AME, Inc.
AME5172
n
Detailed Description
The AME5172 is a fixed frequency synchronous boost
converter in SOT-25 package. The feature include current
mode PWM control with internal frequency compensa-
tion, low R
DS(ON)
MOSFET switch, synchronous rectifica-
tion and 800mA peak switching current limit.
0.85V Single Cell Boost
DC-DC Converter
n
Application Information
Inductor Selection
An inductor value of 4.7µH performs well in most appli-
cation. Larger values of inductance can reduce the induc-
tor ripple current. Choosing inductors with low ESR de-
crease power loss and increase efficiency.
The approximate output current capability versus induc-
tance value is given in the equation below:
Synchronous Rectification
Built-in 0.35Ω NMOS switch and 0.45Ω PMOS syn-
chronous rectification significantly improves efficiency and
eliminates the need for an external Schottky diode. High
switching frequency (at 1.2MHz) also allows easy filter-
ing and faster loop performance.
V
×
D
I
OUT
(max)
=
η
(
1
−
D
)
I
CL
−
IN
2
×
L
×
f
SW
where:
η
=
efficiency
Low Voltage Start-Up
The AME5172 will start-up at typical input voltage of
0.85V. At start-up, switching frequency is determined by
the start-up oscillator and start-up current limit. When
V
OUT
exceeds 2.3V, the start-up circuit will disabled and
the PWM mode is initiated.
D
=
V
OUT
−
V
IN
V
OUT
I
CL
=switching peak current limit
V
IN
=input voltage
L=inductance value
F
SW
=switching frequency
Output Capacitor Selection
Thermal Shutdown Protection
The AME5172 has thermal shutdown protection at
160
O
C (Typ.). If the device temperature rises above 160
O
C,
the internal power MOSFET will turn off. Once the tem-
perature falls below 140
O
C, the device resumes normal
operation. However, device functionality is only guaran-
teed to a maximum junction temperature of +125
O
C.
A single ceramic capacitor of value 4.7µF to 10µF will
provide sufficient output capacitance for most applications.
Larger capacitance with low ESR can be used to improve
ripple and transient response.
An output capacitor with excessive ESR can also re-
duce phase margin and cause instability. In general, if
electrolytic are used, it is recommended to parallel with
ceramic capacitors to reduce ringing, switching losses,
and output voltage ripple.
Input Capacitor Selection
An input capacitor is required to serve as an energy res-
ervoir for the current which must flow into the coil each
time the switch turns ON. This capacitor must have ex-
tremely low ESR, so ceramic is the best choice. We rec-
ommend a nominal value of 4.7µF, but larger values can
be used. Since this capacitor reduces the amount of volt-
age ripple seen at the input pin, it also reduces the amount
of EMI passed back to other circuitry.
Rev. A.02
7
AME [ ANALOG MICROELECTRONICS ]
