MAX1533 Datasheet, PDF (29/38 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
N=
V_SEC + V_FWD
V_OUT5 + V_RECT + V_SENSE
where L_PRIMARY is the primary inductance, N is the
transformer turns ratio, V_SEC is the minimum required
rectified secondary voltage, V_FWD is the forward drop
across the secondary rectifier, V_OUT5(MIN) is the mini-
mum value of the main output voltage, and V_RECT is the
on-state voltage drop across the synchronous-rectifier
MOSFET. The transformer secondary return is often con-
nected to the main output voltage instead of ground to
reduce the necessary turns ratio. In this case, subtract
V_OUT5 from the secondary voltage (V_SEC - V_OUT5) in the
transformer turns-ratio equation above. The secondary
diode in coupled-inductor applications must withstand
flyback voltages greater than 60V. Common silicon recti-
fiers, such as the 1N4001, are also prohibited because
they are too slow. Fast silicon rectifiers such as the
MURS120 are the only choice. The flyback voltage
across the rectifier is related to the V_IN - V_OUT difference,
according to the transformer turns ratio:
V_FLYBACK = V_SEC + (V_IN - V_OUT5) x N
where N is the transformer turns ratio (secondary wind-
ings/primary windings), and V_SEC is the maximum sec-
ondary DC output voltage. If the secondary winding is
returned to V_OUT5 instead of ground, subtract V_OUT5
from V_FLYBACK in the equation above. The diode’s
reverse-breakdown voltage rating must also accommo-
date any ringing due to leakage inductance. The
diode’s current rating should be at least twice the DC
load current on the secondary output.
Transient Response
The inductor ripple current also impacts transient-
response performance, especially at low V_IN - V_OUT dif-
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 total output voltage sag is the sum of the voltage
sag while the inductor is ramping up, and the voltage
sag before the next pulse can occur.
( )²
L ∆I_LOAD(MAX)
( ) ^{VSAG = 2C}_OUT ^V_IN ^{× D}_MAX ^{- V}_OUT ⁺
( ) ^∆I_LOAD(MAX) ^{T - ∆T}
C_OUT
where D_MAX is the maximum duty factor (see the
Electrical Characteristics table), T is the switching period
(1 / f_OSC), and ∆T equals V_OUT / V_IN x T when in PWM
mode, or L x 0.2 x I_MAX / (V_IN - V_OUT) when in skip
mode. The amount of overshoot during a full-load to no-
load transient due to stored inductor energy can be
calculated as:
( ) _{VSOAR =}
2
∆I_LOAD(MAX) L
2C_OUT V_OUT
Setting the Current Limit
The minimum current-limit threshold must be great
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The
peak inductor current occurs at I_LOAD(MAX) plus half
the ripple current; therefore:
I_LIMIT >
I_LOAD(MAX) +
⎛
⎝⎜
∆I_INDUCTOR
2
⎞
⎠⎟
where I_LIMIT equals the minimum current-limit threshold
voltage divided by the current-sense resistance
(R_SENSE). For the default setting, the minimum current-
limit threshold is 70mV.
Connect ILIM_ to V_CC for the default current-limit
threshold. In adjustable mode, the current-limit thresh-
old is precisely 1/10th the voltage seen at ILIM_. For an
adjustable threshold, connect a resistive divider from
REF to analog ground (GND) with ILIM_ connected to
the center tap. The external 500mV to 2V adjustment
range corresponds to a 50mV to 200mV current-limit
threshold. When adjusting the current limit, use 1% tol-
erance resistors and a divider current of approximately
10µA to prevent significant inaccuracy in the current-
limit tolerance.
The current-sense method (Figure 9) and magnitude
determine the achievable current-limit accuracy and
power loss. Typically, higher current-sense limits pro-
vide tighter accuracy, but also dissipate more power.
Most applications employ a current-limit threshold
(V_LIMIT) of 50mV to 100mV, so the sense resistor can
be determined by:
R_SENSE = V_LIMIT / I_LIM
For the best current-sense accuracy and overcurrent
protection, use a 1% tolerance current-sense resistor
between the inductor and output as shown in Figure
9a. This configuration constantly monitors the inductor
current, allowing accurate current-limit protection.
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