The control scheme of the LM53600-Q1 and LM53601-Q1 devices allows this part to operate under a wide range of conditions with a low number of external components. Peak current mode control allows a wide range of input voltages and output capacitance values, while maintaining a constant switching frequency. Stable operation is maintained while output capacitance is changed during operation as well. This allows use in systems that require high performance during load transients and which have load switches which remove loads as system operating state changes. Short minimum on- and off-times ensure constant frequency regulation over a wide range of conversion ratios. These on- and off- times allow for a duty factor window of 13% to 77% at 2.1-MHz switching frequency.
This architecture uses frequency foldback in order to achieve low dropout voltage maintaining output regulation as the input voltage falls close to output voltage. The frequency foldback is smooth and continuous, and activated as off-time approaches its minimum. Under these conditions, the LM53600-Q1 and LM53601-Q1 devices operate much like a constant off-time converter allowing maximum duty cycle to reach 97%, which allows output voltage regulation with 600-mV dropout.
If input voltage exceeds approximately 21 V, frequency is reduced smoothly as a function of input voltage. This frequency reduction allows output voltage regulation and current mode control to operate with duty factor below 13%. Since current mode control continues at high input voltage insensitivity to output capacitance is maintained. This form of fold back will not be active if input voltage is below 18 V, insuring constant frequency operation over normal automotive operating conditions.
High input voltage foldback has two settings; see FSW under 36-V input conditions for detail. Since adjustable output voltage versions fold back under high input voltage conditions as though output voltage were 3.3 V, larger inductance and output capacitance is required if an adjustable device is used with output voltage above 4.2 V. If a 4.7-µH inductor is used in system with greater 4.2-V output using an adjustable device, the converter remains stable but may not achieve full output current when operating at high input voltages, such as 36 V, due to excessive inductor current ripple.
As load current is reduced, the LM53600-Q1 and LM53601-Q1 devices transition to light load mode if SYNC/MODE is low. In this mode, diode emulation is used to reduce RMS inductor current and switching frequency is reduced. Also, fixed voltage versions do not need a voltage divider connected to FB saving additional power. As a result, only 23 µA (typical, while converting 13.5 V to 3.3 V) is consumed to regulate output voltage if output is unloaded. Average output voltage increases slightly while lightly loaded.