Datasheet

MAX16955
36V, 1MHz Step-Down Controller
with Low Operating Current
15
Maxim Integrated
Synchronous rectification reduces conduction losses in
the rectifier by replacing the normal low-side Schottky
catch diode with a low-resistance MOSFET switch. The
internal pulldown transistor that drives DL low is
robust, with a 1.6Ω (typ) on-resistance. This low on-
resistance helps prevent DL from being pulled up dur-
ing the fast rise time of the LX node, due to capacitive
coupling from the drain to the gate of the low-side syn-
chronous rectifier MOSFET. Applications with high
input voltages and long-inductive driver traces can
require additional gate-to-source capacitance. This
ensures that fast-rising LX edges do not pull up the
low-side MOSFET’s gate, causing shoot-through cur-
rents. The capacitive coupling between LX and DL cre-
ated by the MOSFET’s gate-to-drain capacitance (C
GD
= C
RSS
), gate-to-source capacitance (C
GS
= C
ISS
-
C
GD
), and additional board parasitic should not
exceed the following minimum threshold:
Although a low resistive path from DH and DL to the
MOSFET gates is encouraged, there are cases where
series resistors can be added. For instance, a series resis-
tor can be added to the DL path. However, in this case,
should have at least as much resistance in series with the
BST capacitor to help prevent shoot-through current.
High-Side Gate-Drive Supply (BST)
The high-side MOSFET is turned on by closing an inter-
nal switch between BST and DH. This provides the
necessary gate-to-source voltage to turn on the high-
side MOSFET, an action that boosts the gate-drive signal
above V
SUP
. The boost capacitor connected between
BST and LX holds up the voltage across the flying gate
driver during the high-side MOSFET on-time.
The charge lost by the boost capacitor for delivering
the gate charge is refreshed when the high-side
MOSFET is turned off and the LX node swings down
to ground. When the LX node is low, an internal high-
voltage switch connected between BIAS and BST
recharges the boost capacitor to the BIAS voltage.
See the Boost-Flying Capacitor Selection section to
choose the right size of the boost capacitor.
Dropout Behavior During Undervoltage Transients
The controller generates a low-side pulse every four
clock cycles to refresh the BST capacitor during low-
dropout operation. This guarantees that the MAX16955
operates in dropout mode during undervoltage tran-
sients like cold crank.
Current Limiting and Current-Sense Inputs
(CS and OUT)
The current-limit circuit uses differential current-sense
inputs (CS and OUT) to limit the peak inductor current.
If the magnitude of the current-sense signal exceeds
the current-limit threshold, the PWM controller turns off
the high-side MOSFET. The actual maximum load cur-
rent is less than the peak current-limit threshold by an
amount equal to half the inductor ripple current.
Therefore, the maximum load capability is a function of
the current-sense resistance, inductor value, switching
frequency, and duty cycle (V
OUT
/V
SUP
). See the
Current Sensing section.
Design Procedure
Effective Input Voltage Range
Although the MAX16955 controller can operate from
input supplies up to 42V and regulate down to 1V, the
minimum voltage conversion ratio (V
OUT
/V
SUP
) might
be limited by the minimum controllable on-time. For
proper fixed-frequency PWM operation, the voltage
conversion ratio should obey the following condition:
where t
ON(MIN)
is 80ns and f
SW
is the switching fre-
quency in Hz. If the desired voltage conversion does
not meet the above condition, then pulse skipping
occurs to decrease the effective duty cycle. To avoid
this, decrease the switching frequency or lower the
input voltage (V
SUP
).
V
V
tf
OUT
SUP
ON MIN SW
>×
()
VV
C
C
GS TH SUP
RSS
ISS
()
>
⎛
⎝
⎜
⎞
⎠
⎟
Figure 1. Pulse-Skipping/Discontinuous Crossover Point
INDUCTOR CURRENT
TIME
0
V
OUT
V
SUP
f
SW
t
ON(SKIP)
=
ON-TIME
I
PK
I
L
I
LOAD
= I
PK
/2