOR ≤ VSTOR_CHGEN, there are two options for charging the VSTOR
capacitor capacitor, CSTOR, to VSTOR_CHGEN for the main boost charger to turn on. The first option is to
allow the cold start circuit to charge VSTOR to VSTOR_CHGEN. Due to the body diode of the PFET connecting
VSTOR and VBAT, the cold start circuit must charge both the capacitor on CSTOR and the storage element
connected to VBAT up to VSTOR_CHGEN. When a rechargeable battery with an open protector is attached, the
charge time is typically short due to the minimum charge needed to close the FET. When large, discharged
super capacitors are attached, the charge time can be signficant. The second option is to connect a storage
element, charged above VSTOR_CHGEN, to VBAT. Assuming the voltages on VSTOR and VBAT are both
below 100mV, when a charged storage element is attached (i.e. hot-plugged) to VBAT, the IC turns on the
internal PFET between the VSTOR and VBAT pins for t_{BAT_HOT_PLUG} in order to charge CSTOR to
VSTOR_CHGEN. If a system load tied to VSTOR prevents the storage element from charging VSTOR within
t_{BAT_HOT_PLUG}, it is recommended to add an external PFET between the system load and VSTOR. An inverted
VBAT_OK signal can be used to drive the gate of this system-isolating PFET. Once the VSTOR pin voltage
reaches the internal under voltage threshold (VBAT_UV), the internal PFET stays on and the main boost charger
begins to charge the storage element if there is sufficient power available at the VIN_DC pin, as explained below.
If VSTOR does not reach VBAT_UV within 50ms, then the PFET turns off and the cold-start circuit turns on, also
as explained below.
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