MAX8553EEE datasheet(15/24 Pages) MAXIM

15 / 24 page

4.5V to 28V Input, Synchronous PWM Buck Controllers

for DDR Termination and Point-of-Load Applications

______________________________________________________________________________________

Detailed Description

Internal Linear Regulator

An internal regulator produces the +5V supply (VL) that

powers the PWM controller, MOSFET driver, logic, refer-

ence, and other blocks within the IC. This +5V low-

dropout (LDO) linear regulator supplies up to 35mA for

MOSFET gate-drive and external loads. For supply volt-

ages between +4.5V and +5.5V, connect VL to V+. This

bypasses the VL regulator, which improves efficiency

and allows the IC to function at lower input voltages.

On-Time One-Shot and

Switching Frequency

The heart of the PWM is the one-shot that sets the high-

side switch on-time. This fast, low-jitter, adjustable one-

shot includes circuitry that varies the on-time in

response to both input and output voltages. The high-

side switch on-time is inversely proportional to the input

voltage as measured by the EN/HSD (HSD for the

MAX8554) input, and is directly proportional to the out-

put voltage. This algorithm results in a nearly constant

switching frequency despite the lack of a fixed-fre-

quency clock generator. The switching frequency can

be selected to avoid noise-sensitive regions such as

the 455kHz IF band. Also, with a constant switching fre-

quency, the inductor ripple-current operating point

remains relatively constant, resulting in easy design

methodology and predictable output voltage ripple.

The general formula for the MAX8553 on-time (tON) is:

where VEN/HSD and VOUT are the voltages measured at

EN/HSD and the output, respectively, and K = 1.7µs.

The value of N depends on the configuration of FSEL

and is listed in Table 1.

For the MAX8554, the general formula for on-time (tON) is:

where VHSD and VOUT are the voltages measured at

HSD and the output, respectively, and K = 1.7µs. The

value of N depends on the configuration of FSEL and is

listed in Table 2.

tK N

HSD

OUT

=× ×

tK N

EN HSD

OUT

=× ×

0.47

µF

10V

IRF7807V

IRF7821

2.5

µH

10A

Ω

2200pF

12.1k

Ω

6.04k

Ω

2x470

µF

6.3V

VOUT

PGND

4.7

µF

6.3V

CMPSH-3

1.8V AT

17V TO 21V

VIN

470

µF

16V

µF

16V

MAX8554

POK

20k

Ω

OFF

0.22

µF

10V

61.9k

Ω

80.6k

Ω

POK

V+

HSD

REF

ILIM

GND

FSEL

BST

PGND

OUT

Figure 3. Typical Application Circuit 3: 19V Input, 1.8V Output at Up to 8A with 300kHz Switching Frequency

MAX8553EEE Datasheet(PDF) 15 Page - Maxim Integrated Products

MAX8553EEE Datasheet(HTML) 15 Page - Maxim Integrated Products

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL