NCP6335 Datasheet, PDF (12/30 Pages) ON Semiconductor – Configurable 4.0 A Step Down Converter - Transient Load Helper

NCP6335
DETAILED OPERATING DESCRIPTION
Detailed Descriptions
The NCP6335 is voltage mode standalone synchronous
DC−to−DC converter optimized to supply different sub
systems of portable applications powered by one cell Li−Ion
or three cells Alkaline/NiCd/NiMh. The IC can deliver up to
4 A at an I²C selectable voltage ranging from 0.6 V to
1.40 V. It can share the same output rail with another
DC−to−DC converter and works as a transient load helper
without sinking current on shared rail. A 3 MHz switching
frequency allows the use of smaller output filter
components. Synchronous rectification and automatic
PWM/PFM transitions improve overall solution efficiency.
Forced PWM is also configurable. Operating modes,
configuration, and output power can be easily selected either
by using digital I/O pins or by programming a set of registers
using an I²C compatible interface capable of operation up to
3.4 MHz. Default I²C settings are factory programmable.
DC to DC Buck Operation
The converter is a synchronous rectifier type with both
high side and low side integrated switches. Neither external
transistor nor diodes are required for NCP6335 operation.
Feedback and compensation network are also fully
integrated. The converter can operate in two different
modes: PWM and PFM. The transition between PWM/PFM
modes can occur automatically or the switcher can be placed
in forced PWM mode by I²C programming (PWMVSEL0
/ PWMVSEL1 bits of COMMAND register).
PWM (Pulse Width Modulation) Operating Mode
In medium and high load conditions, NCP6335 operates
in PWM mode from a fixed clock 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 NCP6335 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.
Forced PWM
The NCP6335 can be programmed to only use PWM and
disable the transition to PFM if so desired.
Output Stage
NCP6335 is a 2.5 A to 4 A output current capable
integrated DC to DC converter. To supply such a high
current, the internal MOSFETs need to be large. The output
stage is composed of nine modules that can be individually
Enabled / Disabled by setting the MODULE register.
Inductor Peak Current Limitation
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 user can select peak
current to keep inductor within its specifications. The peak
current can be set by writing IPEAK[1..0] bits in LIMCONF
register.
Table 1. IPEAK VALUES
IPEAK[1..0]
00
Inductor Peak Current (A)
3.4 − for 2.5 output current
01
4.0 − for 3.0 output current
10
4.4 − for 3.5 output current
11
5.0 − for 4.0 output current
Output Voltage
Output voltage is set internally by integrated resistor
bridge and error amplifier that drives the PWM/PFM
controller. No extra component is needed to set output
voltage. However, writing in the VoutVSEL0[6..0] bits of
the PROGVSEL0 register or VoutVSEL1[6..0] bits of the
PROGVSEL1 register will change settings. Output voltage
level can be programmed in the 0.6 V to 1.4 V range by
6.25 mV steps.
The VSEL pin and VSELGT bit will determine which
register between PROGVSEL0 and PROGVSEL1 will set
the output voltage.
• If VSELGT = 1 AND VSEL=0 ³ Output voltage is set
by VoutVSEL0[6..0] bits (PROGVSEL0 register)
• Else ³ Output voltage is set by VoutVSEL1[6..0] bits
(PROGVSEL1 register)
Under Voltage Lock Out (UVLO)
NCP6335 core does not operate for voltages below the
Under Voltage lock Out (UVLO) level. Below UVLO
threshold, all internal circuitry (both analog and digital) is
held in reset.
NCP6335 operation is guaranteed down to VUVLO when
battery voltage is dropping off. To avoid erratic on / off
behavior, a maximum 200 mV hysteresis is implemented.
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