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%
