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ADP1052

Data Sheet

Rev. B | Page 28 of 113

a 7-bit CS2 current averaging speed. The 9-bit CS2 current averag-

ing speed has a VS± basic voltage change rate of 1.18 mV/ms.

The 7-bit CS2 current averaging speed has a VS± basic voltage

change rate of 4.72 mV/ms. In addition, the output voltage change

rate can be set by Register 0xFE3A[7:6] to 1×, 2×, 4×, or 8× the

VS± basic voltage change rate.

PULSE SKIPPING

The pulse skipping function reduces the switching loss under

very light load current conditions while keeping the output voltage

stable. Set Register 0xFE67[6] to activate this function.

When light load mode or deep light load mode is enabled, as the

output current drops, the supply enters discontinuous conduction

mode (DCM). In DCM, the modulation value is a function of the

load current. If a very light load current requires a modulation

value (duty cycle) of less than the threshold set by Register 0xFE69,

pulse skipping mode is enabled. In pulse skipping mode, the PWM

output appears intermittently. If the digital compensator signals

an error requiring a modulation value that is less than the threshold

set by Register 0xFE69, no PWM pulses are generated. If the digital

compensator signals an error requiring a modulation value that is

greater than the threshold set by Register 0xFE69, PWM pulses

are generated.

The pulse skipping mode is always blanked during soft start.

PREBIAS STARTUP

The prebias start-up function provides the capability to start up

with a prebiased voltage on the output. It protects the power

supply against existing external voltage on the output during

startup and ensures a monotonic startup before the power supply

reaches full regulation (see Figure 32).

VOUT

PSON

PWM

OUTPUTS

Figure 32. Prebias Startup

The prebias start-up function is enabled by Register 0xFE25[7].

During prebias startup, the ADP1052 soft start ramp starts at

the existing voltage value sensed on the VS± pins, and the soft

start ramp time is reduced proportionally.

The initial PWM modulation value does not begin with zero

but, instead, with a value that builds a balanced relationship

between the input voltage and the output voltage. This balance

avoids the sudden charging or discharging of the output capacitor

and achieves a monotonic and smooth startup. Use the

following equation to calculate the initial modulation value:

NOM

OUT

NOM

MODU

INI

MODU

where:

tMODU_INI is the initial modulation value when the controller begins

to generate PWM pulses during startup.

tMODU_NOM is the modulation value set by Register 0xFE39. It

emulates the modulation value when the input voltage and the

output voltage are in the nominal condition.

VOUT is the sensed output voltage.

VOUT_NOM is the nominal output voltage set by VOUT_COMMAND

(Register 0x21).

VIN_NOM is the nominal input voltage when the VF pin voltage = 1 V.

VIN is the sensed input voltage.

In addition, Register 0xFE6C[1] is set for correct operation. To

sense the input voltage (represented by VF) when the power supply

is off, use additional circuitry, such as an auxiliary power circuit,

to sense the input voltage.

If the input voltage signal is not available when the power is off,

the tMODU_INI value is calculated based on the tMODU_NOM and the

output voltage information. In this case, Register 0xFE6C[1]

clears to 0. The initial modulation value is calculated as

NOM

OUT

NOM

MODU

INI

MODU

where:

tMODU_INI is the initial modulation value when the controller begins

to generate PWM pulses during startup.

tMODU_NOM is the modulation value set by Register 0xFE39. It

emulates the modulation value when the input voltage and the

output voltage are in the nominal condition.

VOUT is the sensed output voltage.

VOUT_NOM is the nominal output voltage set by VOUT_COMMAND

(Register 0x21).

If the closed-loop, line voltage, feedforward function is selected,

the input voltage is introduced from the feedforward loop, and

the VIN value is always included for calculation of the initial

modulation value.

Reverse current protection can also be used in prebias start-up

mode. See the Synchronous Rectification (SR) Reverse Current

Protection section for details. Additionally, to achieve a smooth

transition, the SR soft start can be enabled in this mode. See the

Synchronous Rectification section for details.

OUTPUT VOLTAGE DROOPING CONTROL

The ADP1052 features output voltage drooping control for

current sharing applications. Output voltage drooping is intro-

duced digitally by modifying the value of the digital output

voltage reference, based on the output current value. Two

parameters can be configured independently: the drooping resistor

and the output current averaging speed.

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