this mode, the majority of the internal circuitry (excluding that necessary to maintain regulation) in the device is turned off

to save current. Under no load and with skip mode enabled, the device typically draws 3.5μA for the 3.3V parts, and 6μA

for the 5.0V parts. For load currents > 5mA, the device enters normal skip mode and still maintains very high efficiency.

Output-Voltage Overshoot Protection

In dropout, the output voltage closely follows the input voltage, but is below the regulation point. The device runs at

maximum duty cycle to satisfy the loop, and the internal error-amplifier output is railed high. When the input voltage rises

above the output, the device comes out of dropout, but the internal error-amplifier output takes some time to get back to

steady state. This causes an overshoot in the output voltage. To limit this overshoot, the device clamps the output of the

error amplifier while coming out of dropout, causing it to discharge faster and limiting the output-voltage overshoot. The

actual value of the overshoot depends on the output capacitor, inductor, and load.

Controlled EMI with Forced-Fixed Frequency

In FPWM mode, the device attempts to operate at a constant switching frequency for all load currents. For tightest

frequency control, apply the operating frequency to SYNC. The advantage of FPWM is a constant switching frequency,

which improves EMI performance; the disadvantage is that considerable current can be thrown away. If the load current

during a switching cycle is less than the current flowing through the inductor, the excess current is diverted to AGND.

Extended Input Voltage Range

In some cases, the device is forced to deviate from its operating frequency, independent of the state of SYNC. At high

input voltages above 18V (especially for 2.1MHz operation), the required on-time to regulate its output voltage may be

smaller than the minimum on-time (65ns, typ). In this event, the device is forced to lower its switching frequency by

skipping pulses. If the input voltage is reduced and the device approaches dropout, it continuously tries to turn on the

HSFET. To maintain gate charge on the HSFET, the BST capacitor must be periodically recharged. To ensure proper

charge on the BST capacitor when in dropout, the HSFET is turned off every 20μs and the LSFET is turned on for

approximately 200ns. This gives an effective duty cycle of > 99%, and a switching frequency of 50kHz when in dropout.

Spread-Spectrum Option

Each device has an optional spread spectrum enabled by the SPS pin. If SPS is pulled high, the internal operating

frequency varies by ±3% relative to the internally generated 2.1MHz (typ) operating frequency. Spread spectrum is

offered to improve EMI performance of the device. The internal spread spectrum does not interfere with the external clock

applied on the SYNC pin. It is active only when the device is running with an internally generated switching frequency.

Power-Good (PGOOD)

Each device features an open-drain power-good output. PGOOD is an active-high output that pulls low when the output

voltage is below 92% (typ) of its nominal value. PGOOD is high impedance when the output voltage is above 93% (typ)

of its nominal value. Connect a 20kΩ (typ) pullup resistor to an external supply, or to the on-chip BIAS output.

Overcurrent Protection

Each device limits the peak output current to 3.5A (typ) for 2.1MHz switching frequency parts and 4.7A (typ) for the

400kHz switching frequency parts. The accuracy of the current limit is ±12%, making selection of external components

overcurrent event is detected. The device attempts a soft-start restart every 7ms and stays off if the short circuit has not

been removed. When the current limit is no longer present, it reaches the output voltage by following the normal soft-start

sequence. If the device’s die reaches the thermal limit of 175°C (typ) during the current-limit event, it immediately shuts

off.

Thermal-Overload Protection

Each device features thermal-overload protection. The device turns off when the junction temperature exceeds +175°C

(typ). Once the device cools by 15°C (typ), it turns back on with a soft-start sequence.

MAX25232

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