TC126 Microchip Technology, TC126 Datasheet - Page 4

• Manufacturer Part Number: TC126
• Description: (TC125) PFM Step-Up DC/DC Regulators
• Manufacturer: Microchip Technology

TC125/TC126
DS21372B-page 4
3.0
The TC125/126 are PFM step-up DC/DC regulators for
use in systems operating from two or more cells or in
low voltage, line powered applications. Because Pulse
Frequency Modulation (PFM) is used, the TC125/126
switching frequency (and therefore supply current) is
minimized at low output loads. This is especially
important in battery operated applications (such as
pagers) that operate in standby mode most of the time.
For example, a TC125/126 with a 3V output and no
load will consume a maximum supply current of only
10µA versus a supply current of 40µA maximum when
I
inductor, diode and capacitor to implement a complete
DC/DC converter.
The TC125 is recommended for applications requiring
shutdown mode as a means of reducing system supply
current. The TC125 is powered from the PS input,
which must be connected to the regulated output as
shown in Figure 3-1. PS also senses output voltage for
closed-loop regulation. Start-up current is furnished
through the inductor when input voltage is initially
applied. This action starts the oscillator, causing the
voltage at the PS input to rise, bootstrapping the
regulator into full operation.
The TC126 (Figure 3-2) is recommended for all appli-
cations not requiring shutdown mode. It has separate
V
any source of 2.2V to 10V in the system. The V
of the TC126 may be connected to the V
external DC voltage. Lower values of V
lower supply current, but lower efficiency due to higher
switch ON resistance. Higher V
supply current, but drive the internal switching
transistor harder (lowering RDS
efficiency.
3.1
The TC125 enters a low power shutdown mode when
SHDN is brought low. While in shutdown, the oscillator
is disabled and the internal switch is shut off. Normal
regulator operation resumes when SHDN is brought
high. Because the TC125 uses an external diode, a
leakage path between the input voltage and the output
node (through the inductor and diode) exists while the
regulator is in shutdown. Care must be taken in system
design to assure the input supply is isolated from the
load during shutdown.
OUT
DD
and SENSE inputs, allowing it to be powered from
= 30mA. Both devices require only an external
DETAILED DESCRIPTION
Low Power Shutdown Mode
ON
), thereby increasing
DD
values increase
IN
, V
DD
OUT
result in
DD
, or an
input
3.2
The TC125 and TC126 are designed to operate as
step-up regulators only. As such, V
always be less than the factory-programmed output
voltage setting (V
V
(and corresponding supply current reduction) until V
is again less than V
suspended, V
through the series combination of the inductor and
Schottky diode. Again, care must be taken to add the
appropriate isolation (MOSFET series switch or post
LDO with shutdown) during system design if this V
V
FIGURE 3-1:
FIGURE 3-2:
Supply
OUT
IN
3.3V
Line
Shutdown
1.5V
Control
> V
ON
leakage path is problematic.
100µH Sumida CD54
100µH Sumida CD54
R
OFF
Behavior When V
Than the Factory-Programmed
OUT Setting
causes regulating action to be suspended
L1
L1
IN
is connected to the output voltage node
SENSE
SHDN
LX
LX
R
5
1
4
1
). Operating the TC125/126 with
TC126
TC125
V
TYPICAL TC125 CIRCUIT
TYPICAL TC126 CIRCUIT
R
DD
PS
2
©
2
. While regulating action is
2002 Microchip Technology Inc.
GND
GND
NC
MA735
3
5
NC
4
D1
3
MA735
IN
D1
is Greater
IN
+
+
C
47µF/16V
Tantalum
is assumed to
1
C
47µF/16V
Tantalum
1
3.3V @40mA
V
5V @80mA
OUT
V
OUT
IN
IN
/