40 NXP Semiconductors
22XS4200
FUNCTIONAL DEVICE OPERATION
Open load detection in on state (OL_ON)
Open load in ON state detection (OLON) is performed continuously during the On state for CSR0 over the ranges specified in section
Diagnostic features. An open load in On state fault is detected when the load current is lower than the open load current threshold IOLD(ON).
This happens at IOLD(ON) = 150 mA (typ.) for high-current sense mode (CSR0), and at 7.0 mA (typ.) for Low-current mode. FSB is asserted
low and the OLON bit in the fault register is set to 1 but the channel remains On. FSB goes high as soon as disappearance of the failure
cause is detected, but the OL_ON bit remains set.
In High-current mode (CSR0), open load in On state detection is done continuously during the On state and the OLON-bit remains set
even if the fault disappears.
In High-current mode, the OLON-bit is cleared when the FAULTR register is read during the Off state, even if the fault hasn't disappeared.
The OLON-bit is also cleared when the FAULTR register is read during the ON state, provided the failure cause (load disconnected) has
disappeared.
In Low-current mode (CSR1), OL_ON is done periodically instead of continuously and only operates when fast slew rate is selected. When
the internal PWM module is used with an internal or external clock (case 1), the period is 150 ms (typ.). When the direct inputs are used
(case 2), the period is that of the input signal. The detection instants in both cases are given by the following:
1. In internal PWM (int./ext. clock), Low-current mode (CSR1), open load in ON state detection is not performed each switching
period, but at a fixed frequency of about 7.0 Hz (each tOLLED =150 ms typ.). The function is available for a duty cycle of 100%.
OLON detection is also performed at 7.0 Hz, at the first turn-off event occurring 150 ms after the previous OL_ON detection event
(before OS and OL_OFF).
2. In direct input, Low-current mode (CSR1), OL_ON is performed each switching period (at the turn-off instant) but the duty cycle is
restricted to the values. Consequently, when the signal on the IN[x] pin has a duty cycle of 100%, OL_ON is not performed. To
solve this problem, either the internal PWM function must be activated with a duty cycle of 100%, or the channel’s direct input must
be disabled by setting Dir_dis_s=1 (bit D5 of the CONFR-s register). The OLON-bit is only reset when the FAULTR register is read
after occurrence of an OL_ON detection event without fault presence.
Open load detection in discontinuous conduction mode
If small inductive loads (solenoids / DC motors) are driven at low frequencies, discontinuous conduction mode may occur. Undesired open
load in On state errors may then be detected, as the inductor current needs some time to rise above the open load detection threshold
after turn-on. This problem can be solved by increasing the switching frequency or by disabling the function and activating open load in
Off state detection instead.
When small DC motors are driven in discontinuous conduction mode, undesired open load in Off state detection may also occur when the
load current reaches 0.0 A during the Off state. This problem can be solved by increasing the switching frequency or by enabling open
load in Off state detection only during a limited time, preferably directly after turn-off (see Diagnostic features). The signal on the SYNC
pin can be used to identify the turn-off instant.
Current and temperature sensing
The scaled values of either of the output currents or the temperature of the device’s GND pins (8 and 25) can be made available at the
CSNS pin. To monitor the current of a particular channel or the general device temperature, the CSNS0_en and CSNS1_en bits (see
Table 23) in the General Configuration Register (GCR) must be set to the appropriate values. When overcurrent windows are active,
current sensing is disabled and the SYNCB pin remains high.
Instantaneous and sampled current sensing
The device offers two possibilities for load current sensing: instantaneous (synchronous) sensing mode and track & hold mode (see
Figure 9). In synchronous mode, the load current is mirrored through the current sense pin (Output current monitoring (CSNS)) and is
therefore synchronous with it. After turn-off, the current sense pin does not output the channel current. In track & hold mode however, the
current sense pin continues to mirror the load current as it was just before turn-off. Synchronous mode is activated by setting the T_H_en
bit to 0, and Track & Hold mode by setting the T_H_en bit to 1.
Current sense ratio selection
The load current is mirrored through the CSNS pin with a sense ratio (Figure 17) selected by the CSNS_ratio bit in the OCR register. To
achieve optimal accuracy at low current levels, the lower current sensing ratio, called CSR1, must be selected. In that case, the
overcurrent threshold levels are decreased. The best accuracy that can be obtained for either ratio is shown in Figure 19. The amount of
current the CSNS pin can sink is limited to ICSNS,MAX..The CSNS pin must be connected to a pull-down resistor (470 Ω < R(CSNS)
<10 kΩ, 1.0 kΩ typical), in order to generate a voltage output. A small low-pass filter can be used for filtering out switching transients
(Figure 22). Current sensing operates for load currents up to the lower overcurrent threshold (OCLx A).