TC1225 Datasheet, PDF (3/10 Pages) Microchip Technology – Inverting Dual (-VIN, -2VIN) Charge Pump Voltage Converters

Inverting Dual (–V_IN, –2V_IN)
Charge Pump Voltage Converters
TC1225
TC1226
TC1227
DETAILED DESCRIPTION
The TC1225/1226/1227 dual charge pump convert-
ers perform both a –1x and –2x multiply of the voltage
applied to the V_IN pin. Output ‘– V_IN’ provides a negative
voltage inversion of the V_IN supply, while output ‘-2 V_IN’
provides a negative doubling inversion of V_IN. Conversion
is performed using two synchronous switching matrices
and four external capacitors.
Figure 1 (below) is a block diagram representation of the
TC1225/1226/1227 architecture. The first switching stage
inverts the voltage present at V_IN and the second stage uses
the ‘–V_IN’ output generated from the first stage to produce
the ‘–2V_IN’ output function from the second stage switching
matrix.
Each device contains an on-board oscillator that syn-
chronously controls the operation of the charge pump switch-
ing matrices. The TC1225 synchronously switches at 12KHz,
the TC1226 synchronously switches at 35KHz, and the
TC1227 synchronously switches at 125KHz. The different
oscillator frequencies for this device family allow the user to
trade-off capacitor size versus supply current. Faster oscil-
lators can use smaller external capacitors but will consume
more supply current (see Electrical Characteristics Table).
VIN
+
C1
OSCILLATOR
SWITCH MATRIX
(1st STAGE)
—VIN
COUT1
+
nominal at +25°C and V_IN = +5V. The value of the ‘-2V_IN’
output and is approximately 140Ω nominal at +25°C and V_IN
= +5V. In this particular case, ‘-V_IN’ is approximately – 5V
and ‘–2V_IN’ is approximately –10V at very light loads, and
each stage will droop according to the equation below:
V_DROOP = I_OUT x R_OUT
[-V_IN OUTPUT] = V_OUT1 = – (V_IN – V_DROOP1)
[-2V_IN OUTPUT] = V_OUT2 = V_OUT1 – (V_IN – V_DROOP2)
where V_DROOP1 is the output voltage droop contributed from
stage 1 loading , and V_DROOP2 is the output voltage droop
from stage 2 loading.
Charge Pump Efficiency
The overall power efficiency of the two charge pump
stages is affected by four factors:
(1) Losses from power consumed by the internal oscil-
lator, switch drive, etc. (which vary with input voltage,
temperature and oscillator frequency).
(2) I²R losses due to the on-resistance of the MOSFET
switches on-board each charge pump.
(3) Charge pump capacitor losses due to effective
series resistance (ESR).
(4) Losses that occur during charge transfer (from the
commutation capacitor to the output capacitor) when a
voltage difference between the two capacitors exists.
+
C2
SWITCH MATRIX
(2nd STAGE)
—2VIN
COUT2
+
Figure 1. Functional Block Diagram
Most of the conversion losses are due to factor (2), (3)
and (4) above. The losses for the first stage are given by
Equation 1a and the losses for the second stage are given
by Equation 1b.
P1_{LOSS (2, 3, 4)} = I_OUT1 ² x R_OUT1
where R_OUT1 = [ 1 / [ f_OSC (C1) ] + 8R_SWITCH1 +
4ESR_C1 + ESR_COUT1 ]
Equation 1a.
APPLICATIONS INFORMATION
Output Voltage Considerations
The TC1225/1226/1227 performs voltage conversions
but does not provide any type of regulation. The two output
voltage stages will droop in a linear manner with respect to
their respective load currents. The value of the equivalent
output resistance of the ‘-V_IN’ output is approximately 50Ω
© 2001 Microchip Technology Inc. DS21369A
3
P2_{LOSS (2, 3, 4)} = I_OUT2 ² x R_OUT2
where R_OUT2 = [ 1 / [f_OSC(C2) ] + 8R_SWITCH2 +
4ESR_C2 + ESR_COUT2 ]
Equation 1b.
TC1225/6/7-1 3/24/00