Preliminary Technical Data

ADP2102

Rev. PrA | Page 5 of 7

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