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I made an hbridge using TPS2836 and CSD16404. The schematic is shown below. It does not work properly and I have a few questions about it. It would be great if you can point out where the design is wrong.
1. I gave a VDD-GND of 5V and tried to give the voltages 3.3V (signifying a logical 1) and 0V (signifying a logical 0) to the inputs of the two mosfet drivers. When I try to turn on the left-high side mosfet, the voltage at it's source is 4.5V with no load attached to the driver. The current drawn by the whole circuit is also almost 300 mA. Simultaneously, if the right bottom mosfet is turned on, the voltage at it's drain is not zero but something around 1.5V. Note that I am not giving a PWM to the inputs of the mosfet driver but direct voltage levels. Should there be a pulse train coming in to let the charge pump function properly and fully turn on the mosfet? The voltage at the gate of left-high mosfet is 5.5V which is pretty low, when the CSD16404 turns on only at VGS = 1.8V.
2. If I try to turn on the right top mosfet and the left-bottom mosfet, the whole circuit draws some 1A and things begin to heat up! It could be that the lower-right mosfet is smoked while / after soldering right?
3. Are the resistors between the mosfet driver output and the mosfet gate (not used in this circuit) essential for the functioning? They are only used to control the turn on times right?
1) The CSD16404Q5A must be driven with at least 4.5 volt Vgs to guarantee the Rds(on) in the on state. The low side MOSFET needs to be pulsed to charge the high side bootstrap circuit with these types of MOSFET drivers.
2) I am not sure what is causing your issue but timing will be important in creating a H-bridge with 2 independent drivers.
3) The resistors are used to slow both the turn on and turn off of the MOSFETs.
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The TPS2836 driver uses a floating high-side driver. The high-side driver "floats" on the BOOTLO voltage (also called the Switch Node or Phase Node) such that the driver is always seeing the source voltage of the high-side MOSFET. This driver is powered from the BOOT pin by the BOOTLO to BOOT capacitor (C1 and C2) which are charged through an internal diode through, however if the BOOTLO node is high impedance (low-side drive has not been turned on) the capacitors will not charge properly. It is best to charge these capacitors by turning on the low-side driver (LOWDR) for a short period initially to charge these capacitors.
Note: C1 and C2 are extremely large for this application. This capacitor need only be 0.1uF - 0.22uF. Larger capacitors in these locations offer very little benefit, but can slow their initial charge rate and result in start-up issues.
It's possible that the low-side device is damaged. Check the resistance from both BOOTLO pins to GND when the circuit is unpowered. The internal pull-down resistors should force both MOSFETs off, so one would expect several MegOhms of resistance.
Gate Drive Resistors are intended to limit the gate drive current and the ramp-rate of the gate drive and switching node (BOOTLO) they are not required in most applications, though they can be desirable to limit ringing on the switching node and gate drive or to reduce the high-frequency content of the rising edge of either node.
Peter James Miller
New Product Development Engineer
Power Management - DC/DC Solutions - High Performance Solutions
Member Group Technical Staff 2008
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