Datasheet

LMR10530

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SNVS814A –JUNE 2012–REVISED APRIL 2013

SOFT-START/SHUTDOWN

The LMR10530 has both enable and shutdown modes that are controlled by the EN pin. Connecting a voltage

source greater than 1.8V to the EN pin enables the operation of the LMR10530, while reducing this voltage

below 0.4V places the part in a low quiescent current (300nA typical) shutdown mode. There is no internal pull-

up on EN pin, therefore an external signal is required to initiate switching. Do not allow this pin to float or rise to

0.3V above V

. It should be noted that when the EN pin voltage rises above 1.8V while the input voltage is

greater than UVLO, there is 15µs delay before switching starts. During this delay the LMR10530 will go through a

power on reset state after which the internal soft-start process commences. During soft-start, the error amplifier’s

reference voltage ramps from 0V to its nominal value of 0.6V in approximately 600µs. This forces the regulator

output to ramp up in a controlled fashion, which helps reduce inrush current seen at the input and minimizes

output voltage overshoot.

The simplest way to enable the operation of the LMR10530 is to connect the EN pin to V

which allows self

start-up of the LMR10530 whenever the input voltage is applied. However, when an input voltage of slow rise

time is used to power the application and if both the input voltage and the output voltage are not fully established

before the soft-start time elapses, the control circuit will command maximum duty cycle operation of the internal

power switch to bring up the output voltage rapidly. When the feedback pin voltage exceeds 0.6V, the duty cycle

will have to reduce from the maximum value accordingly, to maintain regulation. The reduction of duty cycle

takes a finite amount of time and can result in a transient in output voltage for a short duration, as shown in

Figure 24. In applications where this output voltage overshoot is undesirable, one simple solution is to add a

feed-forward capacitor C

) across the top feedback resistor R1 to speed Gm Amplifier recovery. In practice, a

27nF to 100nF ceramic capacitor is usually a good choice to remove the overshoot completely or limit the

overshoot to an insignificant level during startup, as shown in Figure 25. Another more effective solution is to

control EN pin voltage by a separate logic signal, and pull the signal high only after V

is fully established. In this

way, the chip can execute a normal, complete soft start process, minimizing any output voltage overshoot. Under

some circumstances at cold temperature, this approach may also be required to minimize any unwanted output

voltage transients that may occur when the input voltage rises slowly. For a fast rising input voltage (100µs for

example), there is no need to control EN separately or add a feed-forward capacitor since the soft-start can bring

up output voltage smoothly as shown in Figure 26.

During startup, the LMR10530 gradually increases the switching frequency from 400kHz (LMR10530X) or

800kHz (LMR10530Y) to the nominal fixed value, as the feedback voltage increases (see Frequency Foldback

section for more information). Since the internal corrective ramp signal adjusts its slope dynamically, and is

proportional to the switching frequency during startup, a larger output capacitance may be required to insure a

smooth output voltage rise, at low programmed output voltage and high output load current.

Figure 24. Startup Response to V

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