TC1142 Datasheet, PDF (4/12 Pages) Microchip Technology – Inductorless -2x Boost/Buck Regulator

TC1142
3.0 DETAILED DESCRIPTION
The TC1142 inductorless -2x boost/buck regulator is an
inverting charge pump that uses a pulse-frequency
modulation (PFM) control scheme to produce a
regulated negative output voltage, -V_R, between -3V
and -5V (depending on the output voltage option) at
20mA maximum load. Output voltage regulation is
achieved by gating ON the clock to the charge pump for
a single half-clock period whenever the output is more
positive than V_R, and gating it OFF when the output is
more negative than -V_R. The resulting PFM of the clock
applied to the charge pump has a high frequency
spectral content consisting only of clock harmonics.
When using an external clock, the transient noise is
then synchronized to the clock and is easier to filter in
sensitive applications.
The TC1142 also can be used as a -1x boost/buck
regulator by omitting the C2 capacitor and connecting
the C2– pin to V_OUT.
The PFM control scheme minimizes supply current at
small loads and permits the use of low value flying
capacitors, which saves on printed circuit board space
and cost. Due to the TC1142’s doubling and inverting
charge pump mechanism, the output voltage is limited
to -2V_IN. To produce a -5V regulated output, for
example, a minimum input voltage of 2.5V is required
at V_IN.
The CCLK pin of the TC1142 has three functions: It can
select the internal 200kHz oscillator (when held HIGH),
put the TC1142 into shutdown (when held LOW), or
provide an external clock input. To achieve this
functionality, an internal counter is reset by any positive
transition at the CCLK pin, but will time out in typically
160 µsec (i.e., a frequency higher than about 3kHz). If
the counter times out following the last positive
transition, then the internal clock will be gated through
to the charge pump if CCLK is HIGH, or the device will
enter shutdown mode if it is LOW. To enter shutdown,
CCLK must be LOW and the counter must have timed
out. These timing diagrams are shown in Figure 3-4.
A functional circuit diagram of the TC1142 is shown in
Figure 3-1. The output voltage V_OUT is compared to an
on-chip reference voltage, and the comparator output
is used to gate the charge pump clock. The charge
pump is a negative voltage doubler and has two
phases of operation which are further illustrated in
Figure 3-2 and Figure 3-3. In phase 1, shown in
Figure 3-2, the flying capacitor C1 charges the flying
capacitor C2 while the device load is totally serviced by
the charge stored on the reservoir capacitor C_OUT. In
phase 2, shown in Figure 3-3, the capacitor C1 is
recharged to V_IN while the capacitor C2 transfers its
charge to the reservoir capacitor C_OUT.
In normal operation, the TC1142 charge pump stays in
phase 2 and only switches to phase 1 as required to
maintain output voltage regulation.
FIGURE 3-1:
FUNCTIONAL CIRCUIT DIAGRAM
C1+
V_IN
V_IN
Shutdown
Clock
Circuit
OSC
Override
C1–
C2+
C2–
+
–
1.2V
+
V_OUT
+ ^COUT
DS21360B-page 4
 2002 Microchip Technology Inc.