NCP3520/NCP3521

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DETAILED OPERATING DESCRIPTION

General

The NCP3520/NCP3521 are LDO Regulator Controllers.

An external NFET device sets the current capability

allowing for designer selection.

Features include an undervoltage lockout for both the

integrated circuit supply Pin VCC, and the supply pin for the

external FET connection to the drain of the FET.

The NRCS (Non Rush Current on Startup) feature

prevents high currents through the external FET

(drain−source). The external capacitor setting component

used in NRCS is also used for short circuit protection (SCP).

The device also has an enable feature allowing it to go into

a low supply current sleep mode demanded by most modern

day feature rich systems when not in use.

Thermal shutdown functionality protects the IC from

damage caused from excessively high temperatures

appearing on the IC.

Output Driver

Output current drive capability is determined by the

designer’s choice of external MOSFET (NFET). Power

dissipated in the driver can be controlled by the voltage

dissipation and high enough to support regulated operation

at the desired output current. It should also be noted the

during high speed transient events until the system loop can

respond to any voltage dips to drive the external FET.

High Speed Control

Unlike most linear regulators whose reaction to

overvoltage events is to turn off the upper driver and let the

external load and resistor feedback network quench the

incident, the NCP3520/21 include a 1.2 mA pulldown

through the VS Pin. This keeps overshoot to a minimum

during powerup. During turn−off and thermal overload, the

pulldown current is increased from 1.2 mA to 220 mA to

provide an even faster turn−off time.

Power On Reset

A 50

ms power on reset circuit is built into the IC acting as

a digital filter and performing housekeeping activity during

a short circuit event.

The timer effects three areas of operation.

1. EN turn on delay. Upon detection of an EN high,

there is a 50

ms delay to when the internal circuitry

turns on and the gate pin (G) goes high. A low on

EN resets the timer.

undervoltage lockout voltage and restored above

its hysteresis value, a 50

ms time is also observed

recognized to be different from the EN turn on

delay by the active circuitry of the voltage

pulldown.

3. Device startup into a short circuit. Further details

are available on this subject under the heading

“Starting Up Into A Short Circuit”.

Normal Powerup/Down

The NRCS (Non Rush Current on Startup) timer controls

the output driver during powerup. The output driver voltage

mimicked to provide a duplicate voltage on VFB. When

1.2 V is reached normal operation of the error amplifier and

feedback network take over. Regulation is maintained in the

loop around 1.2 V. The NRCS pin rises up to 1 V. At 1 V, the

NRCS capacitor is discharged fully at a 300

mA (min) rate.

The IC enters a standby mode capable of short circuit

detection.

A 20

mA pullup current source is used to charge the

external NRCS capacitor linearly and maintain a predictable

powerup. A recommended 0.01

mF will provide a 325 ms

powerup time. Alternative times can be programmed with

this equation:

(eq. 1)

Rush current during startup can be calculated by

The NRCS circuit is not active during powerdown.

Normal circuit operation will be maintained unless

VCC_UVLO or VD_UVLO cause the gate drive output to

turn off.

NRCS

1.0 V

VS / VFB

1.2 V

EN

3 V

0.65 V

Figure 13. Powerup (NCP3520 (1.2 V) Version Shown)

A standby mode for

short circuit is

entered here.

Error amplifier

takes control here.

50 ms