ADP1621ARMZ-R7 datasheet(12/32 Pages) AD

12 / 32 page

ADP1621

Rev. A | Page 12 of 32

THEORY OF OPERATION

The ADP1621 is a fixed-frequency, current-mode, step-up dc/dc

converter controller. It drives an external n-channel MOSFET

to step the input voltage up to a higher output voltage. It can be

used for SEPIC, flyback, boost, buck-boost, forward, and other

converter topologies. It operates at a fixed switching frequency that

is set by an external resistor over a range of 100 kHz to 1.5 MHz,

and it can be synchronized to an external clock by connecting

the SDSN pin to the clock.

The input supply current to the ADP1621 is less than 3 mA

during normal operation and less than 10 μA during shutdown.

The ADP1621 can drive large external MOSFETs, allowing it to

support load currents in excess of 10 A.

CONTROL LOOP

The ADP1621 uses a current-mode architecture to regulate the

output voltage. The output voltage is monitored at FB through

a resistive voltage divider. The voltage at FB is compared to the

internal 1.215 V reference voltage by the internal transconductance

error amplifier to create an error current at COMP. A resistor-

capacitor compensation impedance connected from COMP to

GND converts the error current to an error voltage.

At the beginning of the switching cycle, the MOSFET is turned

on and the inductor current ramps up. The MOSFET current is

measured and converted to a voltage using RCS or RDSON and is

added to the stabilizing slope-compensation ramp. The resulting

voltage sum passes through the current-sense amplifier to generate

the current-sense voltage. When the current-sense voltage is

greater than the COMP error voltage, the MOSFET is turned off

and the inductor current ramps down until the internal clock

initiates the next switching cycle. The duty-cycle of the PWM

modulator is thus adjusted to provide the necessary load current

at the desired output voltage. Because the output voltage ultimately

controls the peak inductor current through the COMP error

voltage, this scheme is referred to as peak current-mode control.

With light loads, the converter can also operate under discon-

tinuous conduction mode and pulse-skipping modulation to

maintain output-voltage regulation. These two forms of operation

are discussed in detail in the Light Load Operation section.

Note that the converter can also be designed to operate in

discontinuous conduction mode at full load if desired.

Overall, the current-mode regulation system of the ADP1621

allows fast transient responses while maintaining a stable output

voltage. By selecting the proper resistor-capacitor network from

COMP to GND, the regulator response can be optimized for a

wide range of input voltages, output voltages, and load currents.

CURRENT-SENSE CONFIGURATIONS

The ADP1621 can sense the current across the on resistance of

the MOSFET to minimize external component count and improve

efficiency by eliminating the power that would be lost in a current-

sense resistor. This lossless technique eliminates the need for an

expensive current-sense resistor. In the lossless mode configuration,

the voltage at the CS pin (or the switch-node voltage at the drain of

the MOSFET) must not exceed 30 V (see Figure 28). This technique

maximizes efficiency and reduces cost. In practice, when the

calculated VSW approaches 30 V, one should build the board and

measure the actual VSW before committing to the lossless mode

design. Because of the parasitic inductance in the diode, output

capacitor, and PCB traces, VSW typically has narrow peaks that

exceed the theoretical maximum voltage at VSW—the sum of

VOUT and the forward-voltage drop of Diode D1. If the measured

peak voltage exceeds 30 V, or if a more accurate current limit is

desired, then the CS pin can be connected to an external current-

sense resistor in the source of the MOSFET (Figure 29). The

maximum power output is limited by the selection of the

external components.

GATE

SDSN

GND

PGND

VIN

VOUT

VSW

ADP1621

Figure 28. CS Pin Connection for VSW < 30 V, Lossless Mode

(No Current-Sense Resistor Needed)

SDSN

GND

GATE

PGND

VIN

VOUT

RCS

VSW

ADP1621

Figure 29. CS Pin Connection for VSW > 30 V, Resistor Sense Mode

with a Current-Sense Resistor, RCS

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