Extending Slow-Start Time

Use n in Table 2.

n

(4)

Switching Frequency (RT)

RT (kW) +

46000

ƒs (kHz) * 35.9

(5)

180° Out-of-Phase Synchronization (SYNC)

TPS54352-EP

,, TPS54353-EP,, TPS54354-EP

TPS54355-EP

, TPS54356-EP, TPS54357-EP

In applications that use large values of output capacitance, there may be a need to extend the slow-start time to

prevent the startup current from tripping the current limit. The current-limit circuit is designed to disable the

high-side MOSFET and reset the internal voltage reference for a short amount of time when the high-side

MOSFET current exceeds the current-limit threshold. If the output capacitance and load current cause the startup

current to exceed the current-limit threshold, the power-supply output will not reach the desired output voltage.

To extend the slow-start time and to reduce the startup current, an external resistor and capacitor can be added

to the ENA pin. The slow-start capacitance is calculated using the following equation:

The TPS5435x has an internal oscillator that operates at twice the PWM switching frequency. The internal

oscillator frequency is controlled by the RT pin. Grounding RT sets the PWM switching frequency to a default

frequency of 250 kHz. Floating RT sets the PWM switching frequency to 500 kHz.

Connecting a resistor from RT to AGND sets the frequency according to the following equation (also see

Figure 30).

RT controls the SYNC pin functions. If RT is floating or grounded, SYNC is an output. If the switching frequency

has been programmed using a resistor from RT to AGND, SYNC functions as an input.

The internal voltage-ramp charging current increases linearly with the set frequency and keeps the feed-forward

modulator constant (Km = 8), regardless of the frequency set point.

Table 3.

SWITCHING FREQUENCY

SYNC PIN

RT PIN

250 kHz, internally set

Generates SYNC output signal

AGND

500 kHz, internally set

Generates SYNC output signal

Float

Terminate to quiet ground

Externally set from 250 kHz to 700 kHz

R = 215 k

Ω to 69 kΩ

with 10-k

Ω resistor

Set RT resistor equal to 90% to 110% of external

synchronization frequency. When using a dual setup (see

Figure 27 for example), if the master 35x device RT pin is

Externally synchronized frequency

Synchronization signal

left floating, use a 110-k

Ω resistor to tie the slave RT pin

to ground. Conversely, if the master 35x device RT pin is

grounded, use a 237-k

Ω resistor to tie the slave RT pin to

ground.

The SYNC pin is configurable as an input or as an output, as noted in the previous section. When operating as

an input, SYNC is a falling-edge-triggered signal (see Figure 3, Figure 4, and Figure 19). When operating as an

output, the signal's falling edge is approximately 180° out of phase with the rising edge of the PH pins. Thus, two

TPS5435x devices operating in a system can share an input capacitor and draw ripple current at twice the

frequency of a single unit.

When operating the two TPS5435x devices 180° out of phase, the total RMS input current is reduced, thus,

reducing the amount of input capacitance needed and increasing efficiency.

When synchronizing a TPS5435x to an external signal, the timing resistor on the RT pin must be set so that the

oscillator is programmed to run at 90% to 110% of the synchronization frequency.

10

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