Integrated analog current sense functionality allows the device to measure and feedback operating parameters including load current. High accuracy and fast response of the analog sense circuit enables closed-loop control of the high side switch. TPS1AH08-Q1 is one HSS that offers analog sense output that can measure supply voltage, output current, and IC temperature, and will be used as an example in this section.

The analog sense circuit converts its measured parameter into a current according to the specified transfer function. The transfer function of TPS1HA08 is shown in Table 4-1.

The sense current I_sns is converted to an analog voltage by the external R_sns resistor and then passed on to an ADC, either discrete or internal to a microcontroller. Designers should select the value of R_sns based on the input range and resolution of the ADC, as well as the possible range of the particular parameter or parameters (if multiplexed) the HSS will see. The tolerance of R_sns should also be considered as it has a direct impact on total analog sense accuracy.

For TPS1HA08-Q1, the analog sense outputs a current I_SNSFH ~7 mA when there is a fault detected. Device specifications for TPS1HA08-Q1 set the maximum steady-state output current at 10 A, though the analog sense is capable of measuring current higher than this. SNS output value should be clamped (e.g. by Zener diode) to avoid exceeding the input range of the ADC if output current of the high side switch jumps unexpectedly.

In a example application, the designer needs the ability to collect current, voltage, and temperature measurements. This fictitious engineer targets 50-mA output current resolution. The supply voltage for logic is 3.3 V, so AVDD 3.3-V supply is assumed for the converter as well. The system limits along with the corresponding sense currents are listed.

• Peak output current: I_OUT = 5 A, I_SNS = 1.10 mA
• Peak Temp: Tj=150°C, I_SNS = 2.25 mA
• Peak Supply Voltage: V_vs = 15 V, I_SNS = 1.29 mA
• SNS Fault current: I_SNSFH = 6.9 mA

We can determine the maximum feasible value of R_sns possible based on the input range of the ADC and the SNS fault current, which is the absolute maximum. In order to minimize the resolution, the max SNS voltage range is set 10% under the input range of the single-ended ADC and a Zener clamps the fault current I_SNSFH to 3.3 V. Without the Zener, the ADC could be damaged as the input voltage would far exceed the device limits. The 300-mV difference allows plenty of headroom to prevent false fault detection.

The analog sense output is gated by both EN and DIAG_EN. The requirement of DIAG_EN allows a controller to multiplex analog sense outputs from several HSS onto a single ADC. After a rising edge of EN, the sense output current, I_SNS, settles after a certain time. For TPS1HA08, the maximum settling time of I_SNS is 180 µs. Since PWM operation implies that EN is constantly driven high or low, the sense settling time affects every switching cycle ON period where diagnostics are enabled.

A low pass is recommend to filter transients or interference on the analog sense output voltage. In TPS1AH08-Q1, a 10 - 15-kΩ resistor R_PROT is suggested to protect the HSS pins. Together with filter capacitor, C_SNS , the RC filter is formed. If there is a long lead or trace from the HSS sense pin to the ADC input, the sense resistor and RC filter should be placed close to the ADC prevent interference.

The transfer function of the sense current to the voltage at the ADC input is

The sense LPF 3dB cutoff frequency is then

R_PROT and C_SNS should be selected such that the f_pwm≪ f_c. By a rule of thumb, setting the cutoff frequency at least one decade above the PWM frequency ensures good input/output matching and no attenuation.

For example, if operating at 1-kHz PWM frequency, we can find a suitable C_SNS value for a fixed R_PROT of 10 kΩ. By using a rule of thumb we set our target cutoff frequency for the LPF at f_c ≥ 10· f_PWM. From there,

From there we found an acceptable value of C_SNS that ensures sufficient bandwidth of the LPF as not to attenuate the SNS signal which is at PWM frequency. The RC filter values can of course be tuned if the designer wishes to filter certain frequencies by the discrete RC filter rather than in the software domain. If a designer wishes to do all filtering digitally within the ADC/controller, a sense LPF does not need to be used.