Charge Pump Frequency Control for NMOS Gate Drive
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Solution Overview
Problem
In battery/power management applications, using NMOSFETs as high side switches can result in excessive driving voltage when battery voltage is high, potentially damaging the switch and reducing power efficiency due to the need for additional voltage clamps to limit the voltage.
Innovation Solution
A circuit with a charge pump and feedback circuit that adjusts the operating frequency of the charge pump based on the control voltage, allowing for a predetermined target voltage to be maintained, thereby reducing power consumption and improving efficiency by dynamically controlling the switch's turn-on state.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If NMOSFET is used as high side switch to reduce cost, then device cost is reduced, but driving voltage may become too high and break down the switch when battery voltage is high
Solution Approach 1:
The charge pump operates at adjustable frequencies to dynamically adapt to varying battery voltages. When battery voltage is low, the charge pump runs at higher frequency to provide sufficient driving voltage. When battery voltage is high, the frequency is reduced to prevent excessive voltage that could break down the NMOSFET, thus maintaining switch safety while using the lower-cost NMOSFET device
Solution Approach 2:
The system changes the operating frequency parameter of the charge pump based on battery voltage conditions. By adjusting this parameter, the system optimizes the driving voltage output to match the actual needs of the NMOSFET gate, preventing both insufficient turn-on at low battery voltage and excessive voltage breakdown at high battery voltage
2Reliability
If voltage clamp is added to limit driving voltage to predetermined value, then switch safety is improved, but power efficiency is degraded due to extra power consumption
Solution Approach 1:
The system employs feedback control where the charge pump's output voltage is monitored and the operating frequency is adjusted accordingly. When the driving voltage approaches the predetermined maximum value, the feedback mechanism reduces the charge pump frequency to maintain voltage within safe limits. This active feedback control replaces the passive voltage clamp, achieving the same voltage limiting function without the continuous power consumption penalty of a clamp circuit
Solution Approach 2:
The invention extracts and removes the voltage clamp component from the system. Instead of using a voltage clamp to limit the driving voltage, the system uses frequency-adjustable charge pump control to achieve the same voltage limiting effect, thereby eliminating the extra power consumption associated with the voltage clamp while maintaining switch safety
3Speed
If charge pump operates at high frequency to quickly turn on switch, then turn-on speed is improved, but power consumption increases
Solution Approach 1:
The charge pump uses periodic switching action at optimized frequencies to transfer charge to the NMOSFET gate. By using periodic charge transfer rather than continuous high-frequency operation, the system achieves fast turn-on when needed while allowing the frequency to be reduced when the switch is already fully on or when battery voltage is high, thereby reducing overall power consumption while maintaining fast response capability
Data Source
AI summary
A circuit includes a charge pump and a feedback circuit. The charge pump coupled to a switch provides a control signal to the switch. The feedback circuit coupled to the charge pump receives the control signal and adjusts an operating frequency of the charge pump based upon the control voltage. The control voltage is adjusted to a predetermined target voltage by adjusting the operating frequency through the feedback circuit.