MAX1536 Datasheet, PDF (13/19 Pages) Maxim Integrated Products – 3.6A, 1.4MHz, Low-Voltage, Internal-Switch Step-Down Regulator with Dynamic Output Voltage Control

3.6A, 1.4MHz, Low-Voltage, Internal-Switch Step-
Down Regulator with Dynamic Output Voltage Control
Thermal Shutdown
The MAX1536 features a thermal fault-protection circuit.
When the junction temperature rises above +165°C, a
thermal sensor shuts down the MAX1536 regardless of
V _SHDN. The MAX1536 is reactivated after the junction
temperature cools to +150°C.
Thermal Resistance
Junction-to-ambient thermal resistance, θ_JA, is highly
dependent on the amount of copper area connected to
the exposed backside pad. Airflow over the board sig-
nificantly reduces θ_JA. For heat-sinking purposes, even-
ly distribute the copper area connected at the IC among
the high-current pins. Refer to the Maxim website
(www.maxim-ic.com) for QFN thermal considerations.
Power Dissipation
Power dissipation in the MAX1536 is dominated by
conduction losses in the two internal power switches.
Power dissipation due to supply current in the control
section and average current used to charge and dis-
charge the gate capacitance of the internal switches
(i.e., switching losses—P_SL) is approximately:
P_SL = C x V_IN2 x f_SW
where:
C = 5nF.
f_SW = switching frequency.
The combined conduction losses (P_CL) in the two
power switches are approximated by:
P_CL = I_OUT2 x R_PMOS
where:
I_OUT = load current.
R_PMOS = PMOS switch on-resistance.
The junction-to-ambient thermal resistance required to
dissipate this amount of power is calculated by:
θ_JA
≤
TJ(MAX) - T_A(MAX)
P_SL + P_CL
where:
θ_JA = junction-to-ambient thermal resistance.
T_J(MAX) = maximum junction temperature = +150°C.
T_A(MAX) = maximum ambient temperature.
Design Procedure
For typical applications, use the recommended compo-
nent values in Table 1. For other applications, take the
following steps:
1) Select the desired PWM-mode switching frequency.
See Figure 4 for maximum operating frequency.
2) Select the constant off-time as a function of input
voltage, output voltage, and switching frequency.
3) Select R_TOFF as a function of off-time.
4) Select the inductor as a function of output voltage,
off-time, and peak-to-peak inductor current.
Setting the Output Voltage
Setting V_OUT with a Resistive Voltage-Divider at FB
The MAX1536 output voltage (V_OUT) is set using FB
and REFIN (Figure 5). The MAX1536 regulates V_FB to
be equal to V_REFIN. Connect FB to a resistive voltage-
divider between V_OUT and AGND to adjust V_OUT from
+0.7V to V_IN. Select an R_B from 10kΩ to 100kΩ, then
calculate R_A based on the desired V_OUT:
1800
V_OUT = 1.5V
1600
V_OUT = 1.8V
1400
1200
1000
800
600 _{VOUT = 2.5V}
400
V_OUT = 3.3V
200
0
3.0 3.5 4.0 4.5
V_IN (V)
V_OUT = 1.0V
NO LOAD
5.0 5.5
Figure 4. Maximum Recommended Operating Frequency vs.
Input Voltage
( ) ^{VOUT = VFB}
1 + ^RA
R_B
LX
PGND
V_REF = +2.0V
C_REF
0.22μF
MAX1536
V_FB = V_REFIN
FB
REF
REFIN
AGND
V_OUT = +2.5V
R_A
2.49kΩ
R_B
10kΩ
Figure 5. Setting V_OUT with a Resistive Voltage-Divider at FB
______________________________________________________________________________________ 13