NCP6360 Datasheet, PDF (12/17 Pages) ON Semiconductor – nullMini Buck Converter for RF Power Amplifiers

NCP6360
OPERATING DESCRIPTION
General Description
The NCP6360 is a voltage−mode standalone synchronous
step−down DC−to−DC converter designed to supply RF
Power Amplifiers (PAs) used into 3G/4G wireless systems
(Mobile/ Smart Phones, Phablets, Tablets, ...) powered by
single−cell Lithium−Ion batteries. The IC can deliver up to
800 mA when operating in PWM mode.
The buck converter output voltage ranging from 0.6 V to
3.4 V can be monitored by the system’s PA output RF power
through the control pin VCON. The control voltage range is
from 0.24 V to 1.36 V and Vout is equal to 2.5 times this
control voltage. VCON allows the PA to have its efficiency
dynamically optimized during communication calls in the
case for example of roaming situation involving a constant
adjustment of the PA output power. The value−added benefit
is an increase of the absolute talk time.
Synchronous rectification and automatic PFM/PWM
operating mode transitions improve overall solution
efficiency. The device operates at 6 MHz switching
frequency.
Buck DC−to−DC Converter Operating
The converter is a synchronous rectifier type with both
high side and low side integrated switches. Neither external
transistor nor diodes are required for NCP6360 operation.
Feedback and compensation network are also fully
integrated. The device can operate in four different modes:
shutdown mode (EN = Low, device off), Sleep Mode when
VCON below about 0.1 V, PFM mode for efficiency
optimization purpose when operating at light load and PWM
mode when operating in medium and high loads. The
transitions between PWM and PFM modes occur
automatically.
Shutdown Mode
The NCP6360 enters shutdown mode when setting the EN
pin Low (below 0.4 V) or when PVIN drops below its
UVLO threshold value (2.35 V typical). In shutdown mode,
the internal reference, oscillator and most of the control
circuitries are turned off. The typical current consumption is
0.7 mA. Applying a voltage above 1.1 V to EN pin will
enable the device for normal operation. A soft−start
sequence is run when activating EN high. EN pin should be
activated after the input voltage is applied.
PWM (Pulse Width Modulation) Operating Mode
In medium and high load conditions, the NCP6360
operates in PWM mode from a fixed clock (6 MHz) and
adapts its duty cycle to regulate the desired output voltage.
In this mode, the inductor current is in CCM (Continuous
Current Mode) and the voltage is regulated by PWM. The
internal N−MOSFET switch operates as synchronous
rectifier and is driven complementary to the P−MOSFET
switch. In CCM, the lower switch (N−MOSFET) in a
synchronous converter provides a lower voltage drop than
the diode in an asynchronous converter, which provides less
loss and higher efficiency.
PFM (Pulse Frequency Modulation) Operating Mode
In order to save power and improve efficiency at low loads
the NCP6360 operates in PFM mode as the inductor drops
into DCM (Discontinuous Current Mode). The upper FET
on time is kept constant and the switching frequency is
variable. Output voltage is regulated by varying the
switching frequency which becomes proportional to loading
current. As it does in PWM mode, the internal N−MOSFET
operates as synchronous rectifier after each P−MOSFET
on−pulse. When load increases and current in inductor
becomes continuous again, the controller automatically
turns back to PWM mode.
Sleep Mode
The NCP6360 device enters the sleep mode in about 4ms
when the control voltage VCON goes below typically
70 mV. Vout is extremely low, close to 0 V and in a state out
of regulation. In this Vout condition the Sleep mode enables
a low current state (40 mA typical range). The buck
converter exits the sleep mode and returns in a regulation
state when VCON goes above 110 mV after typically 5 ms.
Inductor Peak Current limitations
During normal operation, peak current limitation will
monitor and limit the current through the inductor. This
current limitation is particularly useful when size and/or
height constrain inductor power. The High Side Switch
(HSS) peak current limitation is typically 1.5 A, while the
Low Side Switch (LSS) has a peak current up to 0.8 A. The
HSS peak current contributes to limit the current during soft
start sequence in high load conditions.
Under−voltage Lockout (UVLO)
NCP6360 core does not operate for voltages below the
Under Voltage lock Out (UVLO) level. Below UVLO
threshold (typical 2.35 V), all internal circuitry (both analog
and digital) is held in reset. NCP6360 operation is not
guaranteed down to VUVLO when battery voltage is
dropping off. To avoid erratic on / off behavior,a typical
100 mV hysteresis is implemented. Restart is guaranteed at
2.6 V when VBAT voltage is recovering or rising.
Power−Up / Power−Down Sequencing
The EN pin controls NCP6360 start up. EN pin Low to
High transition starts the power up sequencer which is
combined with a soft start consisting to limit the inrush
current at 800 mA while the output voltage is establishing.
If EN is made low, the DC to DC converter is turned off and
device enters shutdown mode.
A built−in pull−down resistor disables the device when
this pin is left unconnected or not driven.
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