Datasheet
EA
REF
CBOOT
LDO2
2.5V
SS
PVIN
AVIN
Input
UVLO
Output
UV, OV
OTP
PGOOD
EN
PWM Control Logic
AGND
PGND
OSC/
Sync
LDO1
5V
OTP
FREQ
2V5
5V0
SW
FB
COMP
L1
C3
C2
C1
C6
C5
R4
R1
C4
C5
SYNC
VIN
VOUT
C7
R1
UVLO
HS
FET
OCP
Z cross
LS FET OCP
PWM
Slope
Comp
OVP
SW
Rev OCP
I
SENSE
PWM
+
-
1.2V
Non-
overlap
Non-
overlap
LM21305
www.ti.com
SNVS639F –DECEMBER 2009–REVISED MARCH 2013
BLOCK DIAGRAM
OPERATION DESCRIPTION
The LM21305 employs peak current-mode control. The 0.598V reference is compared to the feedback signal at
the error amplifier (EA). The PWM modulator block compares the on-time current sense information with the
summation of the EA output (control voltage) and slope compensation. The PWM modulator outputs on/off
signals to the high-side and low-side MOSFET drivers. Adaptive dead-time control is applied to the PWM output
such that shoot through current is avoided. The drivers then amplify the PWM signals to control the integrated
high-side and low-side MOSFETs.
SWITCHING REGULATOR
The LM21305 employs a buck type (step-down) converter architecture. It utilizes many advanced features to
achieve excellent voltage regulation and efficiency. This easy-to-use regulator features two integrated switches
and is capable of supplying up to 5A of continuous output current. The regulator utilizes peak current-mode
control with slope compensation scaled with switching frequency to optimize stability and transient response over
the entire output voltage and switching frequency ranges. Peak current-mode control also provides inherent line
feed-forward, cycle-by-cycle current limiting, and easy loop compensation. The switching frequency can be
adjusted between 300 kHz and 1.5 MHz. The device can operate with a small external L-C filter and still provide
very low output voltage ripple. The precision internal voltage reference allows the output to be set as low as
0.598V. Using an external compensation circuit, the regulator bandwidth can be selected based on the switching
frequency to provide fast load transient response. The switching regulator is specially designed for high efficiency
operation throughout the load range. Synchronous rectification yields high efficiency for low voltage and heavy
load current situations, while discontinuous conduction and diode emulation modes enable high efficiency
conversion at lighter load currents. Fault protection features include: high-side and low-side switch current
limiting, negative current limiting on the low-side switch, over-voltage protection and thermal shutdown. The
device is available in the WQFN-28 package featuring an exposed pad to aid thermal dissipation. The LM21305
can be used in numerous applications to efficiently step-down from a wide range of input rails: 3V to 18V.
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