PT7M7433 Datasheet, PDF (4/7 Pages) Pericom Semiconductor Corporation – Supervisory Circuit

PT7M7433/34/35
P Supervisory Circuit
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Function Description
The PT7M7433/34/35 are available with user adjustable monitoring.
The user sets the threshold voltages for the PT7M7433/34/35, which permit the user to select the hysteresis range, and
consequently the sensitivity of the system to noise. A wide hysteresis range prevents chattering that can result during battery
recovery or load removal. The PT7M7433/34/35 have single low-battery outputs.
The PT7M7433/34/35 combine a 615mV reference with two comparators, logic, and timing circuitry to provide the user with
information about the charge state of the power-supply batteries. The PT7M7433/34/35 monitor separate high-voltage and low-
voltage thresholds to determine battery status. The output can be used to signal when the battery is charged, when the battery is
low, and when the battery is empty. PT7M7433/34/35 are also available with thresholds determined using an external resistor-
divider (see Application information Resistor-Value Selection). The adjustable versions can monitor down to 0.62V, making them
suitable for monitoring single-cell alkaline, NiMH, and NiCd applications (V_CC ≥1.8V).
When the power-supply voltage drops below the specified low threshold, the low-battery output asserts (see Low-Battery Output).
When the voltage rises above the specified high threshold following a 90ms (min) time-out period, the low-battery output is
deasserted (see Threshold Monitoring). This ensures the supply voltage has stabilized before power-converter or microprocessor
activity is enabled.
Low-Battery Output
The low-battery outputs are available in active-low (LBO, push-pull and open-drain) and action-high (LBO open-drain)
configurations. Push-pull outputs are referenced to V_CC (for adjustable threshold versions).
The open-drain devices can be pulled to a voltage independent of V_CC or BATT, up to 5.5V. This allows the device to monitor and
operate from direct battery voltage while interfacing to higher voltage microprocessors.
The PT7M7433/34/35 single-output voltage monitors provide a single low-battery output, LBO/LBO. The PT7M7433/34
adjustable-threshold parts assert LBO/LBO when the LTHIN input drops below the V_LTHIN threshold (615mV).
Threshold Monitoring
The PT7M7433/34/35 HTHIN and LTHIN inputs are high-impedance inputs to comparators. An external resistor divider network
is required between the monitored voltage, HTHIN, LTHIN, and GND to select the desired thresholds (see Application
information Resistor-Value Selection). The PT7M7433/34/35 single-output voltage monitors assert LBO /LBO when LTHIN
drops below the internal LTHIN reference (615mV). LBO/LBO is deasserted when HTHIN rises above the internal HTHIN
reference level (615mV) for at least 90ms (see AC Electrical Characteristics: Fig 1).
Hysteresis
Hysteresis increases the comparator’s noise margin by increasing the upper threshold or decreasing the lower threshold (see blow).
The hysteresis prevents the output from oscillating (chattering) when V_CC is near the low-battery threshold. This is especially
important for applications where the load on the battery creates significant fluctuations in battery voltages.
Adjustable Hysteresis (PT7M7433/34/35)
Block diagram shows the correct connections for the external resistor-dividers. To adjust the low-battery threshold and hysteresis
connect resistor R1 between V_CC and LTHIN, resistor R2 between LTHIN and HTHIN, and R3 between HTHIN and GND. The
hysteresis for the PT7M7433/34/35 is determined by an external resistor-divider network (see Application information Resistor-
Value Selection).
12-07-0002
PT0213-4
07/05/12
4