MCP1252_14 Datasheet, PDF (10/20 Pages) Microchip Technology – Low-Noise, Positive-Regulated Charge Pump

MCP1252/3
4.2 Power Efficiency
The power efficiency, , is determined by the mode of
operation. In boost mode, the efficiency is
approximately half of a linear regulator. In buck mode,
the efficiency is approximately equal to that of a linear
regulator. The following formulas can be used to
approximate the power efficiency with any significant
amount of output current. At light loads, the quiescent
current of the device must be taken into consideration.
EQUATION 4-1:
_BOOST
=
P----_O----U----T-
P_IN
=
V---V-_I^--_N^-O---U-^{---T--2----}^---I--IO--_O^--U--_U^--T--_T^-
=
V---V-_I^--_N^-O---U-^---T--2-
_BUCK
=
P----_O----U----T-
P_IN
=
-V--V-^O---_I-^U-_N---^T-_---^--I---_O^I--O-_U---^U-_T--^T-
=
V----_O----U----T-
V_IN
4.3 Shutdown Mode
Driving SHDN low places the MCP1252 or MCP1253 in
a low-power shutdown mode. This disables the charge
pump switches, oscillator and control logic, reducing
the quiescent current to 0.1 µA (typical). The PGOOD
output is in a high-impedance state during shutdown.
4.4 PGOOD Output
The PGOOD output is an open-drain output that sinks
current when the regulator output voltage falls below
0.93V_OUT (typical). The output voltage can either be
fixed when the selectable output device is chosen
(MCP1252-33X50, MCP1253-33X50) or adjustable
from an external resistive divider when the adjustable
device is chosen (MCP1252-ADJ, MCP1253-ADJ). If
the regulator output voltage falls below 0.93V_OUT
(typical) for less than 200 µsec and then recovers,
glitch-immunity circuits prevent the PGOOD signal
from transitioning low. A 10 k to 1 M pull-up resistor
from PGOOD to V_OUT may be used to provide a logic
output. Connect PGOOD to GND or leave unconnected
if not used.
4.5 Soft-Start and Short-Circuit
Protection
The MCP1252 and MCP1253 feature foldback short-
circuit protection. This circuitry provides an internal
soft-start function by limiting in-rush current during
startup and also limits the output current to 200 mA
(typical) if the output is shorted to GND. The internal
soft-start circuitry requires approximately 300 µsec,
typical with a 5V output, from either initial power-up or
release from shutdown for the output voltage to be in
regulation.
4.6 Thermal Shutdown
The MCP1252 and MCP1253 feature thermal
shutdown with temperature hysteresis. When the die
temperature exceeds 160°C, typically, the device shuts
down. When the die cools by 15°C, typically, the device
automatically turns back on. If high die temperature is
caused by output overload and the load is not removed,
the device will turn on and off, resulting in a pulse
output.
5.0 APPLICATIONS
The MCP1252 and MCP1253 are inductorless,
positive-regulated, charge pump DC/DC converters. A
typical circuit configuration for the fixed output version
is depicted in Figure 5-1. The adjustable version is
depicted in Figure 5-2.
SELECTABLE OUTPUT VOLTAGE
C_FLY
2.7V to 5.5V
MCP1252-33X50
6 _C+
V_OUT ^{2 +5.0V ±2.5%}
5 _C-
+
R_PU
C_OUT
+
C_IN
ON
OFF
Shutdown
Control
3 _VIN
PGOOD ¹
7 _SHDN SELECT ⁸
GND
4
PGOOD Flag
To PIC^®
Microcontroller
C_FLY = 1 µF
C_IN = 10 µF
C_OUT = 10 µF
R_PU = 100 k
FIGURE 5-1:
Typical Circuit Configuration
for Fixed Output Device.
ADJUSTABLE OUTPUT VOLTAGE
C_FLY
2.7V to 5.5V
+
C_IN
ON
OFF
Shutdown
Control
MCP1252-ADJ
6 _C+
V_OUT ²
5 _C-
3 _VIN
PGOOD ¹
7 _SHDN
GND
4
FB ⁸
V_OUT = 1.21V (1 + R₁/R₂)
+4.0V
R_PU
+ C_OUT
R₁
PGOOD Flag
To PIC^®
Microcontroller
R₂
C_FLY = 1 µF
C_IN = 10 µF
C_OUT = 10 µF
R_PU = 100 k
R₁ = 23.2 k
R₂ = 10 k
FIGURE 5-2:
Typical Circuit Configuration
for Adjustable Output Device.
DS20001752C-page 10
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