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In hiccup mode, the device shuts down and attempts to soft start after tHIC. Hiccup mode helps reduce the

device power dissipation under severe overcurrent and short circuit conditions.

7.3.9.4 Thermal Shutdown

Thermal shutdown limits total power dissipation by turning off the internal switches when the IC junction

temperature exceeds 165°C (typical) and power-good (PG) asserts. Thermal shutdown does not trigger

below 155°C. After thermal shutdown occurs, hysteresis prevents the device from switching until the junction

temperature drops to approximately 156°C. When the junction temperature falls below 156°C (typical), the

LM656x5 attempts to soft start.

7.4 Device Functional Modes

7.4.1 Shutdown Mode

The EN pin provides electrical on and off control of the device. When the EN pin voltage is below 0.9V, both the

regulator and the internal LDO have no output voltage and the part is in shutdown mode. In shutdown mode, the

quiescent current drops below 0.85µA and PG pin gets pulled low.

7.4.2 Active Mode

The LM656x5 is in active mode when the following occurs:

• The EN pin is above VEN_TH_R.

• VIN is above VIN_UVLO_R.

• VIN is high enough to satisfy the VIN minimum operating input voltage.

• No other fault conditions are present.

See Section 7.3 for protection features. The simplest way to enable the operation is to connect EN to VIN,

allowing self-start-up when the applied input voltage exceeds the minimum VIN_OPERATE.

In active mode, depending on the load current, input voltage, and output voltage, the LM656x5 is in one of six

sub-modes:

• Continuous conduction mode (CCM) with fixed switching frequency and peak current mode operation.

• Discontinuous conduction mode (DCM) while in auto mode when the load current is lower than half of the

inductor current ripple. If current continues to reduce, the device enters Pulse Frequency Modulation (PFM)

which reduces the switch frequency to maintain regulation while reducing switching losses to achieve higher

efficiency at light load.

• Minimum on-time operation while the on-time of the device needed for full-frequency operation at the

requested low-duty cycle is not supported by TON_MIN.

• Forced pulse width modulation (FPWM) similar to CCM with fixed-switching frequency, but extends the fixed

frequency range of operation from full to no load.

• A current limiting condition where the output voltage remains above 0.4 times the output setpoint.

• Dropout mode when switching frequency is reduced to minimize dropout.

• Recovery from dropout similar to other modes of operation except the output voltage setpoint is gradually

moved up until the programmed setpoint is reached.

7.4.2.1 Peak Current Mode Operation

The following operating description of the LM656x5 refers to Section 7.2 and the waveforms in Figure 7-10.

Both supply a regulated output voltage by turning on the internal high-side (HS) and low-side (LS) NMOS

switches with varying duty cycle (D). During the HS switch on-time, the SW terminal voltage, VSW, swings up to

approximately VIN, and the inductor current, iL, increases with a linear slope. The HS switch is turned off by the

control logic. During the HS switch off-time, tOFF, the LS switch is turned on. Inductor current discharges through

the LS switch, forcing VSW to swing below ground by the voltage drop across the LS switch. The regulator loop

adjusts the duty cycle to maintain a constant output voltage. D is defined by the on-time of the HS switch over

the switching period: D = TON / (TON + TOFF).

In an ideal buck converter, where losses are ignored, D is proportional to the output voltage and inversely

proportional to the input voltage: D = VOUT / VIN.

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