ISL97649A

11

FN7928.0

December 5, 2011

Applications Information

Enable Control

With VIN > UVLO, only the Logic output channel is activated. All

other functions in ISL97649A are shut down when the enable pin

is pulled down. When the voltage at the enable pin reaches H

threshold, the whole chip turns on.

Frequency Selection

The ISL97649A switching frequency can be user selected to

operate at either constant 600kHz or 1.2MHz. Lower switching

frequency can save power dissipation at very light load

conditions. Also, low switching frequency more easily leads to

discontinuous conduction mode, while higher switching

frequency allows for smaller external components, such as

inductor and output capacitors, etc. Higher switching frequency

will get higher efficiency within some loading range depending

on VIN, VOUT, and external components, as shown in Figure 1.

Connecting the FREQ pin to GND sets the PWM switching

Soft-Start

The soft-start is provided by an internal current source to charge

the external soft-start capacitor. The ISL97649A ramps up the

current limit from 0A up to the full value, as the voltage at the SS

pin ramps from 0V to 0.8V. Hence, the soft-start time is 3.2ms

when the soft-start capacitor is 22nF, 6.8ms for 47nF and

14.5ms for 100nF.

Operation

The boost converter is a current mode PWM converter operating

at either 600kHz or 1.2MHz. It can operate in both discontinuous

conduction mode (DCM) at light load and continuous conduction

mode (CCM). In continuous conduction mode, current flows

continuously in the inductor during the entire switching cycle in

steady state operation. The voltage conversion ratio in

continuous current mode is given by Equation 1:

where D is the duty cycle of the switching MOSFET.

The boost regulator uses a summing amplifier architecture

consisting of gm stages for voltage feedback, current feedback

and slope compensation. A comparator looks at the peak

inductor current cycle-by-cycle and terminates the PWM cycle if

the current limit is reached.

An external resistor divider is required to divide the output

voltage down to the nominal reference voltage. Current drawn by

the resistor network should be limited to maintain the overall

converter efficiency. The maximum value of the resistor network

is limited by the feedback input bias current and the potential for

noise being coupled into the feedback pin. A resistor network in

the order of 60kΩ is recommended. The boost converter output

voltage is determined by Equation 2:

This restricts the maximum output current (average) based on

Equation 3:

where

by Equation 4:

Capacitor

An input capacitor is used to suppress the voltage ripple injected

into the boost converter. The ceramic capacitor with a

capacitance larger than 10µF is recommended. The voltage

rating of the input capacitor should be larger than the maximum

input voltage. Some input capacitors are recommended in

Table 1.

Inductor

The boost inductor is a critical part that influences the output

voltage ripple, transient response, and efficiency. Values of

3.3µH to 10µH are used to match the internal slope

compensation. The inductor must be able to handle the following

average and peak currents shown in Equation 5:

Some inductors are recommended in Table 2 for different design

considerations.

Rectifier Diode

A high-speed diode is necessary due to the high switching

frequency. Schottky diodes are recommended because of their

fast recovery time and low forward voltage. The reverse voltage

rating of this diode should be higher than the maximum output

voltage. The rectifier diode must meet the output current and

peak inductor current requirements. Table 3 shows some

recommendations for boost converter diode.

-----------------

1

1D

–

-------------

=

(EQ. 1)

+

--------------------

×

=

(EQ. 2)

TABLE 1. BOOST CONVERTER INPUT CAPACITOR RECOMMENDATION

CAPACITOR

SIZE

MFG

PART NUMBER

10µF/6.3V

0603

TDK

C1608X5R0J106M

10µF/16V

1206

TDK

C3216X7R1C106M

10µF/10V

0805

Murata

GRM21BR61A106K

22µF/10V

1210

Murata

GRB32ER61A226K

TABLE 2. BOOST CONVERTER INDUCTOR RECOMMENDATION

INDUCTOR

DIMENSIONS

(mm)

MFG

PART

NUMBER

NOTE

10µH/

4Apeak

8.3x8.3x4.5 Sumida CDR8D43-100NC

Efficiency

Optimization

6.8µH/

1.8Apeak

5.0x5.0x2.0

TDK

PLF5020T-6R8M1R8

2

--------

–

⎝⎠

--------

×

=

(EQ. 3)

L

--------

D

----

×

=

(EQ. 4)

1D

–

-------------

=

ΔI

L

2

--------

+

=

(EQ. 5)