NCP6324, NCV6324

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DETAILED DESCRIPTION

General

The

NCP/NCV6324,

a

family

of

voltage−mode

synchronous buck converters, which is optimized to supply

different sub−systems of portable applications powered by

one cell Li−ion or three cell Alkaline/NiCd/NiMH batteries.

The devices are able to deliver up to 2 A on an external

adjustable voltage. Operation with 3 MHz switching

frequency allows employing small size inductor and

capacitors. Input supply voltage feedforward control is

employed to deal with wide input voltage range.

Synchronous rectification and automatic PWM/PFM power

save mode offer improved system efficiency.

Operation Mode Selection (NCx6324C)

For NCx6324C with an external mode selection option,

high level (above 1.1 V) at MODE pin forces the device to

operate in forced PWM mode. Low level (below 0.4 V) at

this pin enables the device to operate in automatic

PFM/PWM mode for power saving function.

PWM Mode Operation

In medium and heavy load range, the inductor current is

continuous and the device operates in PWM mode with fixed

switching frequency, which has a typical value of 3 MHz. In

this mode, the output voltage is regulated by on−time pulse

width modulation of an internal P−MOSFET. An internal

N−MOSFET operates as synchronous rectifier and its

turn−on signal is complimentary to that of the P−MOSFET.

PFM Mode Operation

In light load range, the inductor current becomes

discontinuous and the device automatically operates in PFM

mode with an adaptive fixed on time and variable switching

frequency. In this mode, the output voltage is regulated by

pulse frequency modulation of the internal P−MOSFET, and

the switching frequency is almost proportional to the

loading current. The internal N−MOSFET operates as

synchronous rectifier after each on pulse of the P−MOSFET

with a very small negative current limit. When the load

increases and the inductor current becomes continuous, the

controller automatically turns back to the fixed−frequency

PWM mode operation.

Undervoltage Lockout

The input voltage VIN must reach or exceed 2.4 V

(typical) before the NCP/NCV6324 enables the converter

output to begin the start up sequence. The UVLO threshold

hysteresis is typically 100 mV.

Enable

The NCP/NCV6324 has an enable logic input pin EN. A

high level (above 1.1 V) on this pin enables the device to

active mode. A low level (below 0.4 V) on this pin disables

the device and makes the device in shutdown mode. There

is an internal filter with 5

ms time constant. The EN pin is

pulled down by an internal 10 nA sink current source. In

most of applications, the EN signal can be programmed

independently to VIN power sequence.

EN

PG

Vout

95%

90%

1.1V

0.4V

300us

1.15ms

8us

5us

8us

Active

Discharge

8us

100us

Figure 17. Power Good and Active Discharge Timing Diagram

Power Good Output (NCx6324B)

For NCx6324B with a power good output, the device

monitors the output voltage and provides a power good

output signal at the PG pin. This pin is an open−drain output

pin. To indicate the output of the converter is established, a

power good signal is available. The power good signal is low

when EN is high but the output voltage has not been

established. Once the output voltage of the converter drops

out below 90% of its regulation during operation, the power

good signal is pulled low and indicates a power failure. A 5%