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A high-accuracy trimmed band-gap voltage reference is used to regulate the battery charging voltage.
Float VADJ to set the battery voltage
VCSON = 4.2V × number of the cells
Connect VADJ to VREF to set 4.41V × number of cells
Connect VADJ to ground to set 3.99V × number of the cells
Battery charge voltages can also be set by connecting a resistor divider from VREF to ground.
V CELL = 0.175 x V VADJ + 3.99V
Setting the Battery Charge & Input Current Limit Setting the Battery Charge Current Limit The CHLIM input sets the maximum charging current. Setting the Input Current Limit The input current regulator limits the input current by reducing the charging current, when the input current exceeds the input current limit set by ACLIM.
AC/DC Adapter Detection AC Adapter Detection AC Adapter voltage is connected through a resistor divider to ACSET pin to detect AC power is available DC Adapter Detection DC Input is connected through a resistor divider to DCSET pin to detect the lower voltage DC power available. ISL6257
Specification LDO Regulator VDD provides a 5.0V supply voltage from the internal LDO regulator from DCIN and can deliver up to 30mA of current.
The ISL6257 features a low-power shutdown mode. Driving EN low shuts down the ISL6257.
In shutdown, the DC/DC converter is disabled, and VCOMP and ICOMP are pulled to ground.
Snubber Design ISL6257 operates in discontinuous current mode (DCM), when the load current is less than half the P-P current in the inductor, after the low side FET turns-off the phase voltage rings due to high impedance with both FETs off. A snubber circuit (resistor in series with capacitor) from phase node to ground can reduce the rings. PHASE 10V/div INDUCTOR CURRENT 1A/div UGATE 5V/div
Adapter Current Limit Control Loop ISL6257 will reduce the current to battery or output voltage to hold the adapter current at the limit set by ACLIM pin. A filter is added between RS1 and CSIP and CSIN to reduce the switching noise. The filter roll off frequency should be between the cross over frequency and the switching frequency.
External Components Selection Inductor Selection Output Capacitor Selection MOSFET Selection Practical inductor design is based on the inductor ripple current being ±15% to ±20% of the maximum operating DC current at maximum input voltage. Required inductance can be calculated using the below equation. This capacitor is used to absorb the high frequency switching ripple current and supply very high di/dt load transient. It is selected using the equation given. The high-side MOSFET must be able to dissipate the conduction losses plus the switching losses.
Typical Operating Performance LINE AND LOAD REGULATION IN NVDC MODE FREQUENCY RESPONSE OF THE LC OUTPUT FILTER