LTC3104IMSE#TRPBF【PDF 15/20 页】IC REG DL BCK/LINEAR SYNC 16MSOP

LTC3104

15

3104f

APPLICATIONS INFORMATION

V

IN

voltage requirements. If used, note that the external

divider current is not included in the LTC3104 quiescent

current specification.

The rising UVLO threshold can be calculated using the

following equation:

V

UVLO

=0.8V 1+

R6

R5

?/DIV>

For most applications a 0.022礔 will suffice. The capaci-

tor should be placed as close to the respective pins as

possible.

LDO Output Capacitor Selection

The LDO is designed to be stable with a minimum 4.7礔

output capacitor. No series resistor is required when using

low ESR capacitors. For most applications, a 10礔 ceramic

capacitor is recommended. Larger values will improve

transient response and raise the power supply rejection

ratio (PSRR) of the LDO.

LDO Output Voltage Programming

The output voltage is set by a resistive divider according

to the following formula:

V

LDO

=0.6V 1+

R4

R3

?/DIV>

The external divider is connected to the LDO output, V

LDO

,

as shown in Figure 3. Similar to the buck feedback network,

a feedforward capacitor may be placed in parallel with

resistor R4 for improved transient response. For resistor

values of ~1M a 12pF ceramic capacitor will suffice.

Figure 2. Setting the Undervoltage Lockout Threshold

RUN

LTC3104

GND

IN

R5

R6

3104 F02

FBLDO

R4

R3

C

FF2

3104 F03

V

LDO

GND

LTC3104

Figure 3. Setting the LDO Output Voltage

14

13

10

11

12

4

5

3

2

1

V

INLDO

V

LDO

FBLDO

FB

RUN

V

CC

NC

MODE

V

IN

SW

BST

GND

RUNLDO

PGOOD

6

7

9

8

KELVIN TO V

OUT

V

INLDO

V

LDO

3104 F04

UNINTERRUPTED GROUND PLANE

SHOULD EXIST UNDER ALL COMPONENTS

SHOWN AND UNDER THE TRACES

CONNECTING THOSE COMPONENTS

V

OUT

VIA GROUND PLANE

V

IN

Figure 4. PCB Layout Recommendations

Internal V

CC

Regulator

The LTC3104 uses an internal NMOS source follower

regulator off of V

IN

to generate a low voltage internal rail,

V

CC

. The regulator is designed to deliver current only to

the internal drivers and other internal control circuits and

not to an external load. The V

CC

pin should be bypassed

with a 1礔 or larger ceramic capacitor.

Boost Capacitor Selection

The LTC3104 uses a bootstrapped supply to power the

buck switch gate drivers. When the synchronous rectifier

turns on, an internal PMOS switch turns on synchronously

to charge the boost capacitor, C

BST

, to the voltage on V

CC

.

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