ADP2102 Analog Devices, ADP2102 Datasheet - Page 13

• Manufacturer Part Number: ADP2102
• Description: Low Duty Cycle, 600 mA, 3 MHz Synchronous Step-Down DC-to-DC Converter
• Manufacturer: Analog Devices

THEORY OF OPERATION
The ADP2102 is a high frequency, synchronous step-down,
dc-to-dc converter optimized for battery-powered, portable
applications. It is based on constant on-time current-mode
control architecture with voltage feed forward to null frequency
variation with line voltage, creating a pseudofixed frequency.
This type of control allows generation of very low output voltages
at a higher switching frequency and offers a very fast load and
line transient response with minimal external component count
and size. The ADP2102 provides features such as undervoltage
lockout, 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 high-side power switch on-time is determined by
a one-shot timer whose pulse width is directly proportional to
the 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 with the high-side switch turning on,
provided that 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.
The duty cycle for a buck converter operating in continuous
conduction mode (CCM) is given by D = V
definition, D = t
terms of V
t
t
ON
ON
= V
= K(V
OUT
OUT
/V
OUT
/(V
ON
IN
/V
/(t
and t
IN
× f
IN
ON
)
SW
+ t
ON
)
/(t
OFF
ON
). Therefore, equating the duty cycle
+ t
OFF
) gives
OUT
/V
IN
and, by
Rev. B | Page 13 of 24
(1)
(2)
Equating Equation 1 and Equation 2 gives
where K is an internally set on-time scale factor constant resulting
in a constant switching frequency.
As shown in Equation 1, the steady state switching frequency
is theoretically independent of both the input and output voltages
to a first order. This means the loop switches at a nearly 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
return 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 a similar
external filter inductor and capacitor. This is an advantage of
a constant on-time control scheme.
Resistive voltage losses in the high-side and low-side power
switches, package parasitics, inductor DCR, and board parasitic
resistance cause the loop to compensate by reducing the off time
and, therefore, increase the switching frequency with increasing
load current.
A minimum off-time constraint is introduced to allow 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 (CCM). In this mode, irrespective
of the load current, the inductor current stays continuous, and
CCM is the preferred mode of operation for low noise applications.
During this mode, the switching frequency stays close to 3 MHz
typical. In this mode, efficiency is lower at light loads, compared to
the power save mode, but the output voltage ripple is minimized.
POWER SAVE MODE
When the MODE pin is low, the ADP2102 operates in power
save mode (PSM). In this mode, at light load currents, the part
automatically goes into reduced frequency operation where
some pulses are skipped to increase efficiency while 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
There is a first-order dependency of this threshold on the internally
set on-time scale factor indicated in Equation 3. For higher load
currents, the inductor current does not cross zero threshold. The
device switches to the continuous conduction mode, and the
frequency is fixed to the nominal value.
f
I
SW
LOAD
= 1/K
(skip) =
(
V
2
IN
×
−
L
V
×
OUT
V
IN
)
×
×
V
f
OUT
SW
ADP2102
(4)
(3)