MCP1640 12V/50 mA Two Cells Input Boost Converter Reference Design
DS51999A-page 18 © 2011 Microchip Technology Inc.
Attention should be paid to the values of the sense resistors. When testing the board
for other output voltage, a potential issue with higher value resistors is environmental
contamination, which can create a leakage current path on the PCB. This will affect the
feedback voltage and the output voltage regulation. Engineers should use resistors that
are larger than 1 MΩ with precaution. In normal humidity conditions, the V
FB
input
leakage is very low and the resistors’ values will not affect the stability of the system.
When the board is powered up, the internal MCP1640 device’s start-up logic circuitry
turns the internal rectifying switch on until the output capacitor, connected at the V
OUT
pin (C
L
), is charged to a value close to the input voltage. During start-up, the rectifying
switch limits the maximum current. Because at start-up, the external n-channel MOS
(Q) transistor is OFF, a start-up resistor (R
S
) is bypassing it and the C
L
cap is supplied.
MCP1640 starts switching, and the external N-MOS is turned on, cascading the
internal N-MOS switch of the MCP1640 to work with high voltage, more than 6V.
Notice that the MCP1640 is working internally as a synchronous rectifier and delivers
on its output (Pin 5) a part of the total output power. This output is unregulated because
the sense resistors, R
T
and R
B
, are connected to the high output voltage. The
MCP1640 gets its start-up bias from V
IN
. Once the output exceeds the input, bias
comes from the output. The C
L
capacitor is mandatory, because it smooths the output
of MCP1640. This voltage is used to supply the internal blocks after start-up (including
the compensation circuitry). In case of instability, a few mA loads may be required (R
L
is not populated on the board; for 12V output, use a 2k or 3k resistor). On the output of
MCP1640 (VO test point), a maximum 5 mA load can be connected. Exceeding the
load produces unregulated board V
OUT
.
The board works well in Discontinuous mode. However, once entered into Continuous
mode, V
OUT
goes out of regulation.
There is no undervoltage lockout feature for the MCP1640 family of devices. The
device will start up at the lowest voltage possible and run down to the lowest voltage
possible.
Note: Because this configuration is an nonsynchronous converter, the MCP1640
device’s features, true disconnect output and short-circuit protection, will be
lost for this reference board. In case of a short circuit, inductor L, and diode
D, can be broken. Notice that the MCP1640 will not be affected.
