MAX15053_1107 Datasheet, PDF (13/21 Pages) Maxim Integrated Products – High-Efficiency, 2A, Current-Mode Synchronous, Step-Down Switching Regulator

High-Efficiency, 2A, Current-Mode
Synchronous, Step-Down Switching Regulator
FEEDBACK
DIVIDER
ERROR AMPLIFIER
V_OUT
FB
R1
*C_FF
VFB
R2
g_MV
R_OUT
COMP
VCOMP
R_C
C_C
POWER MODULATOR
SLOPE
COMPENSATION
RAMP
V_IN
C
g_MC
COMPARATOR
PWM
CONTROL
LOGIC
Q_HS
L
Q_LS
OUTPUT FILTER
AND LOAD
I_L
V_OUT
DCR
I_OUT
ESR
C_OUT
R_LOAD
V_{COMP GMOD}
I_OUT
R_OUT = 10^AVEA(dB)/20/gMV
REF
*NOTE: CFF IS OPTIONAL AND DESIGNED TO EXTEND THE
REGULATOR’S GAIN BANDWIDTH AND INCREASED PHASE
MARGIN FOR SOME LOW-DUTY CYCLE APPLICATIONS.
NOTE: THE G_MOD STAGE SHOWN ABOVE MODELS THE AVERAGE CURRENT OF
THE INDUCTOR, IL, INJECTED INTO THE OUTPUT LOAD, IOUT, e.g., IL = IOUT.
THIS CAN BE USED TO SIMPLIFY/MODEL THE MODULATION/CONTROL/POWER
STATE CIRCUITRY SHOWN WITHIN THE BOXED AREA.
Figure 1. Peak Current-Mode Regulator Transfer Model
set to 0.3). In addition, the peak inductor current, IL_PK,
must always be below the minimum high-side current-
limit value, IHSCL, and the inductor saturation current
rating, IL_SAT.
Ensure that the following relationship is satisfied:
( ) ^I_{L_PK}
= I_LOAD
+
1
2
∆I_L
< min
I_HSCL,
I_{L_SAT}
Input Capacitor Selection
The input capacitor reduces the peak current drawn from
the input power supply and reduces switching noise in
the device. The total input capacitance must be equal to
or greater than the value given by the following equation
to keep the input ripple voltage within the specification
and minimize the high-frequency ripple current being fed
back to the input source:
C_IN
=
f_SW
×
I_LOAD
∆V_{IN_RIPPLE}
×
V_OUT
V_IN
where DVIN_RIPPLE is the maximum-allowed input ripple
voltage across the input capacitors and is recommend-
ed to be less than 2% of the minimum input voltage,
fSW is the switching frequency (1MHz), and ILOAD is the
output load. The impedance of the input capacitor at
the switching frequency should be less than that of the
input source so high-frequency switching currents do not
pass through the input source, but are instead shunted
through the input capacitor.
The input capacitor must meet the ripple current require-
ment imposed by the switching currents. The RMS input
ripple current is given by:

I_RIPPLE
=


V_OUT
×
(V_IN
V_IN
−
V_OUT
)



I_LOAD
where IRIPPLE is the input RMS ripple current.
Output Capacitor Selection
The key selection parameters for the output capacitor
are capacitance, ESR, ESL, and voltage rating. The
parameters affect the overall stability, output ripple volt-
age, and transient response of the DC-DC converter.
The output ripple occurs due to variations in the charge
stored in the output capacitor, the voltage drop due to
the capacitor’s ESR, and the voltage drop due to the
______________________________________________________________________________________   13