MAX1624 Datasheet, PDF (15/24 Pages) Maxim Integrated Products – High-Speed Step-Down Controllers with Synchronous Rectification for CPU Power

High-Speed Step-Down Controllers with
Synchronous Rectification for CPU Power
error signal. Since average inductor current is nearly
the same as peak current (assuming the inductor value
is set relatively high to minimize ripple current), the cir-
cuit acts as a switch-mode transconductance amplifier.
It pushes the second output LC filter pole, normally
found in a duty-factor-controlled (voltage-mode) PWM,
to a higher frequency. To preserve inner-loop stability
and eliminate regenerative inductor current staircasing,
a slope-compensation ramp is summed into the main
PWM comparator. As the high-side switch turns off, the
synchronous rectifier latch is set. The low-side switch
turns on 30ns later and stays on until the beginning of
the next clock cycle. Under fault conditions where the
inductor current exceeds the maximum current-limit
threshold, the high-side latch resets, and the high-side
switch turns off.
Internal Reference
The internal 3.5V reference (REF) is accurate to ±1%
from 0°C to +85°C, making REF useful as a system ref-
erence. Bypass REF to AGND with a 0.1µF (min)
ceramic capacitor. A larger value (such as 1µF) is rec-
ommended for high-current applications. Load regula-
tion is 10mV for loads up to 100µA. Loading REF
reduces the main output voltage slightly, according to
the reference-voltage load-regulation error (see Typical
Operating Characteristics). Reference undervoltage
lockout is between 2.7V and 3V. Short-circuit current is
less than 4mA.
Synchronous-Rectifier Driver
Synchronous rectification reduces conduction losses in
the rectifier by shunting the normal Schottky diode or
MOSFET body diode with a low-on-resistance MOSFET
switch. The synchronous rectifier also ensures proper
start-up by precharging the boost-charge pump used
for the high-side switch gate-drive circuit. Thus, if you
must omit the synchronous power MOSFET for cost or
other reasons, replace it with a small-signal MOSFET,
such as a 2N7002.
The DL drive waveform is simply the complement of the
DH high-side drive waveform (with typical controlled
dead time of 30ns to prevent cross-conduction or
shoot-through). The DL output’s on-resistance is 0.7Ω
(typ) and 2Ω (max).
BST High-Side Gate-Driver Supply
and MOSFET Drivers
Gate-drive voltage for the high-side N-channel switch is
generated using a flying-capacitor boost circuit (Fig-
ure 5). The capacitor is alternately charged from the
+5V supply and placed in parallel with the high-side
MOSFET’s gate and source terminals.
LEVEL
TRANSLATOR
CONTROL AND
DRIVE LOGIC
MAX1624
MAX1625
BST
DH
R10
LX
V_DD
DL
R9
PGND
V_IN = 5V
D2
N1
C4
N2
R9 AND R10
ARE OPTIONAL
Figure 5. Boost Supply for Gate Drivers
On start-up, the synchronous rectifier (low-side
MOSFET) forces LX to 0V and precharges the BST
capacitor (C4) to 5V through a diode (D2). This pro-
vides the necessary enhancement voltage to turn on
the high-side switch. On the next half-cycle, the PWM
control logic turns on the high-side MOSFET by closing
an internal switch between BST and DH. As the MOS-
FET turns on, the LX node rises to the input voltage, an
action that boosts the 5V gate-drive signal above the
+5V supply. DH on-resistance is 0.7Ω (typical) and 2Ω
(max). Do not bias D2 with voltages greater than 5.5V,
as this will destroy the DH gate driver.
A 0.1µF minimum ceramic capacitor is recommended for
the boost supply. Use a low-power, SOT23 Schottky
diode to minimize reduction of the gate drive from the
diode’s forward voltage. Use a low-leakage Schottky
diode, such as a CMPSH-3 from Central Semiconductor
or a 1N4148, to prevent reverse leakage from discharg-
ing the BST capacitor when the ambient temperature is
high. Place the BST capacitor and diode within 0.4 in.
(10mm) of the BST pin.
Gate-drive resistors (R9 and R10) can often be useful
to reduce jitter in the switching waveforms by slowing
down the fast-slewing LX node and reducing ground
bounce at the controller IC. Low-valued resistors from
around 1Ω to 5Ω are sufficient for many applications.
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