ADP2102-EVAL 查看數據表（PDF） - Analog Devices
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ADP2102-EVAL Datasheet PDF : 7 Pages
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Preliminary Technical Data
ADP2102
THEORY OF OPERATION
The ADP2102 is a high frequency synchronous step down dc-
dc buck converter optimized for battery powered portable
applications. It is based on a constant-ON time current mode
control architecture with voltage feed forward to null frequency
variation with line voltage thus creating a pseudo fixed
frequency.
This type of control allows generation of very low output
voltages at higher switching frequency and offers a very fast
load and line transient response with minimal external
component count and size. The ADP2102 provides features like
Under Voltage Lock Out, Thermal Shutdown and Short Circuit
Protection.
The ADP2102 uses valley current mode control, which helps to
prevent minimum-ON time limitations at very low output
voltages. This allows high frequency operation resulting in low
filter inductor and capacitor values.
CONTROL SCHEME
The ADP2102’s High Side Power Switch ON-time is determined
by a one shot timer whose pulse width is directly proportional
to output voltage and inversely proportional to the input or line
voltage. Another one-shot timer sets a minimum OFF time to
allow for inductor valley current sensing,
The constant ON-time one-shot timer is triggered at the rising
edge of EN and subsequently when the Low Side Power Switch
current is below the valley current limit threshold and the
minimum OFF-time one-shot timer has timed out.
While the constant ON-time is asserted, the high side power
switch is turned on. This causes the inductor current to ramp
positively. After the constant ON–time has completed the high
side power switch turns off and the low side power switch turns
on. This causes the inductor current to ramp negatively until
the sensed current flowing in this switch has reached valley
current limit. At this point, the low side power switch turns off
and a new cycle begins again with the high side switch turning
on provided the minimum OFF-time one shot has timed out.
CONSTANT ON-TIME TIMER
The constant ON-time timer sets the High Side Switch on time.
This fast, low jitter, adjustable one shot varies the ON-time in
response to input voltage for a given output voltage. The High
Side Switch ON –time is inversely proportional to the input
voltage and directly proportional to the output voltage.
Ton = K. (Vout /Vin) & Fsw = 1/K
Where “K” is an internally set On-time scale factor constant
resulting in a constant switching frequency. As can be seen in
the above equation, the steady state switching frequency is
theoretically independent of both the input and output voltages
to a first order. Therefore, with line voltage feed forward the
constant ON- time control scheme is pseudo fixed frequency.
This means the loop switches at a constant frequency until a
load step occurs. When a load step occurs, the constant ON-
time control loop responds by modulating the OFF time up or
down to quickly get back to regulation. This momentary
frequency variation results in a faster load transient response
than a fixed frequency current mode control loop of similar
bandwidth with similar external filter inductor and capacitor.
This is an advantage of constant–ON time control scheme.
As described above the frequency of the constant ON-time
control loop remains constant to a first order with line and
output voltage. There are some second order effects that cause
this frequency to increase slightly with load current.
Resistive voltage losses in the High and Low Side Power
Switches, package parasitics, inductor DCR and board parasitic
resistance cause the loop to compensate by reducing the OFF-
time and there fore increasing the switching frequency with
increasing load current.
A minimum OFF-time constraint is introduced to allow the
inductor valley current sensing on the synchronous switch.
FORCED CONTINUOUS CONDUCTION MODE
When the MODE pin is HIGH, the ADP2102 operates in
Forced Continuous Conduction Mode. In this mode,
irrespective of the load current, the inductor current stays
continuous and is the preferred mode of operation for low noise
applications. During this mode, the switching frequency stays
close to 3MHz typically. In this mode, the efficiency is lower
compared to the Power Save Mode during light loads but the
output voltage ripple is minimized.
POWER SAVE MODE
When the MODE pin is LOW, the ADP2102 operates in Power
Save Mode. In this mode, at light load currents, the part
automatically goes into reduced frequency operation where
some pulses are skipped to increase efficiency while still
remaining in regulation. At light loads, a zero crossing
comparator truncates the low side switch ON-time when the
inductor current becomes negative. In this condition, the part
works in Discontinuous Conduction Mode (DCM). The
threshold between CCM and DCM is approximately
Iload (skip) = (Vin- Vout) / (2 x L) x Ton
For higher load currents, the inductor current does not cross
zero threshold and the device switches to the Continuous
Conduction Mode and the frequency is fixed to the nominal
value. As a result of this auto –mode control technique, the
losses are minimized at light loads, improving the system
efficiency.
CURRENT LIMIT
The ADP2102 has protection circuitry to limit the amount of
current flowing through the High Side and the Low Side
switches. When the current flowing in the forward direction
becomes excessive due to a short on the OUT node to ground
Rev. PrA | Page 5 of 7
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