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MCP1640C Datasheet - Page 11

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4.0
DETAILED DESCRIPTION
4.1
Device Option Overview
The MCP1640/B/C/D family of devices is capable of
low start-up voltage and delivers high efficiency over a
wide load range for single cell, two cell, three cell
alkaline, NiMH, NiCd and single cell Li-Ion battery
inputs. A high level of integration lowers total system
cost, eases implementation and reduces board area.
The devices feature low startup voltage, adjustable out-
put voltage, PWM/PFM mode operation, low I
grated synchronous switch, internal compensation, low
noise anti-ring control, inrush current limit, and soft
start.
There are two feature options for the MCP1640/B/C/D
family:
• PWM/PFM mode or PWM-Only mode
• “True Output Disconnect” mode or Input Bypass
mode
4.1.1
PWM/PFM MODE OPTION
The MCP1640C devices use an automatic switchover
from PWM to PFM mode for light load conditions to
maximize efficiency over a wide range of output
current. During PFM mode, higher peak current is used
to pump the output up to the threshold limit. While
operating in PFM or PWM mode, the P-Channel switch
is used as a synchronous rectifier, turning off when the
inductor current reaches 0 mA to maximize efficiency.
In PFM mode, a comparator is used to terminate
switching when the output voltage reaches the upper
threshold limit. Once switching has terminated, the
output voltage will decay or coast down. During this
period, very low I
is consumed from the device and
Q
input source, which keeps power efficiency high at light
load.
The disadvantages of PWM/PFM mode are higher out-
put ripple voltage and variable PFM mode frequency.
The PFM mode frequency is a function of input voltage,
output voltage and load. While in PFM mode, the boost
converter pumps the output up at a switching frequency
of 500 kHz.
4.1.2
PWM-ONLY MODE OPTION
The MCP1640B/D devices disable PFM mode
switching, and operate only in PWM mode over the
entire load range. During periods of light load opera-
tion, the MCP1640B/D continues to operate at a con-
stant 500 kHz switching frequency, keeping the output
ripple voltage lower than PFM mode.
During PWM-Only mode, the MCP1640B/D P-Channel
switch acts as a synchronous rectifier by turning off (to
prevent reverse current flow from the output cap back
to the input) in order to keep efficiency high.
 2011 Microchip Technology Inc.
MCP1640/B/C/D
For noise immunity, the N-Channel MOSFET current
sense is blanked for approximately 100 ns. With a typ-
ical minimum duty cycle of 100 ns, the MCP1640B/D
continues to switch at a constant frequency under light
load conditions.
Figure 2-12
age versus load current for the pulse skipping threshold
in PWM-Only mode. At lighter loads, the MCP1640B/D
devices begin to skip pulses.
4.1.3
TRUE OUTPUT DISCONNECT
MODE OPTION
The MCP1640B devices incorporate a true output
, inte-
Q
disconnect feature. With the EN pin pulled low, the
output of the MCP1640B is isolated or disconnected
from the input by turning off the integrated P-Channel
switch and removing the switch bulk diode connection.
This removes the DC path that is typical in boost con-
verters, which allows the output to be disconnected
from the input. During this mode, less than 1 µA of cur-
rent is consumed from the input (battery). True output
disconnect does not discharge the output, the output
voltage is held up by the external C
4.1.4
INPUT BYPASS MODE OPTION
The MCP1640C/D devices incorporate the Input
Bypass shutdown option. With the EN input pulled low,
the output is connected to the input using the internal
P-Channel MOSFET. In this mode, the current draw
from the input (battery) is less than 1 µA with no load.
The Input Bypass mode is used when the input voltage
range is high enough for the load to operate in Sleep or
low I
mode. When a higher regulated output voltage is
Q
necessary to operate the application, the EN input is
pulled high, enabling the boost converter.
TABLE 4-1:
PART NUMBER SELECTION
Part
PWM/
Number
PFM
MCP1640
X
MCP1640B
MCP1640C
X
MCP1640D
represents the input volt-
capacitance.
OUT
PWM-
True
Input
Only
Disconnect
Bypass
X
X
X
X
X
X
DS22234B-page 11

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