TC105
PFM/PWM Step-Down DC/DC Controller
FEATURES
s
s
s
s
s
s
s
Space-Saving 5-Pin SOT-23A Package
57
µ
A (Typ) Supply Current
1A Output Current
0.5
µ
A Shutdown Mode
300 KHz Switching Frequency for Small
Inductor Size
Programmable Soft-Start
92% Typical Efficiency
GENERAL DESCRIPTION
The TC105 is a step-down (Buck) switching controller
that furnishes output currents of up to 1A (max) while
delivering a typical efficiency of 92%. The TC105 normally
operates in pulse width modulation mode (PWM), but auto-
matically switches to pulse frequency modulation (PFM) at
low output loads for greater efficiency. Oscillator frequency
is 300 KHz, allowing use of small (22
µH)
inductors. Supply
current draw is only 102
µA
(max), and is reduced to less
than 0.5
µA
when the SHDN input is brought low. Regulator
operation is suspended during shutdown. The TC105 ac-
cepts a maximum input voltage of 10V.
The TC105 is housed in a tiny 5-pin SOT-23A package,
occupies minimum board space, and is ideal for a wide
range of applications.
TYPICAL APPLICATIONS
s
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Palmtops
Battery Powered Systems
Positive LCD Bias Generators
Portable Communicators
Hand-Held Scanners
5V to 3V Down Converters
ORDERING INFORMATION
Part
Number
3.3V
V
OUT
TYPICAL APPLICATION
Output
Voltage*
(V) Package
Osc. Operating
Freq.
Temp.
(KHz)
Range
L
1
22
µH
(Sumida CD54)
R
SS
470K
D
1
MA737
C
SS
0.033
µF
OFF ON
(From System
Control Logic)
TC105503ECT 5.0
TC105333ECT 3.3
TC105303ECT 3.0
5-Pin SOT-23A 300 –40 to +85°C
5-Pin SOT-23A 300 –40 to +85°C
5-Pin SOT-23A 300 –40 to +85°C
5
V
OUT
4
SHDN
C2
47
µF
10V
TANTALUM
NOTE: *Other output voltages available. Please contact Microchip
Technology for details.
PIN CONFIGURATION
5-Pin SOT-23A
TC105333ECT
EXT
V
DD
GND
1
2
3
5
Si 9430
P
4
V
OUT
V
BATT
6V
NiMH
C
1
10
µF/16V
SHDN
TC105
EXT
1
3.3V Regulated Supply Using 6V
NiMH Battery Pack Input
V
DD
2
GND
3
© 2001 Microchip Technology Inc.
DS21349A
TC105-2 5/24/99
PFM/PWM Step-Down DC/DC Controller
TC105
ABSOLUTE MAXIMUM RATINGS*
Voltage on V
DD .........................................................
–0.3V to +12V
EXT Output Current ............................................
±100
mA
Voltage on V
OUT
, EXT, SHDN Pins .... –0.3V to V
DD
+0.3V
Power Dissipation ................................................150 mW
Operating Temperature ............................ –40°C to +85°C
Storage Temperature ............................. –40°C to +125°C
*Static-sensitive device. Unused devices must be stored in conductive
material. Protect devices from static discharge and static fields. Stresses
above those listed under Absolute Maximum Ratings may cause perma-
nent damage to the device. These are stress ratings only and functional
operation of the device at these or any other conditions above those
indicated in the operational sections of the specifications is not implied.
Exposure to Absolute Maximum Rating Conditions for extended periods
may affect device reliability.
ELECTRICAL CHARACTERISTICS:
Note 1, f
OSC
= 300 KHz; T
A
= 25°C, unless otherwise noted.
Symbol
V
DD
V
DDMIN
I
DD
I
STBY
I
SHDN
f
OSC
V
OUT
DTYMAX
DTYPFM
V
IH
V
IL
REXTH
Parameter
Operating Supply Voltage
Minimum Input Voltage
Operating Supply Current
Standby Supply Current
Shutdown Supply Current
Oscillator Frequency
Output Voltage
Maximum Duty Cycle
(PWM Mode)
Duty Cycle
(PFM Mode)
SHDN Input Logic High
SHDN Input Logic Low
EXT ON Resistance to V
DD
Test Conditions
Min
Typ
2.2
0.9
—
—
—
—
—
255
V
R
x 0.975
100
15
0.65
—
—
—
—
—
—
—
—
Max
—
—
57
67
15
16
—
300
V
R
—
25
—
—
17
16
12
15
14
10
92
Unit
10.0
2.2
102
122
27
29
0.5
345
V
R
x 1.025
—
35
—
0.20
24
22
17
20
19
14
—
V
V
µA
µA
µA
µA
µA
KHz
EXT = High; No External Components;
V
OUT
= 0V; SHDN = V
IN
No External Components;
V
R
= 3.0V, 3.3V
V
OUT
= 0V; SHDN = V
IN,
V
R
= 5.0V
No External Components;
V
R
= 3.0V, 3.3V
V
OUT
= SHDN =
V
IN,
V
R
= 5.0V
SHDN = GND
V
IN
= V
OUT
+ 0.3V
Note 2
%
%
I
OUT
= 0 mA
V
OUT
= 0V; No External Components
V
OUT
= 0V; No External Components
No External Components;
V
R
= 3.0
V
R
= 3.3
V
R
= 5.0
V
R
= 3.0
V
R
= 3.3
V
R
= 5.0
V
Ω
Ω
Ω
Ω
Ω
Ω
%
REXTL
EXT ON Resistance to GND
V
OUT
= SHDN = V
IN
; V
EXT
= (V
IN
– 0.4V)
No External Components;
V
OUT
= 0V; SHDN = V
IN
; V
EXT
= 0.4V
η
Efficiency
Note: 1. V
R
= 3.0V V
IN
= 4.5V, I
OUT
= 200mA
V
R
= 3.3V V
IN
= 5.0V, I
OUT
= 220mA
V
R
= 5.0V V
IN
= 7.5V, I
OUT
= 320mA
2. V
R
is the factory output voltage setting.
TC105-2 5/24/99
2
© 2001 Microchip Technology Inc.
DS21349A
PFM/PWM Step-Down DC/DC Controller
TC105
PIN DESCRIPTION
Pin Number
1
Name
EXT
Description
Switch Transistor Control Output. This terminal connects to the gate of an external P-
channel MOSFET (or to the base of an external PNP transistor through a current
limiting resistor).
Power Supply Voltage Input.
Ground Terminal.
Shutdown Input (Active Low). The TC105 enters a low-power shutdown state when this
input is brought low. During shutdown, regulator action is suspended, and supply
current is reduced to less than 0.5
µA.
The TC105 resumes normal operation when
SHDN is again brought high.
Voltage Sense Input. This input senses output voltage for regulation and must be
connected to the output voltage node as shown in the application schematics in this
data sheet.
2
3
4
V
DD
GND
SHDN
5
V
OUT
DETAILED DESCRIPTION
TC105 is a PFM/PWM step-down DC/DC controller for
use in systems operating from two or more cells, or in line-
powered applications. It uses PWM as the primary modula-
tion scheme, but automatically converts to PFM at output
duty cycles less than approximately 10%. The conversion to
PFM provides reduced supply current, and therefore higher
operating efficiency at low loads. The TC105 uses an
external switching transistor, allowing construction of
switching regulators with output currents of up to 1A. The
TC105 consumes only 102
µA,
max, of supply current when
V
IN
= 5V and V
OUT
= 3.3V, and can be placed in a 0.5
µA
shutdown mode by bringing the shutdown input (SHDN)
low. The regulator remains disabled while in shutdown
mode, and output voltage discharges to zero through the
load. Normal operation resumes when SHDN is brought
high. Other features include a built-in undervoltage lockout
(UVLO) and externally programmable soft start time. The
TC105 is housed in a tiny 5-pin SOT-23A package.
Undervoltage Lockout (UVLO)
The TC105 is disabled when V
IN
is below the
undervoltage lockout threshold. This threshold is equal to
the guaranteed minimum operating voltage for the TC105
(i.e. 2.2V). When UVLO is active, the TC105 is completely
disabled.
Input Bypass Capacitors
Using an input bypass capacitor reduces peak current
transients drawn from the input supply and reduces the
switching noise generated by the regulator. The source
impedance of the input supply determines the size of the
capacitor that should be used.
Output Capacitor
The effective series resistance of the output capacitor
directly affects the amplitude of the output voltage ripple.
(The product of the peak inductor current and the ESR
determines output ripple amplitude.) Therefore, a capacitor
with the lowest possible ESR should be selected. Smaller
capacitors are acceptable for light loads or in applications
where ripple is not a concern. The Sprague 595D series of
tantalum capacitors are among the smallest of all low ESR
surface mount capacitors available. Table 1 lists suggested
component numbers and manufacturers.
Low Power Shutdown Mode
The TC105 enters a low power shutdown mode when
SHDN is brought low. While in shutdown, the oscillator is
disabled and the output switch is shut off. Normal regulator
operation resumes when SHDN is again brought high.
SHDN may be tied to the input supply if not used.
Soft Start
Soft start allows the output voltage to gradually ramp
from 0 to rated output value during start-up. This action
minimizes (or eliminates) overshoot, and in general, re-
duces stress on circuit components. Figure 1 shows the
circuit required to implement soft start (values of 470K and
0.033
µF
for R
SS
and C
SS,
respectively, are adequate for
most applications).
© 2001 Microchip Technology Inc.
DS21349A
Inductor Selection
Selecting the proper inductor value is a trade-off be-
tween physical size and power conversion requirements.
Lower value inductors cost less, but result in higher ripple
current and core losses. They are also more prone to
saturate since the coil current ramps faster and could
overshoot the desired peak value. This not only reduces
3
TC105-2
5/24/99
PFM/PWM Step-Down DC/DC Controller
TC105
efficiency, but could also cause the current rating of the
external components to be exceeded. Larger inductor val-
ues reduce both ripple current and core losses, but are
larger in physical size and tend to increase the start-up time
slightly. A 22
µH
inductor is recommended as the best
overall compromise. For highest efficiency, use inductors
with a low DC resistance (less than 20 mΩ). To minimize
radiated noise, consider using a toroid, pot core, or shielded-
bobbin inductor.
possible. The length of the EXT trace must also be kept as
short as possible.
APPLICATIONS
Circuit Examples
Figure 3 shows a TC105 using a PNP switching transis-
tor (Zetex FZT749) that has an h
FE
of 180 and V
CESAT
of 100
mV at I
C
= 1A. Other high beta transistors can be used, but
the values of R
B
and C
B
may need adjustment if h
FE
is
significantly different from that of the FZT749.
The circuit of Figure 4 utilizes a P-Channel MOSFET
switching transistor (Silconix Si9430). This transistor is a
member of the Littlefoot ™ family of small outline MOSFETs.
R
SS
470K
V
IN
4
SHDN/SS
Output Diode
The high operating frequency of the TC105 requires a
high-speed diode. Schottky diodes such as the MA737 or
1N5817 through 1N5823 (and the equivalent surface mount
versions) are recommended. Select a diode whose average
current rating is greater than the peak inductor current and
whose voltage rating is higher than V
DD(max)
.
External Switching Transistor Selection
EXT is a complementary output with a maximum ON
resistance of 22Ω to V
DD
when high and 19Ω to ground
when low. It is designed to directly drive a P-channel
MOSFET or a PNP bipolar transistor through a base current
limiting resistor (Figure 2). A PNP transistor is recom-
mended in applications where V
IN
is less than 2.5V. Other-
wise, a P-Channel MOSFET is preferred as it affords the
highest efficiency because it does not draw any gate drive
current. However, P-Channel MOSFETs are typically more
expensive than bipolar transistors.
P-Channel MOSFET selection is determined mainly by
the on-resistance, gate-source threshold, and gate charge
requirements. Also, the drain-to-source and gate-to-source
breakdown voltage ratings must be greater than V
DD(max)
.
The total gate charge specification should be less than 100
nC for best efficiency. The MOSFET must be capable of
handling the required peak inductor current, and should
have a very low on-resistance at that current. For example,
an Si9430 MOSFET has a drain-to-source rating of –20V,
and a typical on-resistance r
DS(ON)
of 0.07Ω at 2A, with V
GS
= –4.5V. Table 1 lists suppliers of external components
recommended for use with the TC105.
C
SS
0.033
µF
TC105
Shutdown Not Used
R
SS
470K
SHDN
OFF ON
(from System
Control Logic)
4
SHDN/SS
C
SS
0.033
µF
TC105
Shutdown Used
Figure 1. Soft Start Circuit
Board Layout Guidelines
As with all inductive switching regulators, the TC105
generates fast switching waveforms, which radiate noise.
Interconnecting lead lengths should be minimized to keep
stray capacitance, trace resistance, and radiated noise as
low as possible. In addition, the GND pin, input bypass
capacitor, and output filter capacitor ground leads should be
connected to a single point. The input capacitor should be
placed as close to power and ground pins of the TC105 as
TC105-2 5/24/99
4
© 2001 Microchip Technology Inc.
DS21349A
PFM/PWM Step-Down DC/DC Controller
TC105
L
V
IN
P
D
C
L
V
OUT
V
IN
Q
D
L
V
OUT
C
L
R
B
C
B
TC105
EXT
1
V
DD
EXT
1
TC105
P-Channel MOSFET
Figure 2. External Transistor Connection
PNP Bipolar Transistor
V
IN
2.5V
10
µF
16V
FZ749
Q1
22
µH
Sumida CD-54
V
OUT
1.8V
MA737
47
µF
10V Tantalum
R
SS
470K
OFF ON
5
V
OUT
4
SHDN/SS
C
SS
0.033
µF
CB
2200 pF
R
B
500Ω
EXT
1
TC105
V
DD
2
GND
3
Figure 3. Regulator Using PNP Transistor
© 2001 Microchip Technology Inc.
DS21349A
5
TC105-2
5/24/99
