MAX1966 Maxim, MAX1966 Datasheet - Page 7

• Manufacturer Part Number: MAX1966
• Description: Low-Cost Voltage-Mode PWM Step-Down Controllers
• Manufacturer: Maxim

The MAX1966/MAX1967 are BiCMOS switch-mode
power-supply controllers designed to implement sim-
ple, buck-topology regulators in cost-sensitive applica-
tions. The main power-switching circuit consists of two
N-channel MOSFETs (or a dual MOSFET), an inductor,
and input and output filter capacitors. An all N-channel
synchronous-rectified design provides high efficiency
at reduced cost. Gate drive for the N-channel high-side
switch is provided by a flying capacitor boost circuit
that uses a 0.1µF capacitor connected to BST.
Major circuit blocks of the MAX1966/MAX1967 are
shown in Figures 1 and 2:
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In the MAX1996, most blocks are powered from V
the MAX1967, an internal 5V linear regulator steps
down the input voltage to supply both the IC and the
gate drivers. The synchronous-rectified gate driver is
directly powered from 5V V
gate driver is indirectly powered from V
nal diode-capacitor boost circuit.
The MAX1966/MAX1967 use the R
side N-channel MOSFET to sense the current. This
eliminates the need for an external sense resistor usual-
ly placed in series with the output. The voltage mea-
sured across the low-side R
fixed -305mV reference (Figures 1 and
inductor current limit is given by the equation below:
The DH and DL drivers are optimized for driving
MOSFETs with low gate charge. An adaptive dead-time
circuit monitors the DL output and prevents the high-
side FET from turning on until the low-side MOSFET is
Control Logic
Gate Driver Outputs
Current-Limit Comparator
Clock Generator
Ramp Generator
Error Amplifier
Error Comparator
Soft-Start
5V Linear Regulator (MAX1967)
800mV Reference
Thermal Shutdown
I
_______________________________________________________________________________________
PEAK
Resistorless Current Limit
Detailed Description
305
L
MOSFET Gate Drivers
, while the high-side-switch
mV R
DS(ON)
/
DS ON
DS(ON)
is compared to a
(
L
)
plus an exter-
2).
of the low-
Low-Cost Voltage-Mode PWM
The peak
IN
. In
fully off. There must be a low-resistance, low-inductance
connection from the DL driver to the MOSFET gate for
the adaptive dead-time circuit to work properly.
Otherwise, the sense circuitry in the MAX1966/
MAX1967 detects the MOSFET gate as off while there
is charge left on the gate. Use very short, wide traces
measuring no less than 50mils to 100mils wide if the
MOSFET is 1in away from the MAX1966/MAX1967. The
same type of adaptive dead-time circuit monitors the
DH off edge. The same recommendations apply for the
gate connection of the high-side MOSFET.
The internal pulldown transistor that drives DL low is
robust, with a 1.1
prevent DL from being pulled up due to capacitive cou-
pling from the drain to the gate of the low-side synchro-
nous-rectifier MOSFET during the fast rise time of the
inductor node. The gate drivers are capable of driving
up to 1A. Use MOSFETs with combined total gate
charge of less than 200nC and a maximum V
The MAX1966/MAX1967 feature an internally set soft-
start function that limits inrush current. It accomplishes
this by ramping the internal reference input to the con-
troller transconductance amplifier from 0 to the 0.8V
reference voltage. The ramp time is 1024 oscillator
cycles that begins when initial power is applied. At the
nominal 100kHz switching rate, the soft-start ramp is
approximately 10ms. The soft-start does not function if
the MAX1966/MAX1967 are shut down by pulling
COMP/EN low.
Gate-drive voltage for the high-side N-channel switch is
generated by a flying-capacitor boost circuit (Figures 3
and 4). The flying capacitor is connected between BST
and LX.
On startup, the synchronous rectifier (low-side MOS-
FET) forces LX to ground and charges the boost
capacitor to 5V. On the second half-cycle, the
MAX1966/MAX1967 turn on the high-side MOSFET by
closing an internal switch between BST and DH. This
provides the necessary gate-to-source voltage to drive
the high-side FET gate above its source at the input
voltage.
All MAX1967 functions are internally powered from an
on-chip, low-dropout 5V regulator. The MAX1967 has a
maximum regulator input voltage (V
VCC pin must be connected to VL through a 10
tor and VL must be bypassed with a 2.2µF capacitor to
GND. For operation at V
Step-Down Controllers
High-Side Gate-Drive Supply (BST)
Internal 5V Linear Regulator
typical on-resistance. This helps
VIN
< 5V, connect VL to VIN
Internal Soft-Start
VIN
(MAX1967)
) of 28V. The
TH
of 3.5V.
resis-
7