MAX1965 Maxim Integrated Products, MAX1965 Datasheet - Page 15

• Manufacturer Part Number: MAX1965
• Description: (MAX1964 / MAX1965) Tracking/Sequencing Triple/Quintuple Power-Supply Controllers
• Manufacturer: Maxim Integrated Products

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Figure 3. “Valley” Current-Limit Threshold Point
The DL low-side drive waveform is always the comple-
ment of the DH high-side drive waveform (with con-
trolled dead time to prevent cross-conduction or
“shoot-through”). A dead-time circuit monitors the DL
output and prevents the high-side FET from turning on
until DL is fully off. In order for the dead-time circuit to
work properly, there must be a low-resistance, low-
inductance path from the DL driver to the MOSFET
gate. Otherwise, the sense circuitry in the MAX1964/
MAX1965 will interpret the MOSFET gate as “off” when
gate charge actually remains. Use very short, wide
traces (50mils to 100mils wide if the MOSFET is 1 inch
from the device). The dead time at the other edge (DH
turning off) is determined by a fixed internal delay.
Gate-drive voltage for the high-side N-channel switch is
generated by a flying-capacitor boost circuit (Figure 1).
The capacitor between BST and LX is alternately
charged from the VL supply and placed parallel to the
high-side MOSFET’s gate and source terminals.
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 switch-
mode power supply turns on the high-side MOSFET by
closing an internal switch between BST and DH. This
provides the necessary gate-to-source voltage to turn
on the high-side switch, an action that boosts the 5V
gate-drive signal above the input voltage.
All MAX1964/MAX1965 functions, except the current-
sense amplifier, are internally powered from the on-
chip, low-dropout 5V regulator. The maximum regulator
input voltage (V
Internal 5V Linear Regulator (VL)
IN
) is 28V. Bypass the regulator’s output
______________________________________________________________________________________
High-Side Gate-Drive Supply (BST)
Tracking/Sequencing Triple/Quintuple
TIME
Power-Supply Controllers
I
PEAK
= I
VALLEY
(VL) with a ceramic capacitor of at least 1µF to GND.
The V
when V
The internal linear regulator can source up to 20mA to
supply the IC, power the low-side gate driver, charge
the external boost capacitor, and supply small external
loads. When driving particularly large FETs, little or no
regulator current may be available for external loads.
For example, when switched at 200kHz, a large FET
with 40nC total gate charge requires 40nC x 200kHz, or
8mA.
If VL drops below 3.5V, the MAX1964/MAX1965
assumes that the supply voltage is too low to make
valid decisions, so the undervoltage lockout (UVLO)
circuitry inhibits switching, forces POK low, and forces
the DL and DH gate drivers low. After VL rises above
3.5V, the controller powers up the outputs (see Startup
section).
Externally, the MAX1964/MAX1965 start switching when
VL rises above the 3.5V undervoltage lockout thresh-
old. However, the controller is not enabled unless all
four conditions are met: 1) VL exceeds the 3.5V under-
voltage lockout threshold, 2) the internal reference
exceeds 92% of its nominal value (V
the internal bias circuitry powers up, and 4) the thermal
limit is not exceeded. Once the MAX1964/MAX1965
assert the internal enable signal, the step-down con-
troller starts switching and enables soft-start.
The soft-start circuitry gradually ramps up to the refer-
ence voltage in order to control the rate of rise of the
step-down controller and reduce input surge currents
+
[
(V
IN
IN
IN
-to-VL dropout voltage is typically 200mV, so
- V
L
is less than 5.2V, VL is typically V
OUT
)
( )
V
V
IN
OUT
ƒ
OSC
]
-I
I
I
LOAD
VALLEY
PEAK
Undervoltage Lockout
REF
> 1.145V), 3)
IN
- 200mV.
Startup
15