TC125 Datasheet, PDF (4/12 Pages) Microchip Technology – PFM Step-Up DC/DC Regulators

TC125/TC126
3.0 DETAILED DESCRIPTION
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_OUT = 30mA. Both devices require only an external
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_DD and SENSE inputs, allowing it to be powered from
any source of 2.2V to 10V in the system. The V_DD input
of the TC126 may be connected to the V_IN, V_OUT, or an
external DC voltage. Lower values of V_DD result in
lower supply current, but lower efficiency due to higher
switch ON resistance. Higher V_DD values increase
supply current, but drive the internal switching
transistor harder (lowering RDS_ON), thereby increasing
efficiency.
3.1 Low Power Shutdown Mode
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.
3.2 Behavior When V_IN is Greater
Than the Factory-Programmed
OUT Setting
The TC125 and TC126 are designed to operate as
step-up regulators only. As such, V_IN is assumed to
always be less than the factory-programmed output
voltage setting (V_R). Operating the TC125/126 with
V_IN > V_R causes regulating action to be suspended
(and corresponding supply current reduction) until V_IN
is again less than V_R. While regulating action is
suspended, V_IN is connected to the output voltage node
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_IN/
V_OUT leakage path is problematic.
FIGURE 3-1:
TYPICAL TC125 CIRCUIT
L1
100µH Sumida CD54
D1
MA735
1.5V
ON OFF
Shutdown
Control
5
4
LX
GND
TC125
SHDN PS NC
1
2
3
V_OUT
3.3V @40mA
+
C₁
47µF/16V
Tantalum
FIGURE 3-2:
TYPICAL TC126 CIRCUIT
3.3V
Line
Supply
L1
100µH Sumida CD54
D1
MA735
4
5
LX
GND
TC126
SENSE V_DD NC
1
2
3
+
C₁
47µF/16V
Tantalum
V_OUT
5V @80mA
DS21372B-page 4
© 2002 Microchip Technology Inc.