MAX1549ETL Datasheet, PDF (30/35 Pages) Maxim Integrated Products – Dual, Interleaved, Fixed-Frequency Step-Down Controller with a Dynamically Adjustable Output

Dual, Interleaved, Fixed-Frequency Step-Down
Controller with a Dynamically Adjustable Output
The absolute worst case for MOSFET power dissipation
occurs under heavy overload conditions that are greater
than I_LOAD(MAX) but are not high enough to exceed the
current limit and cause the fault latch to trip. To protect
against this possibility, “overdesign” the circuit to tolerate:
I_LOAD
=
I_LIM
-
⎛ I_LOAD(MAX)LIR⎞
⎝⎜
2
⎠⎟
where I_LIM is the peak current allowed by the current-limit
circuit, including threshold tolerance and sense-resis-
tance variation. The MOSFETs must have a relatively
large heatsink to handle the overload power dissipation.
Choose a Schottky diode (D_L) with a forward-voltage
drop low enough to prevent the low-side MOSFET’s
body diode from turning on during the dead time. As a
general rule, select a diode with a DC current rating
equal to 1/3rd the load current. This diode is optional
and can be removed if efficiency is not critical.
Boost Capacitors
The boost capacitors (C_BST) must be selected large
enough to handle the gate-charging requirements of
the high-side MOSFETs. Typically, 0.1µF ceramic
capacitors work well for low-power applications driving
medium-sized MOSFETs. However, high-current appli-
cations driving large, high-side MOSFETs require boost
capacitors larger than 0.1µF. For these applications,
select the boost capacitors to avoid discharging the
capacitor more than 200mV while charging the high-
side MOSFETs’ gates:
C_BST
=
N × Q_GATE
200mV
where N is the number of high-side MOSFETs used for
one regulator, and Q_GATE is the gate charge specified
in the MOSFET’s data sheet. For example, assume one
IRF7811W N-channel MOSFET is used on the high
side. According to the manufacturer’s data sheet, a sin-
gle IRF7811W has a maximum gate charge of 24nC
(V_GS = 5V). Using the above equation, the required
boost capacitance is:
C_BST
=
1 × 24nC
200mV
=
0.12µF
Selecting the closest standard value, this example
requires a 0.1µF ceramic capacitor.
Applications Information
Duty-Cycle Limits
Minimum Input Voltage
The minimum input operating voltage (dropout voltage)
is restricted by the maximum duty-cycle specification
(see the Electrical Characteristics table). However,
keep in mind that the transient performance gets worse
as the step-down regulators approach the dropout volt-
age, so bulk output capacitance must be added (see
the voltage sag and soar equations in the Design
Procedure section). The absolute point of dropout
occurs when the inductor current ramps down during
the off-time (∆I_DOWN) as much as it ramps up during
the on-time (∆I_UP). This results in a minimum operating
voltage defined by the following equation:
( ) V_IN(MIN)
=
V_OUT
+ V_CHG
+
⎛
h_⎝⎜
1
D_MAX
−
⎞
1 _⎠⎟
V_OUT + V_DIS
where V_CHG and V_DIS are the parasitic voltage drops in
the charge and discharge paths, respectively. A rea-
sonable minimum value for h is 1.5, while the absolute
minimum input voltage is calculated with h = 1.
Maximum Input Voltage
The MAX1549 controller includes a minimum on-time
specification, which determines the maximum input
operating voltage that maintains the selected switching
frequency (see the Electrical Characteristics table).
Operation above this maximum input voltage results in
pulse-skipping operation, regardless of the operating
mode selected by SKIP. At the beginning of each
cycle, if the output voltage is still above the feedback-
threshold voltage, the controller does not trigger an on-
time pulse, effectively skipping a cycle. This allows the
controller to maintain regulation above the maximum
input voltage, but forces the controller to effectively
operate with a lower switching frequency. This results
in an input-threshold voltage at which the controller
begins to skip pulses (V_IN(SKIP)):
V_IN(SKIP)
=
V_OUT
⎛
1
⎞
⎜
⎝
f_OSCt_ON(MIN)
⎟
⎠
where f_OSC is the switching frequency selected by FSEL.
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