MAX8553 Maxim, MAX8553 Datasheet - Page 15

• Manufacturer Part Number: MAX8553
• Description: 4.5V to 28V Input / Synchronous PWM Buck Controllers for DDR Termination and Point-of-Load Applications
• Manufacturer: Maxim

Figure
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.
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
4.5V to 28V Input, Synchronous PWM Buck Controllers
3. Typical Application Circuit 3: 19V Input, 1.8V Output at Up to 8A with 300kHz Switching Frequency
for DDR Termination and Point-of-Load Applications
OFF
POK
ON
20kΩ
61.9kΩ
80.6kΩ
R1
R4
0.22µF
10V
C7
R5
______________________________________________________________________________________
Detailed Description
Internal Linear Regulator
POK
V+
EN
HSD
REF
ILIM
GND
On-Time One-Shot and
MAX8554
Switching Frequency
FSEL
PGND
OUT
BST
DH
DL
VL
LX
FB
CMPSH-3
D1
C4
0.47µF
10V
VL
C3
4.7µF
6.3V
Q1
IRF7807V
Q2
IRF7821
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 (t
where V
EN/HSD and the output, respectively, and K = 1.7µs.
The value of N depends on the configuration of FSEL
and is listed in
For the MAX8554, the general formula for on-time (t
where V
HSD and the output, respectively, and K = 1.7µs. The
value of N depends on the configuration of FSEL and is
listed in
EN/HSD
R6
3Ω
Table
HSD
C8
2200pF
2.5µH
t
10A
L1
ON
and V
t
ON
2.
Table
and V
=
=
K N
OUT
1.
× ×
K N
OUT
R2
12.1kΩ
R3
6.04kΩ
C2
470µF
16V
× ×
are the voltages measured at
are the voltages measured at
V
EN HSD
V
HSD
1
/
1
C6
2x470µF
4V
×
×
V
17V TO 21V VIN
OUT
V
1.8V AT
OUT
8A
C1
10µF
16V
C5
10µF
6.3V
VOUT
PGND
ON
ON
) is:
) is:
15