Datasheet

PFMModeatLightLoad
PFMRipple
PWMModeatHeavyLoad
NominalDCOutputVoltage
TPS62620, TPS62621
TPS62622, TPS62623
TPS62624, TPS62625
www.ti.com SLVS848C –JULY 2009– REVISED AUGUST 2011
Figure 39. Operation in PFM Mode and Transfer to PWM Mode
MODE SELECTION
The MODE pin allows to select the operating mode of the device. Connecting this pin to GND enables the
automatic PWM and power-save mode operation. The converter operates in regulated frequency PWM mode at
moderate to heavy loads and in the PFM mode during light loads, which maintains high efficiency over a wide
load current range.
Pulling the MODE pin high forces the converter to operate in the PWM mode even at light load currents. The
advantage is that the converter operates with a fixed frequency that allows simple filtering of the switching
frequency for noise-sensitive applications. In this mode, the efficiency is lower compared to the power-save
mode during light loads.
For additional flexibility, it is possible to switch from power-save mode to forced PWM mode during operation.
This allows efficient power management by adjusting the operation of the converter to the specific system
requirements.
ENABLE
The device starts operation when EN is set high and starts up with the soft start as previously described. For
proper operation, the EN pin must be terminated and must not be left floating.
Pulling the EN pin low forces the device into shutdown, with a shutdown quiescent current of typically 0.1μA. In
this mode, the P and N-channel MOSFETs are turned off, the internal resistor feedback divider is disconnected,
and the entire internal-control circuitry is switched off.
SOFT START
The TPS6262x has an internal soft-start circuit that limits the inrush current during start-up. This limits input
voltage drops when a battery or a high-impedance power source is connected to the input of the converter.
The soft-start system progressively increases the on-time from a minimum pulse-width of 35ns as a function of
the output voltage. This mode of operation continues for c.a. 100μs after enable. Should the output voltage not
have reached its target value by this time, such as in the case of heavy load, the soft-start transitions to a second
mode of operation.
The converter then operates in a current limit mode, specifically the P-MOS current limit is set to half the nominal
limit, and the N-channel MOSFET remains on until the inductor current has reset. After a further 100 μs, the
device ramps up to the full current limit operation if the output voltage has risen above 0.5V (approximately).
Therefore, the start-up time mainly depends on the output capacitor and load current.
OUTPUT CAPACITOR DISCHARGE
The TPS6262x device can actively discharge the output capacitor when it turns off. The integrated discharge
resistor has a typical resistance of 15 Ω. The required time to discharge the output capacitor at the output node
depends on load current and the output capacitance value.
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