MAX1917
Tracking, Sinking and Sourcing, Synchronous Buck
Controller for DDR Memory and Termination Supplies
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Detailed Description
Internal Linear Regulator (VL)
An internal regulator produces the 5V supply (VL) that
powers the PWM controller, MOSFET driver, logic, ref-
erence, 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 input, and is directly propor-
tional to the VTT output voltage. This algorithm results in
a nearly constant switching frequency despite the lack
of a fixed-frequency clock generator. The switching fre-
quency can be selected to avoid noise-sensitive regions
such as the 455kHz IF band. Also, with a constant
switching frequency, the inductor ripple-current operat-
ing point remains relatively constant, resulting in easy
design methodology and predictable output voltage rip-
ple. The general formula for on time (t
ON
) is:
where V
HSD
and V
DDR
are the voltages measured at
EN/HSD and DDR, respectively, and K = 1.7µs. The
value of N depends on the configuration of FSEL and is
listed in Table 1.
The actual switching frequency, which is given by the
following equation, varies slightly due to voltage drop
across the on-resistance of the MOSFETs and the DC
resistance of the output inductor:
where I
O
is the output current, R
DSONH
is the on-resis-
tance of the high-side MOSFET, R
DSONL
is the on-
resistance of the low-side MOSFET, and R
DC
is the DC
resistance of the output inductor. The above equation is
valid only when FSEL is connected to ground. The ideal
switching frequency for V
DDR
= 2.5V is about 550kHz.
The switching frequency, which is almost constant,
results in relatively constant inductor ripple current
regardless of input voltage and predictable output volt-
age ripple. This feature eases design methodology.
Switching frequency increases for positive (sourcing)
load current and decreases for negative (sinking) load
current, due to the changing voltage drop across the
low-side MOSFET, which changes the inductor-current
discharge ramp rate. The on times guaranteed in the
Electrical Characteristics tables are also influenced by
switching delays caused by the loading effect of the
external power MOSFETs.
VTTR Reference
The MAX1917 VTTR output is capable of sourcing or
sinking up to 25mA of current. The V
TTR
output voltage
is one half of the voltage applied to the DDR input.
Bypass VTTR with at least a 1.0µF capacitor.
EN/HSD Function
In order to reduce pin count and package size, the
MAX1917 features a dual-function input pin, EN/HSD.
When EN/HSD is connected to ground, the internal cir-
cuitry powers off, reducing current consumption to less
than 5µA typical (circuit of Figure 6). To enable normal
operation, connect EN/HSD to the drain of the high-side
MOSFET. If EN/HSD is not grounded, it becomes an
input that monitors the high-side MOSFET drain voltage
(converter input voltage) and uses that measurement to
calculate the appropriate on time for the converter.
Therefore, EN/HSD must be connected to this node in
order for the controller to operate properly.
.
