ISL9305IRTWCLBZ-T View Datasheet(PDF) - Renesas Electronics

Part Name

Description

Manufacturer

ISL9305IRTWCLBZ-T

3MHz Dual Step-Down Converters and Dual Low-Input LDOs with I2C Compatible Interface

Renesas Electronics 

ISL9305

Theory of Operation

DCD1 and DCD2 Introduction

Both the DCD1 and DCD2 converters on ISL9305 use the

peak-current-mode pulse-width modulation (PWM) control

scheme for fast transient response and pulse-by-pulse current

limiting. Both converters are able to supply up to 800mA load

current. The default output voltage ranges from 0.8V to 3.6V

depending on the factory pre-set configuration and can be

programmed via the I2C interface in the range of 0.825V to 3.6V

at 25mV/step with a programmable slew rate. An open-drain

DCDPG (DCD Power-Good) signal is also provided to monitor the

DCD1 and DCD2 output voltages. Optionally, both DCD1 and

DCD2 can be programmed to be actively discharged via an

on-chip bleeding resistor (typical 115Ω) when the converter is

disabled.

Skip Mode (PFM Mode) for DCD1/DCD2

Under light load condition, the DCD1 and DCD2 can be

programmed to automatically enter a pulse-skipping mode to

minimize the switching loss by reducing the switching frequency.

Figure 2 illustrates the skip mode operation. A zero-cross sensing

circuit monitors the current flowing through the SW node for zero

crossing. When it is detected to cross zero for 16 consecutive

cycles, the regulator enters the skip mode. During the 16

consecutive cycles, the inductor current could be negative. The

counter is reset to zero when the sensed current flowing through

the SW node does not cross zero during any cycle within the 16

consecutive cycles. Once the converter enters the skip mode, the

pulse modulation is controlled by an internal comparator while

each pulse cycle remains synchronized to the PWM clock. The

P-Channel MOSFET is turned on at the rising edge of the clock

and turned off when its current reaches ~20% of the peak

current limit. As the average inductor current in each cycle is

higher than the average current of the load, the output voltage

rises cycle-over-cycle. When the output voltage is sensed to reach

1.5% above its nominal voltage, the P-Channel MOSFET is turned

off immediately and the inductor current is fully discharged to

zero and stays at zero. The output voltage reduces gradually due

to the load current discharging the output capacitor. When the

output voltage drops to the nominal voltage, the P-Channel

MOSFET will be turned on again, repeating the previous

operations.

The regulator resumes normal PWM mode operation when the

output voltage is sensed to drop below 1.5% of its nominal

voltage value as shown in Figure 3.

CLOCK

IL

0

VOUT

16 CYCLES

20% PEAK CURRENT LIMIT

1.015*VOUT_NOMINAL

VOUT_NOMINAL

FIGURE 2. SKIP MODE OPERATION WAVEFORMS

vEAMP

vCSA

d

iL

vOUT

FIGURE 3. PWM OPERATION WAVEFORMS

FN7605 Rev 2.00

February 9, 2015

Page 7 of 17

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