TPS62050, TPS62051, TPS62052, TPS62054, TPS62056

SLVS432F – SEPTEMBER 2002 – REVISED JUNE 2015

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Feature Description (continued)

9.3.5 Power Good Comparator

The power good (PG) comparator has an open-drain output capable of sinking typically 1 mA. The PG function is

only active when the device is enabled (EN = high). When the device is disabled (EN = low), the PG pin is pulled

to GND.

The PG output is only valid after a 250-µs delay after the device is enabled and the supply voltage is greater

than 2.7 V. Power good is low during the first 250 µs after shutdown and in shutdown.

The PG pin floats high when the output voltage exceeds typically 98.5% of its nominal value. Leave the PG pin

unconnected, or connect it to GND when not used.

9.3.6 Synchronization

If no clock signal is applied, the converter operates with a typical switching frequency of 850 kHz. It is possible to

synchronize the converter to an external clock within a frequency range from 600 kHz to 1200 kHz. The device

automatically detects the rising edge of the first clock and synchronizes to the external clock. If the clock signal is

stopped, the converter automatically switches back to the internal clock and continues operation. The switchover

is initiated if no rising edge on the SYNC pin is detected for a duration of four clock cycles. Therefore, the

maximum delay time can be 8.3 µs if the internal clock has its minimum frequency of 600 kHz. During this time,

there is no clock signal available. The device stops switching until the internal circuitry is switched to the internal

clock source.

When the device is switched between internal synchronization and external synchronization during operation, the

output voltage may show transient overshoot or undershoot during switchover. The voltage transients are

minimized by using 850 kHz as an initial external frequency, and changing the frequency slowly (>1 ms) to the

value desired. The voltage drop at the output when the device is switched from external synchronization to

internal synchronization can be reduced by increasing the output capacitor value.

If the device is synchronized to an external clock, the power-save mode is disabled and the device stays in

forced PWM mode.

Connecting the SYNC pin to the GND pin enables the power-save mode. The converter operates in the PWM

mode at moderate to heavy loads and in the PFM mode during light loads maintaining high-efficiency over a wide

load current range.

9.4 Device Functional Modes

9.4.1 Soft-Start

The TPS6205x device have an internal soft-start circuit that limits the inrush current during start-up. This

prevents possible voltage drops of the input voltage if a battery or a high impedance power source is connected

to the input of the TPS6205x devices.

The soft-start is implemented as a digital circuit increasing the switch current in steps of 200 mA, 400 mA, 800

mA and then the typical switch current limit of 1.2 A. Therefore the start-up time mainly depends on the output

capacitor and load current. Typical start-up time with a 22-µF output capacitor and a 200-mA load current is 1

ms.

9.4.2 Constant Frequency Mode Operation (SYNC = HIGH)

In the constant frequency mode, the output voltage is regulated by varying the duty cycle of the PWM signal in

the range of 100% to 10%. Connecting the SYNC pin to a voltage greater than 1.5 V forces the converter to

operate permanently in the PWM mode even at light or no load currents. The advantage is the converter

operates with a fixed switching 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 (see

Figure 7). The N-MOSFET of the devices stays on even when the current into the output drops to zero. This

prevents the device from going into discontinuous mode. The device transfers unused energy back to the input.

Therefore, there is no ringing at the output that usually occurs in the discontinuous mode. The duty cycle range

in constant frequency mode is 100% to 10%.

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