MIC2660 Datasheet, PDF (4/8 Pages) Micrel Semiconductor – IttyBitty Charge Pump

MIC2660
Block Diagram
IN ^MIC2660
EN
OSC
D1 ^2×
XLO
Q1
C1
Q2
XLO
3×
D2 D3
Q3
C2
Q4
OUT
GND
Micrel
Functional Description
Refer to the block diagram.
The MIC2660 charge pump consists of an oscillator and a
voltage tripler. A logic-high applied to EN activates the
charge pump. The charge pump produces an output voltage
that is higher than the input voltage.
Supply Input
IN (supply input) is rated for +2.7V to +5.5V.
Ouput
OUT is connected to IN, less 3 diode drops, at all times.
Enable
EN (enable) is a CMOS input. A logic low turns the oscillator
off. The threshold is approximately half the supply voltage. A
floating EN input may cause unpredictable operation.
Oscillator
The oscillator produces a square wave at approximately
18MHz. It has a noninverting and an inverting output.
Crossover Lockout
The charge pump contains two crossover lockout (XLO)
circuits. Each crossover lockout circuit drives a totem pole,
consisting of a P-channel MOSFET and an N-channel MOS-
FET. The crossover lockout alternately switches the MOS-
FETs with no significant transition current (shoot-through
current from supply to ground).
Tripler
Voltage stepup is performed by charging an internal capaci-
tor then switching the charged capacitor in series with the
supply voltage to produce a higher voltage. A description of
the nominal voltage tripler output is:
V_OUT = 3V_IN – 3V_D.
where:
V_OUT = output voltage
V_IN = supply voltage
V_D = voltage drop across forward biased diode
All formulas are simplified. Refer to the last paragraph of this
subsection about the actual output voltage.
The following sequence describes the basic operation of the
tripler by showing how the voltage at the “2×” and “3×” nodes,
V_2× and V_3×, increases.
Q2 turns on, completing the ground path to charge C1 (and
the 2× node) to the supply voltage, less a diode voltage drop.
V_{2× (charging)} = V_IN – V_D1
After Q2 turns off, Q1 turns on. The Q1-Q2 side of C1 is
switched (offset upward) from ground to V_IN. The 2× node,
that was nominally at the supply voltage, becomes nominally
twice the supply voltage.
V_2× = V_IN – V_D1 + V_IN
While Q1 is on, Q4 is also on. When Q4 is on, the nominally
doubled voltage at the 2× node is applied across C2, through
D2.
V_{3× (charging)} = V_IN – V_D1 + V_IN – V_D2
After Q4 turns off, Q3 turns on. The Q3-Q4 side of C2 is
switched from ground to V_IN. The 3× node, that was nomi-
nally twice the supply voltage, becomes nominally three
times the supply voltage.
V_3× = V_IN – V_D1 + V_IN – V_D2 + V_IN
The tripled voltage is available at the output through D3.
V_OUT = V_IN – V_D1 + V_IN – V_D2 + V_IN – V_D3
The output is nominally 3 times the supply voltage less the
voltage drop across three diodes.
The actual output is lower. These simplified formulas do not
show that the voltage across the capacitors decreases when
charge flows to the following stage or output. An actual
device also has some internal loss.
ESD Protection
Zener diodes are provided at IN, EN, and OUT to limit ESD
voltage.
MIC2660
4
August 1999