MAX1533A Datasheet, PDF (33/38 Pages) Maxim Integrated Products – High-Efficiency, 5x Output, Main Power-Supply Controllers for Notebook Computers

High-Efficiency, 5x Output, Main Power-Supply
Controllers for Notebook Computers
where C_RSS is the reverse transfer capacitance of N_H,
and I_GATE is the peak gate-drive source/sink current
(1A typ).
Switching losses in the high-side MOSFET can become
a heat problem when maximum AC-adapter voltages
are applied, due to the squared term in the switching-
loss equation (C x V_IN² x f_SW). If the high-side MOSFET
chosen for adequate R_DS(ON) at low battery voltages
becomes extraordinarily hot when subjected to
V_IN(MAX), consider choosing another MOSFET with
lower parasitic capacitance.
For the low-side MOSFET (N_L), the worst-case power
dissipation always occurs at maximum battery voltage:
( ) PD (N_L
⎡
Resistive) = ^⎢1 -
⎣⎢
⎛
⎜
⎝
V_OUT
V_IN(MAX)
⎞⎤
⎟
⎠
⎥
⎦⎥
I_LOAD
2 _RDS(ON)
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 cir-
cuit to tolerate:
I_LOAD =
I_LIMIT
-
⎛
⎝⎜
ΔI_INDUCTOR
2
⎞
⎠⎟
where I_LIMIT is the peak current allowed by the current-
limit circuit, including threshold tolerance and sense-
resistance 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 =
Q_GATE
200mV
where Q_GATE is the total gate charge specified in the
high-side MOSFET’s data sheet. For example, assume
the FDS6612A n-channel MOSFET is used on the high
side. According to the manufacturer’s data sheet, a sin-
gle FDS6612A has a maximum gate charge of 13nC
(V_GS = 5V). Using the above equation, the required
boost capacitance is:
C_BST =
13nC
200mV
=
0.065μF
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 (ΔI_DOWN) as much as it ramps up during
the on-time (ΔI_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 MAX1533A/MAX1537A controllers include a mini-
mum on-time specification, which determines the maxi-
mum 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
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