500mV to 5.5V Input, Ultrasonic

Boost Converter with Short-Circuit

Protection and True Shutdown

MAX18002

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Analog Devices | 13

Ultrasonic Mode (USM)

The MAX18002 features a USM in which the converter seamlessly operates at a switching frequency of 29kHz (typical)

to stay away from acoustic audible noise interference while operating at light loads. The USM circuit operates by clamping

the minimum switching frequency.

When the load current reduces, the switching frequency goes down until it reaches 29kHz (typical). USM is turned on at

this stage. In USM, the MAX18002 switches at 29kHz (typical) until the output voltage reaches 2% above the output

voltage target. The device then switches at about 29kHz (typical) and keeps the boost discharge cycle on even after the

inductor current crosses 0A resulting in the output capacitor discharging to the input. This extra cycle is necessary to

prevent the output voltage from ramping up. The MAX18002 overregulates to 5% above the output voltage target when

the part operates in no load.

VOUT

ILOAD

FUSM

VOUT_TARGET

1.02 x VOUT_TARGET

FUSM

ILX

USM OPERATION

LOAD DE PENDANT

REGULATION POINT

Figure 3. USM Operation

Thermal Shutdown

When the junction temperature exceeds TSHUT_R (+165°C typical), the converter is turned off until the temperature drops

to TSHUT_F (+150°C typical) after which the part starts up again. If the fault condition persists, power is cycled on and off

until the fault is cleared.

Over-Current Protection

The MAX18002 features a cycle-to-cycle peak current limit of 3.6A (typical) to protect the IC and the system from overload

conditions. When the IC is in boost mode and the inductor current hits the current limit, the pMOS is turned on to start a

discharge cycle and bring the inductor current down.

When the IC hits current limit in pass-through mode, the pMOS turns off to protect the part causing the inductor current

to fall to zero. The part then goes into pMOS slew mode gradually causing the inductor current to rise and bring output

back to the VIN level.

Short-Circuit Protection

When the MAX18002 is hard shorted and the output voltage falls below 0.5V, it enters hard-short state. In the hard-short

condition, the MAX18002 stops switching and pMOS switch limits short-circuit current from the input to 0.7A (typical).

If the fault persists, the heat generated in the pMOS switch due to the short-circuit current may cause the part to enter

thermal shutdown.

When the fault is removed, the MAX18002 goes into pMOS slew until VIN = VOUT. After the pMOS slew is complete, the

MAX18002 skips the boost slew phase (that is observed during start-up) and enters the boost mode of operation. This

operation could lead to an overshoot in output voltage of about 10% (typical) when VIN and VOUT are close to each other.