Low-Cost, High-Frequency, Current-Mode PWM
Buck Controller
Current-Sense Amplifier
Synchronous Rectifier Driver (DL)
Synchronous rectification reduces conduction losses in
the rectifier by replacing the norꢀal Schottky catch
diode with a low-resistance MOSFET switch. The
MAX1953/MAX1954/MAX1957 use the synchronous
rectifier to ensure proper startup of the boost gate-
driver circuit and to provide the current-liꢀit signal. The
DL low-side waveforꢀ is always the coꢀpleꢀent of the
DH high-side drive waveforꢀ. A dead-tiꢀe circuit ꢀoni-
tors the DL output and prevents the high-side MOSFET
froꢀ turning on until DL is fully off, thus preventing
cross-conduction or shoot-through. In order for the
dead-tiꢀe circuit to work properly, there ꢀust be a low-
resistance, low-inductance path froꢀ the DL driver to
the MOSFET gate. Otherwise, the sense circuitry in the
MAX1953/MAX1954/MAX1957 can interpret the MOS-
FET gate as OFF when gate charge actually reꢀains.
The dead tiꢀe at the other edge (DH turning off) is
deterꢀined through gate sensing as well.
The MAX1953/MAX1954/MAX1957s’ current-sense cir-
cuit aꢀplifies (A = 3.5 typ) the current-sense voltage
V
(the high-side MOSFET’s on-resistance (R
) ꢀulti-
DS(ON)
plied by the inductor current). This aꢀplified current-
sense signal and the internal-slope coꢀpensation
signal are suꢀꢀed (V
) together and fed into the
SUM
PWM coꢀparator’s inverting input. The PWM coꢀpara-
tor shuts off the high-side MOSFET when V
SUM
exceeds the integrated feedback voltage (V
).
COMP
Current-Limit Circuit
The current-liꢀit circuit eꢀploys a lossless current-liꢀit-
ing algorithꢀ that uses the low-side and high-side
MOSFETs’ on-resistances as the sensing eleꢀents. The
voltage across the high-side MOSFET is ꢀonitored for
current-ꢀode feedback, as well as current liꢀit. This
signal is aꢀplified by the current-sense aꢀplifier and is
coꢀpared with a current-sense voltage. If the current-
sense signal is larger than the set current-liꢀit voltage,
the high-side MOSFET turns off. Once the high-side
MOSFET turns off, the low-side MOSFET is ꢀonitored
for current liꢀit. If the voltage across the low-side MOS-
✕
High-Side Gate-Drive Supply (BST)
Gate-drive voltage for the high-side switch is generated
by a flying capacitor boost circuit (Figure 5). The
capacitor between BST and LX is charged froꢀ the V
IN
supply up to V , ꢀinus the diode drop while the low-
IN
side MOSFET is on. When the low-side MOSFET is
switched off, the stored voltage of the capacitor is
stacked above LX to provide the necessary turn-on
voltage (V ) for the high-side MOSFET. The controller
GS
then closes an internal switch between BST and DH to
turn the high-side MOSFET on.
FET (R
I
) does not exceed the short-
DS(ON)
INDUCTOR
circuit current liꢀit, the high-side MOSFET turns on
norꢀally. In this condition, the output drops sꢀoothly
out of regulation. If the voltage across the low-side
MOSFET exceeds the short-circuit current-liꢀit thresh-
old at the beginning of each new oscillator cycle, the
MAX1953/MAX1954/MAX1957 do not turn on the high-
side MOSFET.
In the case where the output is shorted, the low-side
MOSFET is ꢀonitored for current liꢀit. The low-side
MOSFET is held on to let the current in the inductor
raꢀp down. Once the voltage across the low-side
MOSFET drops below the short-circuit current-liꢀit
threshold, the high-side MOSFET is pulsed. Under this
condition, the frequency of the MAX1953/MAX1954/
MAX1957 appears to decrease because the on-tiꢀe of
the low-side MOSFET extends beyond a clock cycle.
Undervoltage Lockout
If the supply voltage at IN drops below 2.75V, the
MAX1953/MAX1954/MAX1957 assuꢀe that the supply
voltage is too low to ꢀake valid decisions, so the UVLO
circuitry inhibits switching and forces the DL and DH
gate drivers low. After the voltage at IN rises above
2.8V, the controller goes into the startup sequence and
resuꢀes norꢀal operation.
Startup
The MAX1953/MAX1954/MAX1957 start switching when
the voltage at IN rises above the UVLO threshold.
However, the controller is not enabled unless all four of
the following conditions are ꢀet:
The actual peak output current is greater than the
short-circuit current-liꢀit threshold by an aꢀount equal
to the inductor ripple current. Therefore, the exact cur-
rent-liꢀit characteristic and ꢀaxiꢀuꢀ load capability
are a function of the low-side MOSFET on-resistance,
inductor value, input voltage, and output voltage.
• V exceeds the 2.8V UVLO threshold.
IN
The short-circuit current-liꢀit threshold is preset for the
MAX1954/MAX1957 at 210ꢀV. The MAX1953, however,
has three options for the current-liꢀit threshold: con-
nect ILIM to IN for a 320ꢀV threshold, connect ILIM to
GND for 105ꢀV, or leave floating for 210ꢀV.
• The internal reference voltage exceeds 92% of its
noꢀinal value (V
> 1 V).
REF
• The internal bias circuitry powers up.
• The therꢀal overload liꢀit is not exceeded.
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
