MAX1761 Maxim, MAX1761 Datasheet - Page 15

• Manufacturer Part Number: MAX1761
• Description: Small / Dual / High-Efficiency Buck Controller for Notebooks
• Manufacturer: Maxim

The MAX1761 features a thermal fault protection circuit.
When the temperature rises above +160°C, the DL low-
side gate-driver outputs latch high until ON1 is toggled
or V+ is cycled. The fault threshold has 10°C of thermal
hysteresis, which prevents the regulator from restarting
until the die cools off.
Power-on reset (POR) occurs when V+ falls below
approximately 2V, resetting the fault latch and prepar-
ing the PWM for operation once the power is cycled. VL
undervoltage lockout (UVLO) circuitry inhibits switching
and forces the DL gate driver low until VL rises above
4.25V, whereupon an internal digital soft-start timer
begins to ramp up the maximum allowed current limit.
The ramp occurs in five steps: 20%, 40%, 60%, 80%,
and 100%; 100% current is available after 1.7ms.
Firmly establish the input voltage range and the maxi-
mum load current before choosing the inductor operat-
ing point (ripple current ratio). The following three
factors determine the SMPS design using the
MAX1761:
1) Input Voltage Range. The maximum value
2) Maximum Load Current. There are two values to
Table 4. Operating Mode Control Summary
Shutdown
ON1 Enabled
Forced PWM
Normal Operation
(V+(max)) must accommodate the worst-case high
AC adapter voltage. The minimum value (V+(min))
must account for the lowest battery voltage after
drops due to connectors, fuses, and battery selec-
tor switches. If there is a choice at all, lower input
voltages result in better efficiency.
consider, the peak load current (I
the continuous load current (I
current determines the instantaneous component
stresses and filtering requirements and thus drives
output capacitor selection, inductor saturation rat-
ing, and the design of the current-limit circuit. The
continuous load current determines the thermal
stresses and thus drives the selection of input
capacitors, MOSFETs, and other critical heat-con-
tributing components. Modern notebook CPUs gen-
MODE
______________________________________________________________________________________
Thermal Fault Protection
ON1 < 0.5V
2.0V < ON1 < V+
2.0V < ON1 < V+
2.0V < ON1 < V+
Design Procedure
ON1
POR and Soft-Start
LOAD
). The peak load
LOAD(MAX)
Buck Controller for Notebooks
X
ON2 < 0.5V
Floating
2.0V < ON2 ≤ VL
Small, Dual, High-Efficiency
) and
ON2
3) Inductor Operating Point. This choice provides
The MAX1761’s pulse-skipping algorithm initiates skip
mode at the critical conduction point. So, the inductor
operating point also determines the load-current value
at which PWM/skip mode switchover occurs. The opti-
mum point is usually found between 20% and 50% rip-
ple current.
The inductor ripple current also impacts transient-
response performance, especially at low V
ferentials. Low inductor values allow the inductor
current to slew faster, replenishing charge removed
from the output filter capacitors by a sudden load step.
The amount of output sag is also a function of the maxi-
mum duty factor, which can be calculated from the on-
time and minimum off-time:
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as fol-
lows:
Example: I
2.5V, f = 350kHz, 35% ripple current or LIR = 0.35:
erally exhibit I
trade-offs between size and efficiency. Low induc-
tor values cause large ripple currents, resulting in
the smallest size, but poor efficiency and high out-
put noise. The minimum practical inductor value is
one that causes the circuit to operate at the edge of
critical conduction (where the inductor current just
touches zero with every cycle at maximum load).
Inductor values lower than this grant no further size-
reduction benefit.
Both OUT1 and OUT2 off, VL and REF disabled
OUT1 on in normal mode, OUT2 off
Both OUT1 and OUT2 on in forced PWM mode
Both OUT1 and OUT2 on in normal mode
V
SAG
LOAD(MAX)
L
=
=
2C
V+
LOAD
F
× ƒ ×
×
V
( I
= 2.5A, V+(max) = 20V, V
∆
OUT
DUTY V (
= I
DESCRIPTION
LOAD(MAX)
LIR I
LOAD(MAX)
(
V
+
×
-
+
LOAD(MAX)
Inductor Selection
V
(MIN)
OUT
)
2
)
✕
×
- V
L
80%.
OUT
IN
)
- V
OUT
OUT1
dif-
15
=